# Punch the Throttle, what can I expect?



## leeklm (Mar 11, 2012)

Besides my circle track racing days a few years back, I have not done much tuning to a carburated street car, so am not certain what I should, or should not expect...

Many of you are familiar with my setup, which includes:

--New iron head 455, mild cam & 500lbs of TQ at approx 3,500rpm, 400hp
--800 cfm qjet, factory intake, RAM exhaust- 2.5" w/H pipe 
--Muncie M21 with 2.20 first gear
--12 bolt posi w/3.31 gears
--255/60/15 rear tires (Eagle GT - seem to have good traction)

Before putting in the posi, I could roll in first gear at an idle, punch the throttle, which would break the one tire loose, wind up the engine in a hurry, and go as fast as my one tire will let me through the gears.

Now with the posi, i roll in first gear, punch the throttle, get a squeak from the tires, the engine bogs a little, then picks up RPM over a second or two, and goes like a banchee again.

Before I go chasing carb adjustments night & day, what can I really expect here? With the added traction and relatively tall gears, should the engine be able to provide me with that instant response to break things loose and scream?

Of course if I wind it up some and dump the clutch, no problem, everything is up to speed and we are off to the races.

For you guys that have been down this road a time or two, what are your thoughts?


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## BearGFR (Aug 25, 2008)

leeklm said:


> Now with the posi, i roll in first gear, punch the throttle, get a squeak from the tires, the engine bogs a little, then picks up RPM over a second or two, and goes like a banchee again.
> 
> Before I go chasing carb adjustments night & day, what can I really expect here? With the added traction and relatively tall gears, should the engine be able to provide me with that instant response to break things loose and scream?


Full title banshee mode is available at the hit of the throttle - my car does that and it's also an 800 QJet. Do you have the Cliff Ruggles book about them? Tuning a QJet for initial throttle response under those conditions is one of the more tricky things to accomplish, but it's not impossible.


Bear


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## leeklm (Mar 11, 2012)

Hey Bear, i do have cliffs book, and have bought 3 kits from him over the past year, experimenting along the way with the different carbs. I have tried a few things that could help initial response like pump arm position and stiff primary rod spring, but that is about it. I have not tried pump return springs yet. I should check in with cliff for some general direction before i chase my tail around too much.

Regarding your car, we could probably just bolt on a worn out rochester 2bbl, and the thing would still scream...  

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## geeteeohguy (Feb 2, 2008)

M21 muncies have a very tall 1st gear, and were designed to work with 3.90-4.33 rear ends. With your 3.31 rear gear, an M20 trans with a stiffer first gear ratio would be a big help off the line, with no other changes. With the stock m-20 in my '65 GTO, with a 3.36 posi and a 389 tripower (less power/torque than you have) I can punch it from a roll in 1st gear and annihilate the rear tires. Part of the reason is the steep 1st gear in my early (2.56 1st gear) M-20. Your match of an M-21 with a 3.31 rear end is less than optimum.


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## leeklm (Mar 11, 2012)

I was also thinking the 2.20 1st gear was part of the problem. Sad thing is that I thought the previous 2.56 gear was too steep with the old engine, so when needing to replace the muncie cluster gear for a 1" shaft over the winter, i decided to try a 2.20 ratio. Oops. I guess for a few hours work and a new cluster gear and input shaft i could fix that issue. 

If i am looking at the final drive ratio correctly, i assume that going from a 2.56 to 2.20 1st gear, would have the same impact on acceleration as going from a 3.31 rear axle gear to a 2.95 rear gear?

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## jmt455 (Mar 26, 2012)

Another way to look at the first gear performance is to look at the product of the trans ratio times the axle ratio.
For reasonable off-the-line performance, you want that number to be 7 at an absolute minimum. 

With your 2.20 first gear and the 3.31 axle, the result is only 7.282.
The 2.56 first in the M20 and the 3.31 yields an overall 8.47; that would be noticeable.

I'd probably go for a 3.42 or 3.55 rear gear if you want to keep that trans, but your engine will then be spinning pretty high revs at 70 mph.


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## BearGFR (Aug 25, 2008)

Now that you've got the posi and some traction, there's a chance you could be having the same problem I did.

Does it seem to hit hard just for an instant, then stumble, then catch-up with itself and take off?

Did the problem just start with the posi?

Bear


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## leeklm (Mar 11, 2012)

I do recall reading about that Bear a while back. I think mine is more of a bog than a stumble. It did just start with the posi, which i assumed was because the one wheeler allowed the engine to instantly rev?

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## BearGFR (Aug 25, 2008)

Ok then.... here's what I'm suspicious of. Better traction, harder leave, more of a 'jolt' when it does. My car was doing the same thing and it was driving me batty (er). I played with the air valve spring tension, jets, rods, vacuum diaphragm pull-off rate.... everything. Nothing helped. What fixed it? A 1-inch piece of black electrical tape.
Turns out the car was leaving hard enough to slosh fuel to the back of the bowl where it was splashing up through the holes where the secondary rods pass through and over into the secondaries, creating a momentary rich condition and associated bog. I used the tape to make a seal around the rods (put the tape on the airhorn gasket, make some pin-holes in the tape, then upon reassembling the carb and reinstalling the seondary rods poke them through the tape. 'Work it" some so that the tape doesn't bind up the rods.
The tape makes a seal, no more fuel slosh-over, no more bog.

I wish I could take credit for thinking of this but I can't. Cliff actually told me about it. Once you know to look, you can find references to the problem and the solution on the 'net.
Here's just one thread:
Quadrajet spilling fuel at launch [Archive] - V8Buick.com

Bear


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## geeteeohguy (Feb 2, 2008)

Sigh.....the _problems _you guys have when you pull the front wheels off the ground!


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## leeklm (Mar 11, 2012)

Tried the tape trick this evening, but no go. While I had the airhorn off, I installed the stiffest power valve spring (had one lighter before), which seemed like it helped a little, but maybe just my imagination. Tried the different pump rod positions again, and it does feel better when using the inside rod hole, but still stumbles when punching the throttle. 

I was hit by a few rain sprinkles that dampened the road just a little, and hey, it had great throttle response then by easily breaking the tires loose!

I think my next step will be to wire my secondaries shut to keep them out of the equation and play around with primary jetting, etc.


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## PontiacJim (Dec 29, 2012)

"....and stiff primary rod spring," I may be thinking of something else, but shouldn't the spring be weaker? If this is the spring that lifts the rods up inside the carb, it is operated by the vacuum of the engine. Depending on cam, if you had a weaker vacuum signal, the stiff spring would want to lift the rods up sooner -possibly too soon? Adjusting the secondary air valve opening may help, and its simple to adjust. You may want to tighten the tension on the small spring to delay the opening a small amount to ensure a better transition between the primary and secondary opening of the carb. If it opens too quick, this will cause it to bog. Quick and easy adjustment that costs you nothing and you may be able to tune it to your stiffer gearing. If you have to go into the carb, then Bear makes a good point and I have done this as well per one of the books on Q-jet tuning.


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## BearGFR (Aug 25, 2008)

PontiacJim is right. A stronger spring will have more of a tendency to push the metering rods open, causing the car to run rich. The right way to set this up is two-fold:
1: Measure your car's idle vacuum using a good gauge.
2: Using a hand held vacuum pump, apply "that much" vacuum to the port leading to the power piston (you'll probably have to remove the carb's base plate to access the port) and verify that amount of vacuum is enough to hold the piston fully seated. Change springs until you get the 'right' one.

You -can- tune the secondary air-valve opening rate using the spring tension, but that's really not the right way to do it. The problem is you can get to a point where "enough" tension to slow them down to the point where you lose the bog might also be "too much" tension to allow them to open fully. The right way to tune the opening rate is to alter the calibration on the choke pull off diaphragm on the front of the carb to change the rate at which it 'relaxes' when vacuum is removed. As a starting point, you probably want it to relax somewhere in the neighborhood of 2 seconds. You'll need some very small wire-sized drill bits for this operation. 
Cut some short (1/2 inch or so) lengths of brass tubing that are the right size to fit the vacuum hose on the diaphragm.
Plug all the tubes with epoxy (like JB-weld) and let them dry.
Now take various sizes of tiny drill bits and drill different sized holes in the epoxy plugs - smaller holes for slower opening rates, (slightly) larger holes for faster rates.
Clip the hose going to the diaphragm and insert various restrictions you just made until you find one that gives you the opening rate you want.

This way, you can tune the air-valve spring the way it needs to be (just barely enough tension to hold the air-valves closed) and use the restrictions to tune how quickly it opens when you dump the throttle (and all the manifold vacuum disappears). Experiment until you find the sweet spot your engine likes.

HOWEVER: before you start playing with all this, the first order of business is to make sure your idle, part throttle, and full throttle fuel mixtures are right. Otherwise you'll wind up chasing your tail and not getting anywhere. Read Lar's QJet tuning papers for a good procedure (or buy yourself an Innovate LM-2). Cliff's book has a good way to get close too (read about his "tip-in" procedure).

Fun 

Bear


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## leeklm (Mar 11, 2012)

Thanks for the suggestions guys. I have been down most, if not all of these paths, but probably not in the right order! BTW, here is a note from Cliff on the topic...

"Everything changes for secondary tuning once you find traction. The engine is loaded longer at lower rpm's, so you basically need to slow things down some. Primary side tuning would have little if any effect here, go after the secondaries instead."

Regarding piston springs, I had been going by the "book" of running the strongest spring possible that keeps the piston seated at idle, which is about 18" of vacuum, and part throttle gets up around 23". I have my choke pull-off tuned at about 2 seconds or a little less. I ordered a set of $5 wire bits on ebay a while back just for this purpose  I may install a stock pull-off, which is about 3 or 4 seconds delay to see how it changes things.

I do have Lars paper, and like his tune-in procedure, but must admit, I have not followed it 100%. I am going to get my little hand-held garmin GPS out and use it to measure "max speed" on short runs so that I can better measure performance. However, I am more interested in seat-of-the-pants feel than I am max speed (which of course will mostly coincide).

Regarding the wideband O2 sensor, I actually installed an Innovate 5-wire a couple of weeks back. My idle is hanging around 13-13.5 and part throttle curise between 14.4 & 15, depending on weather. We have been flopping between cool & dry, and warm and muggy lately (if you call 74 warm).

I am doing more research to get a better idea of suggested A/F guidelines, but overall, I am probably a little rich at full throttle. One change on the secondaries was going from a .035 tip to .040, which made a positive improvement at higher speed.

It feels like I am 90% there with this carb, as my "only" concern is the throttle response in an off-idle, high traction, no tire spin condition. 

I am sure that a change of my cluster gear and input shaft to a 2.56 ratio would cover up most of my carb issues, (at least until I bolted on a set of slicks) but really do not feel like spending another $400 in parts and pulling the thing out, again. However, if the trans needs pulled for another reason, I will order up the parts and install them. Anyone need an almost new 1" shaft cluster gear & input shaft for a 2.20 ratio muncie? 

Dang, I almost wrote a novel there, sorry guys...


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## leeklm (Mar 11, 2012)

While Bear & others are out having fun on the Power Tour, many of us are home cranking away on our cars... I spent more time tuning the carb, and am seeing improvements. The power & A/F readings are looking good. I am still rich on the rolling start with secondaries wired shut, and am thinking I need to look at my accel pump and maybe try a softer spring?.

I was getting irritated with my 2.20 1st gear in the Muncie, so purchased a new input and cluster gear, and coverted it to a 2.52 wide ratio setup. Swaped the cluster out today, and is a much nicer setup with my 3.31 rear gear! I think this change alone will double the life of my clutch!


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## BearGFR (Aug 25, 2008)

If you're rich at WOT on the primaries only, and good at cruise, then the solution might be to go with smaller primary jets. You may need smaller primary rods also in order to preserve your present part throttle/cruise mixture when the rods are "down". That's if it stays rich for more than a few seconds. If it's just a momentary thing though, then my vote is for a smaller pump shot. Move the rod to the outer hole if it's not already there. You can also bend the tip of the pump arm "down" so that you get s shorter pump stroke and therefore less fuel.

How long does it stay rich? 

Bear


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## leeklm (Mar 11, 2012)

Had my son riding along watching the A/F readout while I ran through the gears. My bog and rich mixture of about 10.2 last maybe 2 seconds until things rev up & go while secondaries wired shut. This was testing with the 2.20 gear. It is better, but still a little boggy with the 2.52 gear. 

When running full rpm on primaries only, my A/F is about 13 with a 72 jet (started with a 73 that was reading in the low 12s.) Performance was hard to read between the two, but seemed more reponsive with the 72 jet. I am guessing a 71 jet will be reading in the high 13s or 14 with the A/F, so did not change it. 

Getting closer...


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