# Another timing question



## WhittP (Jun 19, 2013)

Just got the 67 GTO back from the shop after an engine rebuild and transmission swap. The car runs really well but I'd like to get my vacuum advance into the mix. The engine is a 461 with a HR cam, Eheads and about 10.4 compression. The car only sees street use so I would like to gain the efficiency of the extra timing advance under partial load and idle. I dusted off my timing light and with the vacuum pot disconnected the initial advance is 23 degrees. I didn't dial it back far enough or rev it far enough to see what the total advance was but it was at least another 12 degrees if not more. Is there any possible way the vacuum pot can be used without introducing pinging under light load?


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## BearGFR (Aug 25, 2008)

Engines rarely if ever ping under light load, but considering that you're already running quite a bit of initial lead I'd recommend starting out with the can connected to a ported vacuum source as opposed to a non-ported source. It's not going to hurt anything to hook it up and give it a try to see if your engine like it.

Bear


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## WhittP (Jun 19, 2013)

Thank you for the quick reply! I'll plug the vacuum line into the ported vacuum on the Holly carb and see how it does.


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## Lemans guy (Oct 14, 2014)

Just don’t have much more than 12 on top of 23…that is 35 already so not much up room.

if you knocked that centrifugal up to about 20….ran 16 base + 10 from the vac cam at idle, you would have a nice smooth runner. Have the springs bring in all of the 20 by 3000 Rpm or so.


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## WhittP (Jun 19, 2013)

Hooked up the canister with no adjustments to the timing. No audible ping under light load/partial throttle driving. Even low rpm up a hill. Mission accomplished?


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## Atarchus (Aug 4, 2020)

Don't forget your Eheads are a "fast burn" design and usually run less timing. Most people with the fast burn heads run 32-34 total from what I've seen. From the googling I've done it seem like KRE heads don't like more than 32 and Edelbrock don't like more than 34, but it varies of course.

Check around your "all in" rpm maintaining speed up a hill. That's where I first start to get some detonation with my Edelbrock heads. I'm all in at 3000 and if I run too much timing, the vac advance plus max timing will induce some ping/miss holding speed up a hill around 3000rpm.

Any issues starting with 23 initial? I run 18 initial but I've always been reluctant to run more with the aluminum fast burn heads.


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## Lemans guy (Oct 14, 2014)

With 23 initial and then adding vac advance of what 20 or more?…..that is 43? Maybe? But somewhere too high….just as you start moving even using ported vacumn. Many views on it, but seems way too much timing to be any good.

Bear is right that detonation under light throttle is not a problem, but detonation can occur under acceleration with too much timing, plus it is completely inefficient for anything.

you don’t get any tremendous benefit from just adding timing. It has to be the right mix and amount for each of the delivery systems, Centrifigal, Vacumn and base. It is like an orchestra.
you just can’t bang the drum louder and think it is better.

How much timing does the vac can bring in? And when? Is your cam so radical in duration that it needs tremendous base timing? Some do….

you are on it, so you will get it. You first have to figure out where you want to go with it. And yes 32 or 34 are safer total amounts. You can start there and see. But with 23 base that means 9 or 10 Centrifigal…….you think it is that low. Maybe, but rarely see that. Not saying a race shop couldn’t have set it that way.


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## WhittP (Jun 19, 2013)

The engine fires up with less than one revolution it seems like. No hard starting. I was very surprised to see that much initial timing as well. I'll get some time in the next few days to get all these numbers down and figure things out. I'll let you know what it looks like.


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## Lemans guy (Oct 14, 2014)

You will get it right Whitt, sounds like a nice build.


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## WhittP (Jun 19, 2013)

Checked the vacuum advance and it is 23 degrees. Is there any scenario where this would not be too much timing? I thought this was the adjustable vacuum pot
but I guess it's not. It's an MSD "ready to run" unit with the vacuum advance. I set initial to 18 degrees with 16 degrees mechanical by about 2800.


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## Atarchus (Aug 4, 2020)

Just have to listen to your motor. 23 is a lot though. My motor doesn't like more than about 8 degrees advance. It will start to miss when cruising around 2500rpm+ up hills and such (usually at highway speeds). But every motor is different.


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## Lemans guy (Oct 14, 2014)

One common mistake on setting timing is assuming that the “all in” is all of it at 3000 RPM or so. There may be more in there but if you are sure then you know that you have 34 total timing.

Most vac can, including MSD ready to run cans have 20 or more degrees of timing and yes that is way too much. The adjustability of those cans is for RATE OF VACUMN only, not for timing. For that you need to get a fixed stop. MSD has a prototype in development.

Or you can get one from Lars @ [email protected]. Either one will work.

once the corrector is on then set your vac can rate to 2HG below your idle vac and hook it to full manifold vacumn. I always toss those adjustable cans and use a fixed can with the corrector, much better keeps it all solid. But you can make it work.

once set up you will have 18 base and 10 from vac for 28 degrees at idle. These settings I usually use for the most radical cams. But since these electronic modules now retard crank timing by 4 degrees, so you don’t get starter kickback, it is more forgiving. And can be used.

I would probably set yours up more in the mid twenties at idle, with something like 14 Base + 10 vac for 24 degrees of idle timing…then 20 for Centrifigal timing for the same 34 total.

The reason the racer or builder sets that idle at 18 is because he is eliminating vacumn advance and therefore has to run the initial up high to get a good idle. The engines really were not sold or designed for the street that way, they had vacumn advance to compensate for load, that is your foot or driver demand.

Sounds like you are getting to a better place with it either way….


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## Atarchus (Aug 4, 2020)

"The reason the racer or builder sets that idle at 18 is because he is eliminating vacumn advance and therefore has to run the initial up high to get a good idle. The engines really were not sold or designed for the street that way, they had vacumn advance to compensate for load, that is your foot or driver demand."

This also applies to some EFI systems. My timing is controlled by my ECU, I run vacuum advance but the computer doesn't apply advance at idle. So, I run 18-20 initial, 33 @ 3000 with advance starting at 1200. I only run 8 degrees of advance because going up a hill at highway speeds will see around 38 degrees of advance. I'd like to run more to try and squeeze a few more mpg, but it's not worth the risk of inducing detonation. Especially on a car that isn't driven often.


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## WhittP (Jun 19, 2013)

Checked everything again and ran the rpms up past 3k. It turns out that "all in" the curve is actually making 23 degrees mechanical. I pushed the initial back to 15 degrees and drove the car and I didn't have any audible signs of pinging. That's 38 total and I remember the car running pretty well on that. Now there's there's the issue of the vacuum advance. It wasn't plugged in on this test drive but I'd sure like to put it into the equation. I measured it at 23 degrees total advance. That seems like a ton of advance I'd get at partial throttle with my 15 initial plus 23 mechanical along with the vacuum pot.


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## Lemans guy (Oct 14, 2014)

Yes I suspected that you were under counting your centrifugal advance, RPM’s have nothing to do with determining that. It is a total mechanical fixed point.

you now believe it is 23 not 16. And set your base at 15 for 38 total timing.

you do not want more than 8 to 10 degrees of Vacumn advance. With today’s gasoline that is the perfect number. I described how to achieve that above. With a 10 degree vac can hooked to full manifold vac you will have 26 degrees of idle timing.

it will idle perfect there. 38 is aggressive, but some engines can take it.

to be sure of your total centrifugal, you can remove one spring from the distributor weights put the cap back on hook up the timing light and retest until the timing advances no more. That way you will know that it really has reached it’s mechanical limit. And you won’t have to rev it too high to determine that.


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## Lemans guy (Oct 14, 2014)

What is your idle Vacumn reading? 

Automatic in drive…manual in neutral…


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## Lemans guy (Oct 14, 2014)

Oh and don’t forget to put the spring back on if you run that test


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## WhittP (Jun 19, 2013)

It pulls around 16" at idle. Better than it did before the rebuild oddly enough. The engine seems to run very well with the initial and mechanical I have dialed in right now. I'd sure love to get this vacuum working though. Can you tell me how I can limit this to 10 degrees? Its the MSD 8528 ready to run model.


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## PontiacJim (Dec 29, 2012)

WhittP said:


> It pulls around 16" at idle. Better than it did before the rebuild oddly enough. The engine seems to run very well with the initial and mechanical I have dialed in right now. I'd sure love to get this vacuum working though. Can you tell me how I can limit this to 10 degrees? Its the MSD 8528 ready to run model.
> [/
> 
> Make a stop? Look at my pic #1. Cut a strip of metal that will fit inside/between the allen screw and what appears to be a raised dimple (?). This will keep the strip from moving side-to-side.
> ...


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## Lemans guy (Oct 14, 2014)

Yes read post #12 above, I tell you how to get a vacumn corrector.


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## Baaad65 (Aug 29, 2019)

I had the same distributor and same problem so I contacted MSD and they sent me out a prototype limiter you could try but it's not quite as good as a better can with the Lars limiter that LeMans guy was describing, something about the quality of the vacuum signal I can't remember.


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## WhittP (Jun 19, 2013)

Thank you very much for the information gentlemen. I'll report back when I make some progress on it.


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## WhittP (Jun 19, 2013)

My solution involved $3 in parts from the local hardware store. Needed a couple of adjustments as the metal bar limited the travel to 4 degrees at first so I shaved a little off the corner and got it to 8 degrees. I set my initial to 14 so I have 22 at idle with the vacuum to a manifold source. The mechanical advance is still at 23 degrees which seems a little high. I guess you have to buy new weights that are different shapes to change the limit on that? One model of this MSD uses different size bushings but I don't think that applies on this one.


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## Lemans guy (Oct 14, 2014)

Yes you would need new weights from a curve kit. But 23 is perfect, you won’t gain anything unless your cam is so radical that it needs 28 to 30 degrees idle timing.
You are running 37 degrees total timing, ok as long as you are not pinging. If you are dial the base back 2 degrees

good work on that stop!


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## WhittP (Jun 19, 2013)

Thanks, I'm going to retighten my screw and use some blue Loctite unless you think I should shave off a tiny bit to get the vacuum advance to 10 degrees.


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## Lemans guy (Oct 14, 2014)

8 to 12 is good……10 is the median, if you decide to shave it you can go up to as much as 4 degrees at Idle, but 10 would give you 24 that would Be smooth and cool. 8 is ok 10 a hair better on your set up I would say


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## WhittP (Jun 19, 2013)

I wanted to update the folks interested in this. I'm not one to just leave something alone after it's running ok so I decided to tinker again. I took the mechanical advance down to 18 degrees so I could start with more initial. I went with 18 and it starts and idles just fine. Of course I messed with my vacuum pot as well and it's pulling 12 degrees now. I'm at 36 total and my initial plus vacuum at idle is 30. I know that's a little high but I don't have any audible knocking but I'll check the plugs after a bit. The engine seems to run much nicer with these settings. I had the feeling that 14 or 15 was a little low for this motor and it was contributing to a small but noticeable off idle stumble. I like to tune my Holly idle screws to max vacuum per the guidelines but it always seemed to want another 1/4 turn to get rid of the stumble (accelerator pumps are set correctly too by the way). That fixed it but I always had the rich idle partially burned gas smell all over me when I took a cruise. Well, a little timing and a little less fuel did the trick. No gas smell and no stumble. Now it's time to get my catch can and wagner pcv valve into the mix. Thanks for the guidance on this.


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## Lemans guy (Oct 14, 2014)

Great job whitt 30 is Ok. As long as the starter does not kick back and the idle is smooth it will be nice. An off idle stumble from a stop is usually indicative of the transfer slot not being perfectly set. The carb transfer slot actually is the first thing that adds fuel/air to the idle circuit as you transition from stop to moving. A slower pedal does not engage the accelerator pump them and if the transfer slot was not right it can stumble.

When you set to highest vacumn it sometimes causes you to close the throttle and thereby open the transfer slot too much to the manifold. Now the slot is already adding mixture at idle, already drawing it. So when you transition it does not help, because you are already drawing it.

So setting the vac gauge say 2 HG below highest can help to make it right. Another way is to square the slot with carb turned over and don’t touch it at all. Just use the mixture screws.

But for you, you now have it right, I would not touch it drive it and enjoy that perfect timing!


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## Baaad65 (Aug 29, 2019)

Nice job, I ended up putting a 14° bushing in my MSD from FBO ignitions, set the base at 20°, all in at 34° about 2500 rpms and pulling 11° vacuum so 31° at idle to burn off the fuel and the plugs are looking good, still starts good even after letting the temperature peak and runs like a beast with no detonation....now if I could just eliminate the secondary hesitation from idle to wot I could sleep better. Tried everything, cam position, bigger tube nozzles, floats are right on as are the accelerator pumps, tried secondary idle adjustment, went up a couple jet sizes, have a 5.5 pv. It's an 850 QFT on an Performer RPM with a 1/2" spacer but just can't get the air moving fast enough to start pulling fuel when I mat the peddle after that split second it goes like a scalded dog.


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## Lemans guy (Oct 14, 2014)

Have you increased the size of your “Power valve channel restrictors? PVCR’s are two mini jets behind each power valve. On a Quick fuel they are easy screw in just like a jet.

when you drop below 5.5 HG on the power valve those are what feeds the fuel… they come in various sizes. At WOT all your main jets, primary and secondary and all of your PVCR’s are open and pulling. If you have two PV you have 4 PVCR.

Normally hesitation like this is a transfer slot or accelerator pump, the cam or nozzles or adjustment of same. But you say that tried those, so you might want to look at the PVCR they may need to be larger.

pull the bowl they are right under the PV they have a number just like a jet. Your carb sheet that comes with it will tell you the size as well.


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## Baaad65 (Aug 29, 2019)

Haven't touched those and don't know the size but no one has suggested that before also I was mistaken the PV is a 6.5 with the secondary pv blocked off. FBO thought it might be out of balance so they said turn the main idle up 200 rpms then use the secondary idle screw to bring it back down, tried this twice with no luck so I reversed the procedure just once. From about 1500 rpms there's no hesitation and none up to the secondarys. The builder doesn't have any more suggestions and I guess I shouldn't have used Ebay for this but his stuff looks professional with a 100 feedback and I talked with him a few times and seemed to know his stuff since it's all he does but I suppose every carb needs to be tuned to the motor it's on, he says it's not to big either...here's the build sheet and sorry I'm drifting off of the timing post so maybe I'll start another one but I did change one advance spring back to how I got the motor one light blue and one light silver instead of two light blues to see if it was an advance problem, now I'm all in at 34° @ about 2500 so hopefully that's ok. Got a couple of shows this weekend so I'll see how it feels.


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## Baaad65 (Aug 29, 2019)

Lemans guy said:


> Have you increased the size of your “Power valve channel restrictors? PVCR’s are two mini jets behind each power valve. On a Quick fuel they are easy screw in just like a jet.
> 
> when you drop below 5.5 HG on the power valve those are what feeds the fuel… they come in various sizes. At WOT all your main jets, primary and secondary and all of your PVCR’s are open and pulling. If you have two PV you have 4 PVCR.
> 
> ...


Don't know if you got a chance to look at my build sheet on the carb but will it have pvcr's if the secondary power circuit is blocked off? I tried one lighter advance spring and driving it this weekend it still hesitates on wot so don't think it's timing.


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## Lemans guy (Oct 14, 2014)

Yes I see it is a double pumper with one 6.5 power valve with one blocked off. So under the one power valve up front you have 2 PVCR’s, yes the valve is high flow, but the PVCR are part of flow too. You can look on the quick fuel tech website and see what the stock one is. You can call your carb builder and ask him what it is, or you can pull off the front bowl unscrew the power valve and look for yourself. They are two small screw in mini- jets.

These are part of fuel fuel during any operation where the PV vacumn is below the rated number, 6.5 HG. It basically opens below that and allows fuel to flow thru the PVCR, they come in various sizes. They are not normally a “Hesitation” thing, so not sure it will solve your issue, but since you tried everything else and they are in the fuel equation when you go to WOT that extra fuel may be needed. It sounds like a lean bog. But again if this does not happen under load and only in neutral not sure it is a valid concern.


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## Baaad65 (Aug 29, 2019)

Yes it's happening under load all the time but there's no hesitation stabbing the throttle up to the secondarys only when I put the pedal to the floor quickly so does that mean the problem is in the secondarys and will there be PVCR's in the secondary if the PV is blocked off? I went up two jet sizes in the secondary to 84 and up one to 75 in the primary, I know one size doesn't do much but it's what I had in stock now I bought some #76 and #86 I could try. The carb builder wasn't that friendly the first time I called and even less the second time so I'm done with him. I emailed with a Holley site tech because they own Quick Fuel and they were no help and stopped answering me.


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## Lemans guy (Oct 14, 2014)

Right no PVcr in secondary because no Power valve, just two up front. You could up the jets watch the AFR on a meter…12.2 for power…

you also have high speed air bleeds on top of the air horn 2 for the primaries 2 for the secondaries. On QFT those are mini jets as well and easy to change. But you are getting into some sophisticated tuning and an AFR meter is really needed, temp or permanent.


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## Baaad65 (Aug 29, 2019)

Ok, don't have an AF meter ..I did increase the rear jets from 82 to 84, would it be a jet problem that quickly into flooring the pedal? The plugs are on light side after about a 100 miles and I don't ever remember seeing black smoke when this happens so maybe I'm still lean, I did try larger pump nozzles but they do have a shorter duration so maybe I should try a smaller nozzle for a longer squirt duration?


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## Lemans guy (Oct 14, 2014)

Did you have this hesitation before you changed the timing per FBO?…..the 14 Centrifigal and 31?…..

FBO says the carb is not too large, but you say that you cannot get the velocity thru the Ventura’s to draw the fuel…

..those two things cannot both be true…..

it has to be one way or the other…..


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## Baaad65 (Aug 29, 2019)

Yes it started hesitating last year soon as it got hot and humid so I called the builder who wasn't to helpful and he said I could try up jetting it, then called him this summer and he said check to see if the fuel was boiling out of the squirters which it wasn't so I asked him if it was to big he said no the 750 has the same bores, FBO never said it was to big when I asked him. FBO said they can't help me anymore without putting their 5 gas analyzer on it so idk how I would accomplish that. I'm just guessing on the air velocity speed theory by what I've read in the Holley books I have, and like I mentioned I changed out one spring to see if getting my advance in a little sooner would help but it didn't, it feels good and I don't hear any detonation so I think I'll leave it there.


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## Baaad65 (Aug 29, 2019)

I have a good video of the problem but I haven't been able to post videos here so I could put it on YouTube or if you want to PM me with an email I'll send it that way.


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## Lemans guy (Oct 14, 2014)

Well now that you have identified it as the secondaries, the most likely culprit is the secondary accelerator pump. Richer in some form, bigger, longer duration, different cam….or some combination.

a 5 Gas is great an AFR is almost as good lot’s easier to read. The goal of the 5 gas is to get to the perfect AFR anyway, but both tools together is great. As particular as you are I would put wideband 02 sensors in each exhaust and get a dash mounted gauge set. I use Daytona digital meter. Then you easily see if you are too lean or too rich in the secondaries and adjust according.


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## Baaad65 (Aug 29, 2019)

Ok so probably not the jets because I have a set of 86's in stock I could try vs the 84's. Both cams are in the same position and both nozzles are #31 sooo maybe try the secondary cam in the number 2 position or a smaller nozzle in the secondary or both??? but of course one experiment at a time. I don't want an AFR gauge in the dash but I could keep it in the glove box and pull it out when needed, once this is figured out I shouldn't need to be looking at it all the time I feel, if it's running good and the plugs stay looking good I don't see the point, I'm not going to go making adjustments every time I see the gauge go a little rich or lean...like my friend said it can drive you crazy. Maybe a winter project if I can't get this figured out by then. Do you want to see the video? I'll try and post it to YouTube under "carb hesitation"


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## PontiacJim (Dec 29, 2012)

Did you highjack the OP's thread? Might want to consider starting a new post since the OP seems to have gotten his engine squared away?


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## Baaad65 (Aug 29, 2019)

Just drifted off course from the timing issue, will correct that, sorry boss 👍


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## PontiacJim (Dec 29, 2012)

Baaad65 said:


> Just drifted off course from the timing issue, will correct that, sorry boss 👍



Good deal. I know you got the timing pretty much under control, so now you can address the carb. Start a new post as I have a couple questions for you.


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