# Sticky  GTO Tuning and Setup Tips



## lars (Sep 28, 2004)

*Lars’s Tuning Secrets: The Common Tuning Tour Observations and Fixes*

During the _Tuning for Beer World Tour Event Series_, I run across some common themes and problems during the tuning. Thought I’d share the most common performance tuning and upgrade issues that we typically see during the Tours. Here are some comments, tips, and things to look at to help you in your tuning and setup:

First trick to any tuning, and I emphasize this constantly during the Seminars, is to tackle the ignition system first. Never, ever, try tuning a carb without first having tuned the timing curve. 90% of all carb problems are ignition problems.

Get a good dial-back timing light and learn how to use it. This is the most valuable and indispensable tool you will ever own. Sears has them for $69.95, or you can get a nice digital one with built in tach through Summit for around $110. Invest in good tools, and get a timing light.

Unless your distributor has recently been rebuilt and set up by someone competent, your distributor is messed up. I guarantee it. Whether you have a 200,000-mile points unit, an HEI, or a brand new MSD ProBillet, the distributor does not have a good advance curve in it. Yank it out, shim out the endplay with Moroso part number 26140 (setting shaft end play to .005 - .007” for cast iron units and .010 - .015” for aluminum housings), clean it up, and make sure the advance system is working right.

Most point-style distributors are missing the advance stop bushing (little rubber bushing on a pin under the cam advance plate). You can get a brass replacement bushing in Mr. Gasket kit part number 928G. Smack the bushing with a hammer before installing it to make it fit very snug on the pin. This kit also has springs that you can use for your advance curve. For HEI distributors, use the 2 gold springs in the kit. For points systems and MSD distributors, use one black and one silver spring.

Set up your timing to 36 degrees total advance, and install a set of advance springs that make the total timing come in around 2500-2800 rpm (faster if your engine can handle it without detonation). Disregard what the initial timing is – initial timing is irrelevant as long as it’s in a general acceptable range. For a performance engine, I like to see initial timing in the 16 – 18 degree range. Mild-cammed engines can get by with a little less. Big cammed engines like 20 to 24 degrees initial. But set total to 36 as a starting point.

Use a vacuum advance control unit that does not pull in any more than 16 degrees of vacuum advance, and make sure it is “soft” enough to pull its full advance in at the vacuum reading you get at idle. You can run off of either ported or manifold vacuum, depending on what your engine wants for best quality idle, but most engines will respond best (and run cooler) with the vacuum advance attached to manifold vacuum.

Set your carb up to the stock specs for the carb number you are using and make intelligent jetting choices from there. If you are running headers, free-flowing exhaust, good intake, etc., you can bump the carb jetting up about 2 jet sizes as a starting point for your tuning.

On Holley and BG carbs without secondary power valves, secondary jet size should be 8 sizes larger than primary jet size. The Holley Street Avenger carbs are jetted extremely lean, and respond well to an increase in primary jetting by 2 to 4 jet sizes with an 8-jet split to the secondary side.

On vacuum secondary carbs, run the softest spring you can find for the secondary. This is the short white spring in the Holley spring kit.

Beware of commercially rebuilt Q-Jets: These carbs are seriously messed up. Typically, we find that the common screw-ups are: Incorrect floats; cut/modified power pistons; incorrect jetting; and plugged idle air bleed holes. These issues must be corrected before the carb can be made to run right.

Make sure your Q-Jet secondary throttle blades do not open over-center. The secondary throttles should only open to the point that the top edge of the blades points directly at the lower edge of the air baffle. This is short of vertical.

On Q-Jet equipped GTOs, the most common performance issue is that the throttle linkage does not allow enough travel to fully open the secondaries. Check throttle travel on any GTO. Remove the floor mat to correct it in most cases. If this does not do enough, grab the gas pedal and bend it upwards until you can get WOT at the carb.

Holley and BG carbs flow idle fuel on the secondary side. Therefore, you must also have air flow on the secondary side at idle. All Holley and BG carbs have secondary idle speed screws. Use this screw to make the secondary side throttle opening identical to the primary side opening at idle, and make all idle speed adjustments equally to both primary and secondary sides.

Set Holley and BG float levels to the bottom of the sight hole with the engine hot.

Initial bench setting for Holley and BG idle mixture screws is 1 turn out from lightly seated. Make all idle mixture adjustments in small increments, adjusting all mixture screws evenly for best quality idle.

Give the car what it wants: Listen, feel and smell. The car will “talk” to you as you make changes, and you need to listen.

Happy tuning!!

Lars


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## Rukee (Feb 8, 2007)

Wow, what an excellent post. I`m gunna check my distributor right now. Thank you very much!


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## Guest (May 23, 2007)

*Info*

GOOD INFORMATION, but most of the people on this site do not have carborated engines they are most F.I


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## Guest (May 23, 2007)

*tune*



lars said:


> *Lars’s Tuning Secrets: The Common Tuning Tour Observations and Fixes*
> 
> During the _Tuning for Beer World Tour Event Series_, I run across some common themes and problems during the tuning. Thought I’d share the most common performance tuning and upgrade issues that we typically see during the Tours. Here are some comments, tips, and things to look at to help you in your tuning and setup:
> 
> ...


I would NEVER set a float level with a hot engine,,you will spill a little gas


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## Rukee (Feb 8, 2007)

dizzy1 said:


> GOOD INFORMATION, but most of the people on this site do not have carborated engines they are most F.I


Not us original classic Muscle Car guys. I even have one of those points distributors he was talking about.


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## Rukee (Feb 8, 2007)

dizzy1 said:


> I would NEVER set a float level with a hot engine,,you will spill a little gas


I do it all the time, I wouldn`t do it while smoking or having the tourch going near by or running with no exhaust manifolds on, but that`s the way it`s done right.


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## lars (Sep 28, 2004)

dizzy1 said:


> GOOD INFORMATION, but most of the people on this site do not have carborated engines they are most F.I


I've never seen a real GTO with F.I. Are you sure you're on the right Site...? All classic GTOs use carbs - F.I. was never used. And those of us who run them know how to spell it, too... :lol:


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## lars (Sep 28, 2004)

dizzy1 said:


> I would NEVER set a float level with a hot engine,,you will spill a little gas


Nonsense. Float levels are always set on a hot engine. Just don't light a match when you're doing it. Gas spillage is not a problem - the engine surface temp is below 200 degrees. Gasoline does not ignite at 200.


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## VettenGoat (Jun 19, 2007)

dizzy1 said:


> GOOD INFORMATION, but most of the people on this site do not have carborated engines they are most F.I



Is that like a carbonated engine? Gosh I never heard of a real Fuelie GTO. I just have three of those carbonated things.............

Lars, good write up. Any tips for tri-powers? I am going to be dropping in a 421 SD with 1967 2.11/1.77 ported heads (yes we're using 428 pistons to align the valve reliefs) and am wondering how best to approach setting up the carbs; any pearls of wisdom? 
Mike


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## 70455goat (Nov 5, 2007)

Same here. carb and points!!!


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## 70455goat (Nov 5, 2007)

Hey Lars, great post. Normally, I take my engines to a pro to dial them in. I do a lot of work myself, but every time I have tried to dial in my own engines, they run worse, use more gas, and usually wind up pi--ing me off.

I had my son's 350 in his 69 chevelle rebuilt with all good performance parts from bottom up adn replaced all bolt on items under the hood. I couldn't believe how crappy the car ran when I got it back. I took it to a guy in the area known to have the best carb shop. It costed me $400, but it was probably the best $400 I spent. He found 2 different size secondary jets in a brand new Edlebrock carb. recurved the elect. distributor to match the cam, changed the gap on the plugs, and a few other tricks and the chevelle seemed like it had an extra 100 horsepower.

I have a question, what is the difference between manifold or ported vacuum? Is it the difference between taking it from the base of the carb or from the manifold?


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## 05GTO (Oct 6, 2004)

70455goat said:


> I have a question, what is the difference between manifold or ported vacuum?


Manifold vacuum is highest when the throttle blades are closed and rolls off as the blades open.

Ported vacuum is zero at idle, and rises slightly under part throttle and then goes to zero at wide open throttle.


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## 70455goat (Nov 5, 2007)

Lars,
Where do you connect to get ported or manifold vacuum?

Second question: I was checking my stock Q-Jet the other night and when I open the throttle, the secondaries don't open. I hit the throttle on a hard romp and they looked like they wanted to but just barely moved. After looking at the carb, I noticed the lever on the passenger's side of the carb (that keeps the rear flaps from opening) was not releasing. If I manually lifted the lever and hit the throttle, the 4 barrell opened up completely.

Is something wrong witht the release lever?
What makes the lever release so the 4 barrel will kick in?

Thanks for all the great info.
Dale


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## Rukee (Feb 8, 2007)

Ported vacuum will be on the carb, one of the ports at the base of the carb. Manafold vacuum will be plumbed into the intake manifold.
The secondaries will not open untill the motor is warmed up and the choke is fully open. You should be able to hold the choke plate fully open and then that locking lever should move out of the way and allow the secondaries to open.


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## 70455goat (Nov 5, 2007)

Thanks, I don't think my choke is working because, it doesn't idle up at start. I have to keep it running when it first starts. It stays running after a few seconds, but never has a high idle. Could the choke cause the 4 barrels from kicking in?


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## Rukee (Feb 8, 2007)

If the choke doesn`t fully close, or the fast idle cam isn`t adjusted right, it will cause your no high idle, if it doesn`t fully open, it`ll cause your no secondary action. It may just need some lube in the linkage or possaibly the choke stat is bad or way out of adjustment.


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## 70455goat (Nov 5, 2007)

Thanks. I will be checking the carb out this week end. You have been a lot of help.


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## 70455goat (Nov 5, 2007)

Hey Lars,

Thanks for all of the info. Another question: How are you at auto trannys? When I give my goat enough gas to down shift (I don't have to be romping hard even), it won't shift back into the higher gear until I am almost letting completely off of the gas. Like if I punch it and it downshifts into first, it is almost like I have the shifter pulled into first until I let completly off of the gas. But when I am just cruising easily, it upshifts perfectly. It did seem to shift a little better after I drove about 40 miles home from work the other night. I thought of maybe a vacuum leak, but wouldn't it have problems shifting altoghter?

Dale,


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## Rukee (Feb 8, 2007)

When was the last time the fluid and filter were changed? If it`s a turbo 350 in there you could try to adjust the detent cable. There should be a little like a horse shoe sleeve thing on the outer cable at the back side of the bracket that holds it at the carb. Pull that adjusting sleeve up with a screwdriver or whatever then roll the throttle to wide open, let the detent cable slide in or out where ever it wants to be, then while still at WOT push the adjusting sleave back down to lock the detent cable in place. If that doesn`t help, check the vacuum hoses and module at the tranny for manifold vacuum. If it has vacuum check to be sure there is no tranny fluid leaking into the vacuum line, if none, then check inside the hose nipple there, some vacuum modules are adjustable with a thin screwdriver, you may be able to adjust it in or out to make it up shift better to your liking.
If it`s a turbo 400, check the kickdown switch at the gas peddle (electrical) to make sure it`s adjusted right and doesn`t stick in the kickdown position. I`ve seen some that stick and won`t slide easly back and forth


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## cpeahl (Nov 9, 2008)

Same here. 69 convertible, thanks for a great article


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## Bensjammin66 (May 12, 2009)

Great article!! I like the guy that said most of us run EFI.. LOL!! We dont have Australian rear wheel drive grand prix's with GTO badges... We have 60's and 70's GTO's, Lemans and Tempests. I gotta say tho, this i do not agree with *"On vacuum secondary carbs, run the softest spring you can find for the secondary. This is the short white spring in the Holley spring kit."* Thats gonna make the secondarys open REAL fast which is good, dont get me wrong!! BUT, only if your engine can deliver and ignite the fuel that fast. Not everyone runs a race car.... The trick is to supply extra fuel and air to the engine right when it needs it and can handle it. Not too soon and definetly not too late. People who just use the lightest spring in a mild built poncho will feel a flat spot then a big jump in power. "Thats the back barrells kickin in!!!" No its not... Its fuel bogging the engine then igniting it late and making power in a sudden surge. Take your spring out and move softer down the color chart till you feel that bog in the rpm curve, when you feel the bog/surge, use the stiffer spring right above it and youll have smooth operation and power. Just my two cents tho...


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## 646904GTO (Feb 10, 2008)

One opinion I would like to share with all of you about this subject...(Great article) it has to do with the vacuum advance. As we all know the vacuum advance can be as much as 12-15 degrees in a stock unit. What I like to do to the stock units is limit the vacuum advance rod travel so when running 13-15 initial I don't end up with to much advance at sustained low rpm, thus the chance of low RPM detonation. Most of us know that if we check total timing on a say, 1969 GTO YS 400 it is set at 9 BTDC in Drive @650 RPM. The vacuum advance is above the throttle plate so it is a non factor at idle. (I know the '69 had a thermostatic idle switch that allowed manifold vacuum to reach the advance plate when the engine was under 140 degrees) Anyway if one checks the total at 2600 RPM they find some engines have as much as 45-51 degrees! (Can you say early death?)
To eliminate this factor I pull the vacuum pod and weld up the slot a little to control the travel. Reinstall it and check total again. If your off a little either add some weld or file it down a little (or change your intial a little). I use a handheld vacuum pump on the unit when I check the total timing so I then can see how much to limit the travel. All the cars I have done around the area I live have commented on how crisp the throttle became after this mod.(No retard from loss of maniflod vacuum) 
Also as far as Total Timing goes I usually set them up at 34 Total instead of 36 (and all in by 2600 RPM). My reasoning is I used to race a lot of circle track cars that ran Pontiacs and Chevy engines. We dynoed all of them, ran them on race gas and never saw a HP or torque gain after 34 degrees total. Just my experence, hope this is useful information.:seeya:


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## BCsGTO (May 17, 2010)

646904GTO said:


> One opinion I would like to share with all of you about this subject...(Great article) it has to do with the vacuum advance. As we all know the vacuum advance can be as much as 12-15 degrees in a stock unit. What I like to do to the stock units is limit the vacuum advance rod travel so when running 13-15 initial I don't end up with to much advance at sustained low rpm, thus the chance of low RPM detonation. Most of us know that if we check total timing on a say, 1969 GTO YS 400 it is set at 9 BTDC in Drive @650 RPM. The vacuum advance is above the throttle plate so it is a non factor at idle. (I know the '69 had a thermostatic idle switch that allowed manifold vacuum to reach the advance plate when the engine was under 140 degrees) Anyway if one checks the total at 2600 RPM they find some engines have as much as 45-51 degrees! (Can you say early death?)
> To eliminate this factor I pull the vacuum pod and weld up the slot a little to control the travel. Reinstall it and check total again. If your off a little either add some weld or file it down a little (or change your intial a little). I use a handheld vacuum pump on the unit when I check the total timing so I then can see how much to limit the travel. All the cars I have done around the area I live have commented on how crisp the throttle became after this mod.(No retard from loss of maniflod vacuum)
> Also as far as Total Timing goes I usually set them up at 34 Total instead of 36 (and all in by 2600 RPM). My reasoning is I used to race a lot of circle track cars that ran Pontiacs and Chevy engines. We dynoed all of them, ran them on race gas and never saw a HP or torque gain after 34 degrees total. Just my experence, hope this is useful information.:seeya:


Wow very cool posts, makes me wanna tear into my dist and carb. I have a question that may sound stupid but what the heck I've never actually looked. I just inherated my dads 67 gto 400 335hp m21 4.11 posi CA smog pump car that has the factory CDI ignition. Do I even have points? We've had the car since new but it's been in storage for 30+ years. It runs ok but not like I remember it as a kid. I know dad had replaced the carb way back when (1975/76) when the floats stuck one day. I have since purchased a correct 7037263 carb for it. After reading your post I'm wondering if I should take it apart first.


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## 646904GTO (Feb 10, 2008)

The factory CDI stands for capitive discharge ignition, so its the electronic ignition of the day. Since I posted that I have learned that somebody sells a little clip that does the same thing as the weld does. Except it is held down with one of the vacuum pod screws on one side only so the screw head can be broken off easily.


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## OrbitOrange (Jul 5, 2010)

Great information. I went and bought a digital w/tach timight light this morning. Gonna try to set mine up. At what rpm should the TOTAL timing be all in?


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## dastoria (Nov 24, 2007)

To Lars and all the other great contributors on this site,

Thanks to your posts I have been able to: swap out my distributor to HEI, (retained the stock unit for preservation purposes), use a dial back timing light and get my GTO running; braking firmly and surely (thanks again for all the help on that project), Intake manifold gaskets properly sealed (and the intake) , replace the quadrajet (no worries it was with another quadrajet) and a host of smaller items. I had minimal mechanic experience beforehand and the instructions/Chiltons books are basic help at best. Tuning is definitely challenging at 7000 feet, but thanks to all your posts and advice I was finally able to take my son who loves "his GTO" out for a ride this weekend - seeing the joy in his face is priceless. So thanks again.


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## Bensjammin66 (May 12, 2009)

Great post, just read it... I do disagree with this though.. 

"On vacuum secondary carbs, run the softest spring you can find for the secondary. This is the short white spring in the Holley spring kit."

I strongly disagree.. Opening the secondarys up asap is NOT going to work for every application. Why do you think they sell that kit? What worked for me was start soft and go for a ride, gradually going stiffer till the bogs gone. You want smooth aceleration into the secondarys, dumping all the fuel you can as early as you can is not the answer. Just my two cents.


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## Dave67 (Jan 12, 2011)

Great article Lars. 

Glad you posted it and glad it is still easy to find on this forum


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## Sloan66 (May 7, 2010)

Can someone explain where and how to measure the end play spoke of in the post? Thx


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## NEEDLEZ (Oct 21, 2009)

I know what I'm buying at sears today! I plan on pulling the motor this winter and am just gonna bite the bullet and purchase a timing light instead of having to borrow someones.


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## yousaygo (Apr 3, 2006)

Hi, So what is the process for setting timing and advanced timing? I see you mention 36 degrees to start, but how do i then "know" i am good there? The engine is a Pontiac 1965 421, Ross pistons, eagle rods, dyno to 400/425 or so, crank was balanced, the cam is a Crane, do not have specs in front of me, the distributor is a MSD Pro Billet, and i have an MSD Digital 6 ignition, a Muncie m20 4 speed. Car runs cool, never any heating issues. The carb is a Holley 4150 600cfm. I do get about 7 MPG which does seem low, i feel like i am just consuming massive fuel, but do not really smell any out the exhaust, although the tips are getting blackened too. 

1. Do i set the initial timing at 36, then see if it sounds good to me?
2. Do i set initial timing to the cam specs?
3. I have my advance timing at the default for the distributor, right in the middle.
4. I heard i use a vacuum gauge to maximize boost at idle, when do i do this procedure?

My skill is just not high in this area. The car runs really well except the carb is getting "blackened", probably rich?
It idles pretty smooth although i would really like a rougher idle. 
I hope this makes some sense and appreciate your help. I just want the car to be safe, like i am not killing it without know it.
Thanks, Shane


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## BearGFR (Aug 25, 2008)

If you want to "play" with timing, you need at least two things: an "advance" timing light of some sort that's capable of measuring total advance, not just initial. You also need a way to measure your results. By that a mean a dyno, drag strip, etc. -- some way to measure what happens that's more accurate than just the seat of your pants. Once you have those things, then the general procedure is pretty simple. Start "low", meaning a setting that you can be pretty sure is going to be "less than" optimum for your engine. Make a run/dyno pull/whatever to establish a base line measurement. Advance the timing by one or two degrees and repeat. Performance will improve up to a point, then begin to "fall off" again. When that happens, return to your previous setting (which you now know was the "peak") and you're done. Very important that you duplicate the conditions from test to test as much as you can so you don't get false results. I mean engine temperature, weather, road conditions, etc --- the same for every test. You've probably also heard of "recurving the distributor". That means changing the rate at which the centrifugal/mechanical advance comes in - on a stock or stock "style" ignition that involves using different springs and/or weights to change how it behaves. It can also involve limiting the amount of total advance that's "in" the distributor. On many digital/electronic systems all that's controlled by software in the unit itself.

Your description of the "black" does sound like the car's running rich. Although it's possible to get it right without it if you know what you're doing, it's probably easier to spend some time on a chassis dyno to get the carb mixture correct. I'd do that before embarking on trying to optimize the timing.

Using a vacuum gauge as you described is really just for setting the fuel mixture at idle. It's important for idle quality and also for the first part of the transition coming off idle, but it's not going to make a noticable difference anywhere else. Do that after you get the rest of the carb mixture profile right.

Sounds like you've got a really nice car!

Bear


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## lars (Sep 28, 2004)

For those of you asking about how to set up and check the total timing, drop me an e-mail request for my tech papers on "How to Set Timing" and "Vacuum Advance Specs." These papers contain in-depth how-to steps and technical explanations on the entire process and subject, written by me and other automotive authorities. Also, for those asking about "ported" versus "manifold" vacuum, you can write to me for my "Engine Vacuum Explained" paper.

I'm available by e-mail for any other technical questions on carbs and ignition issues. You can also request a full listing of the tech articles I have available on multiple topics.

Lars
[email protected]


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## lars (Sep 28, 2004)

Just a couple of comments and clarifications on the Holley Secondary Vacuum Spring that some of you made some remarks about:

I do a lot of dyno testing of engines that I build, so I have an opportunity to test and observe a lot of different carbs. Generally, a Holley (or BG) vacuum secondary carb, even one that is smaller than the engine's cfm requirements, will never get the secondaries wide open, regardless of engine rpm, load, and spring tension. Typically, to see an engine's actual power potential, we will manually force the secondary throttle to the wide open position on a vacuum secondary carb. This will always add singificantly to the top end of the engine's power curve. Use of the lightest spring possible will always produce a better power and torque curve on the dyno.

On the street, however (and especially in a heavier car like a GTO), the use of an excessively light spring can cause a bog going into the secondaries if the engine's rpm is too low to justify the secondaries opening. I will generally advocate use of a very light spring, and encourage the vehicle owner to choose the "right" rpm (downshift, dammit) before smashing the go-pedal into the secondaries. Getting the secondaries to open as much as possible will always add power to the top end. If the bog is excessive, or if it is a problem for the way the vehicle is driven, then go to a tighter spring and tune to eliminate the bog. If it were me, I'd increase the squirter size and do what I could to run the lightest spring possible to make darned sure you're using the carb to its full potential. A stiffer spring than the "Tall Yellow" would very seldom be required. In order of preference and testing, try the "White" spring first, then regress to the "Short Yellow" and then the "Tall Yellow" for "secondary bog tuning."

Lars


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## ppurfield001 (Jan 21, 2008)

Rukee said:


> Not us original classic Muscle Car guys. I even have one of those points distributors he was talking about.


Points, plugs and condenser???? Only the old guys (like me) remember that.


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## Mississippi Marine (May 25, 2012)

:shutme Hey im building up my 66 389. It needs a clean up bore and some other things. I was wondering what kinda heads to go with and if the ram air 3 and 4 were that great and would they even work on my motor. Also who might make a good replacment set up for three 2BB and an intake. Anything will help and more.


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## BearGFR (Aug 25, 2008)

Ram Air IV's are definitely great, but get ready to drop some serious coin for a set. Much more than you'd pay for a good ported set of Edelbrock aluminum heads that would also outperform the Ram Air IV's. Ram Air III's are pretty decent, but not really "all that" better than a good set of cast iron D-port heads. They'll all fit your engine just fine.
Edelbrock also recently introduced a D-port aluminum head. There are other aftermarket head vendors as well.

There are several aftermarket reproduction tri-power systems "out there". I've got no direct experience with any of them myself personally.

Bear


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## RunninLeMans (Apr 3, 2014)

I replaced the weights, springs and vacuum can on my distributor (stock with HEI) this morning, and using a simple timing light (no advance or tach) and with the vacuum line disconnected at the carb and the carb ports plugged, I set the total timing at 35 degrees, seems to be all in at ~3,000 rpm, and it backs down to about 18 degrees at idle. But, when I plug in the vacuum line, the timing goes to ~ 55 degrees at idle. I read the thread about limiting the vacuum advance travel, I'll try that, but just so I'm clear, shouldn't the total reading be the 18 degrees at idle plus maybe 12 degrees vacuum for total of 30 degrees at idle?

In case it matters, my motor is a mild ~280 hp 326.

Thanks.


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## RunninLeMans (Apr 3, 2014)

I know, seems like a dumb question, but the 55 degrees is so far from what it should be I thought I was missing something....


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## BearGFR (Aug 25, 2008)

Not a dumb question at all. There seems to be a lot of confusion and misinformation on the topic.

Setting up the "timing curve" for an engine consists of two steps. The first is the most important because it is associated with running the engine at full throttle, maximum load, maximum power conditions. This step you do with the vacuum advance completely disconnected, plugged, and out of the picture because when the engine is actually running at full throttle, max load it also doesn't have any manifold vacuum (well, actually just a miniscule amount - but still far less than what would be needed to activate the vacuum advance at all). Since you can't recreate full throttle, max load conditions while sitting still in your driveway, you "simulate" it by physically disabling the vacuum advance system. When setting up the full throttle/max power advance curve, you're interested in two things: the maximum amount of advance your engine "likes" and also how quickly it reaches that maximum value as rpm starts to increase from idle. The "best" way to determine the total value is to go the track and start making passes, varying the setting until you find the one that consistently produces the best time slips. If you can't do that, then a reasonable approximation for most traditional Pontiac engines is to set the total advance (again, vacuum disconnected) to 34-35 degrees if you have the "later model" cast iron open chamber heads, 37-39 degrees if you have closed chamber heads (like the 1967 670's), and 36-38 degrees if you're running aftermarket aluminum heads. The "initial" setting (at idle) really doesn't matter much - what you care about is that total figure and the rpm where it occurs - getting that right. If after getting the total set to your engine's "happy place" you find that it's difficult to start because you've got "too much" initial, then the solution is to modify the distributor so that it has more travel in the advance mechanism so that when you set the total to where it needs to be, it results in less initial. If the total is coming in "too quickly" and/or at too low an rpm, then you either go with lighter weights or stiffer springs to "slow down the rate" at when the maximum advance number is attained.

Step two: Vacuum advance. The purpose of vacuum advance is to add additional timing _only under part throttle, lightly loaded, cruise conditions_ to aid fuel economy (primary) and engine cooling (secondary). You'll find people who swear that vacuum advance always should be connected directly to unported manifold vacuum (which means it will be "active" any time the engine is running) and you'll also find people who swear it always has to be connected to ported vacuum (which means it won't start to come in until the throttle is opened slightly past idle). My opinion on that is that the answer is to try both and use the one that your engine seems to like. If the amount of advance that the vacuum system adds is too much, then either use one of the adjustable advance units, or modify the one you have so that the plunger can't move as far.

In your case, I'd try connecting the vacuum advance to a ported source and see how it goes...

Bear


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## geeteeohguy (Feb 2, 2008)

There is a limiter bushing on the advance rod that rots and falls off, causing too much advance (55+ degrees) at cruise. The bushing can be replaced, or an adjustable advance unit can be installed. These engines actually like 48-50 degrees under light throttle cruise for fuel economy, but tend to surge and buck with too much advance. It takes some tweaking to get it in the sweet spot.


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## RunninLeMans (Apr 3, 2014)

Thanks, Bear and Jeff. Bought a new timing light with all the trimmings, hope to have time Sunday to try to sort it out. Work, wife, band in the way for a few days....


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## Tron70 (Oct 5, 2014)

Come over and tune my junk!! 20* initial is ok with cam and exhaust? I lack the intelligence to use the final timing light.


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## Goat Roper (Oct 28, 2014)

Well after searching for the data on the Mr. Gasket kit for my distributor and finding no/conflicting info here is what I found.
The gold/copper colored springs were all in at around 1,700 and change, the silver ones all in at 2,100 and change and the black were all in at 2,700 and change.
I went with the black and timing at 34 at 3K.
Idle at 750 and with the TH-400 it goes into gear smoothly and the idle drops to 700.
Runs great and no overheating like with the tropical fishes.
Pretty much stays at 195.
I think she is ready now for the Route 66 to Williams run I just need to put a couple hundred more miles on the break in oil and then change it before I go.
I can't wait to get it on the hwy and see what she does.
I think I have everything, Grand Funk, Bonetime by James Harman, Lee Michaels etc.


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## tooth (Jun 5, 2017)

This is an old thread, but my head is spinning after reading it. I was planning on doing a "tune up" on a 68 I picked up yesterday, but I think I have allot to learn to get it right.


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## integrity6987 (May 10, 2017)

*Help understanding Lars recommendation*

Lars - "Yank it out, shim out the endplay with Moroso part number 26140 (setting shaft end play to .005 - .007” for cast iron units and .010 - .015” for aluminum housings), clean it up, and make sure the advance system is working right."

Following Lars statement that this is an issue for most of us - the recommended fix is ~$10 and some garage time. I ordered the Moroso shim kit, but would like to confirm my understanding of where these shims get installed. Based on Figure 6E-26 in the 68 service manual these look like they could go between the washer above the drive gear and the stationary housing tube.

Is the end-play I'm trying to limit between .005 - .007 accomplish a more consistent gap between the rotor and distributor cap?

So it seems the trick will be to assess how much initial end play there is to begin (dial indicator?) then install the correct shim(s) to leave .007" with a cold engine.

Thanks in advance for any help.


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## integrity6987 (May 10, 2017)

To continue - have the distributor apart and worked out the end play question above. I also got the moroso spring and weight kit. Going to use the medium springs to get full 23 degree mechanical advance by 3000rpm with the stock weights. That kit came with a brass bushing the is supposed to be pressed onto a pin in the lower surface of the rotor mount. My stock distrib did not have this and it is not shown or discussed in the 68 service manual. Car ran fine below 5000rpm before this - so thinking this is a generic kit and this bushing is used on small block chevys or other applications. 

Can someone please confirm the Pontiac distributor does NOT use this bushing. It looks like it would fit - but also would use up a bunch of the rotational travel/practical mechanical advance. My inclination is to not install - but this is the first time I've ever done this, and don't want experiment or do it twice to get it correct.


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## tjs72goat (Jul 20, 2017)

Geez, I started looking at my out of engine distributer today and have .067 end play. Wow! I also noticed a chunk broke off the boss that gets clamped on. Previous owner had a bunch of silicone on it. Now I know why. It probably leaked. I had been thinking I might go with something electronic to put in my rebuild. I just might.


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## integrity6987 (May 10, 2017)

tjs72goat said:


> Geez, I started looking at my out of engine distributer today and have .067 end play. Wow! I also noticed a chunk broke off the boss that gets clamped on. Previous owner had a bunch of silicone on it. Now I know why. It probably leaked. I had been thinking I might go with something electronic to put in my rebuild. I just might.


I had that much end play too. Now will be .018" PJ also pm'd me and provided this (link below) - I found very useful and after measuring my parts decided I will install the bushing and secure with an e-clip. Thanks PJ!

How to Optimize the Centrifugal Advance in Pontiacs - High Performance Pontiac - Hot Rod Network


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## 64GTOConvertible (Aug 28, 2016)

Thanks for sticky-ing this. I used this guide this weekend and the results are fantastic!


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## 64GTOConvertible (Aug 28, 2016)

I figured I should tell the whole story. Some of you will get a kick out of it...

I recognized the need to recurve my distributor. In setting the timing I noticed the timing would advance about 8 degrees as I tightened the distributor down. I also noticed the vacuum advance wasn't working at all. So I went and bought the items listed in the OP.

I pulled the distributor, remembering that I needed to mark the position of the distributor before I yanked it out. I forgot I needed to mark the inner part until afterwards, but I had marked the position of the rotor by scratching a mark in the advance weight. Brilliant, right? Yeah, the rotor only installs one way, and I changed the weights...

Anyhow, After setting the end play and changing the advance, weights and springs, I reinstalled the points and reinstalled the distributor. Nothing. No worries, I figured I had it 180* out of phase, still nothing. Checked for spark using a screwdriver - nothing.

OK, out it came again, making sure the wires were connected properly (they were), then I found the points weren't actually closing. Oh, I should also tell you I had bought a point/condenser integral unit, and the condenser effectively blocks access to the points with a feeler guage. I had to 'eyeball' it.

I reinstalled the distributor, pulled the coil wire and stuck a screwdriver in it, holding the driver close to the block while teh wifey cranked it to check for spark. This time there was plenty. I told her to stop cranking and started moving away from the block, only to become the ground and getting about 10 jolts before she let off the key. Once the tingling wore off I had her bump it to TDC, looked at where the rotor pointed and reset the cap and wires. 

I asked her to crank while I moved the distributor, which was working great until the distributor came up out of the block. I had forgotten the clamp that tightens it down, and now the dizzy needed to get indexed to the oil pump shaft too. Crap.

I had to crawl on back there to be able to see the shaft. I figured I was in for an hour of 'move it, try it again', but the damned thing dropped right in on the first try!! This time I set the clamp on first, found TDC, set the cap and wires, and it was running about 5 minutes later.

Moral of the story? Let someone who knows what they are doing at it next time. Any 'real' mechanic would have had it done in an hour. It took me a week...


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## Richard Painter (Oct 21, 2016)

Bear, thank you for sharing your knowledge and experience! I have a 1967 GTO with the original YS code engine that has been rebuilt to HO specifications with the Melling SP7 "068" cam, Ram Air Exhaust Manifolds, original 67 intake, and a 1970 7040264 Quadrajet with a fresh kit and #73 jets. It has ICON forged pistons and forged steel rods. It is balanced. Heads are the 670. It has stock single point distributor with a new Vacuum Advance {V280}. I don't plan to race it, just like to do cruise ins, local car shows, and some cruising around our lovely part of upstate SC. My question is would setting my total timing at 34 degrees be enough to keep it running cool, and run OK on 93 Octane with ethanol? It has an aluminum radiator, shroud, 7 blade fan, and a severe duty fan clutch, 160 thermostat. It will run about 180 to 190 at 45 mph or so, but jumps up to 195-205 when slowing down or stopped.
Thank you for your time and help!
Richard


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## Instg8ter (Sep 28, 2010)

Is your water pump plate that goes over the impellers clearances to .040 (most important)? Also shroud should be set centered over fan blades. I am running the same set up and even stuck in stop and go at Woodward Dreamcruise for 2 hrs never got hotter than 180.


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## Richard Painter (Oct 21, 2016)

Instg8ter said:


> Is your water pump plate that goes over the impellers clearances to .040 (most important)? Also shroud should be set centered over fan blades. I am running the same set up and even stuck in stop and go at Woodward Dreamcruise for 2 hrs never got hotter than 180.


my engine builder set up the clearance on the plate to the impellers and said he got it as close as he could. Fan blades are approximately half way in the shroud. This is an 8 bolt water pump. I am thinking of getting a flowkooler pump and trying that. Thank you for your reply!


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## Rosewood (Jan 3, 2016)

what plugs and gap are recommended for a HEI distributor set up?


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## Dansjudge (Aug 14, 2016)

lars said:


> *Lars’s Tuning Secrets: The Common Tuning Tour Observations and Fixes*
> 
> During the _Tuning for Beer World Tour Event Series_, I run across some common themes and problems during the tuning. Thought I’d share the most common performance tuning and upgrade issues that we typically see during the Tours. Here are some comments, tips, and things to look at to help you in your tuning and setup:
> 
> ...


Thanks for the good post. I just found out about the throttle linkage problem. I was getting a bog when I punched it. I thought my secondary throttle blades were not opening. Then I tried smashing it to the floor and bog all gone. It seems like I need a little more travel.


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## RMTZ67 (Mar 12, 2011)

Dansjudge said:


> Thanks for the good post. I just found out about the throttle linkage problem. I was getting a bog when I punched it. I thought my secondary throttle blades were not opening. Then I tried smashing it to the floor and bog all gone. It seems like I need a little more travel.


There might even be more travel. Have someone hold the pedal all the way down then pull the throttle at the carb back and see if it still has enough more to do something about. I notched another groove farther back on my cable holder so I could be all in.


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