Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

Quadrajet tuning

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rocks:
Have the following Edlebrock 1910 quadrajet. Pontiac 455, 10.0:1 compression, iron 4x heads, performer rpm, headers, very large mystery cam. I think it may be a comp cam 292 or around there. The car has to idle at 900-1000 rpms. Get about 14-16" of vacuum.

I decided to put a velocity stack on it for fun, this has required multiple jet changes from what I had, it had a 10x2" air filter. Air filter off it would run around 11.0:1 afr. Filter on it was to lean.

With the velocity stack i started with 72 main jet and CH secondary rods.
It now has 74 main jet with DA secondary rods.
This seems to provide a very responsive throttle, almost instant secondary opening, as in I cant tell when when the transition happens it just does. The car launches very hard now.

My afr at WOT is around 12.0:1. Idle afr is 13.5-14.0.
Cruise is 13.0-13.2 with the APT screw all the way down.

Can I change the rods on the power piston to fix my Cruise AFR?. With a 73 jet I can get it to 14.5 on the highway and it stays there, but with a 73 main jet, its to lean at wot, and throttle response is bad.

I currently have 71-74 main jets, CE,DA,CH secondary rods. Ordered a CK secondary to try it out.

What am I missing? I did go through the carb with all new gaskets and accelerator pump from cliffs.

77cruiser:
What rods are in it? Yea a bigger rod & then you will be able to tune it in with the APT.

Cliff Ruggles:
Yes you can install a larger primary rod to provide more control with the APT system.  Edelbrock should offer them, we don't stock those here......Cliff

rocks:
I found the rods at jegs, can only get three out of four of them. One set is not longer made it seems. So ill be ordering the two bigger rods for now. Thanks for the help. Eventually im going to do fuel injection with megasquirt and get a cowl hood for it.


Some pictures of it.

https://photos.app.goo.gl/UT1r7T1iDmdDgl8h1
https://photos.app.goo.gl/VcPaiehSsaX0xgkc2

Cliff Ruggles:
No interest in fuel injection here, too costly and same end result as a carburetor aside from you may have to "pat" the throttle once to set the choke for a cold start.

When tuning with your q-jet I would establish the ideal primary jet size FIRST.

It should have been 72, 73, or 74 for that carb number.  It has small MAB's so we often find the factory installed 74's just a tad rich at heavy part throttle when mated up to metering rods with .036" tips.

You should not have experience "bad throttle response" by simply moving from 74's down to 73's as the 50M metering rod could simply be raised up to compensate and delivery the same part throttle fuel as the 74's were with the same rods.

Once we determine the ideal jet size we move to the primary metering rod or APT, depending on carb type.  Once the APT is dialed in we go to the track and do some full throttle runs.  I take along precision machined metering rods and find out which ones it likes the best by ET and MPH. 

I may use an A/F meter for some of this tuning but really don't use it for much more than reference. 

I also spend considerable time with the distributor and the vacuum advance when part throttle tuning as you can pick up considerable efficiency there by adding timing at light engine load.  Not at all uncommon to see A/F ratios well into the 15's and the engine run flawlessly once the APT is dialed in with the proper amount of timing.

Each engine will be a little different, so looking at A/F readings on a wide ban only tell me where I'm at for A/F, not particularly what the engine is the most happy with. 

For WOT I typically stay "fat" just to keep EGT's down and reduce any potential for detonation.  You aren't really chasing much power or vehicle performance there anyhow.  At the track I've tuned from "pig" rich to so lean the engine started to surge a bit and the entire spread of ET/MPH from one end to the other was less than .2 and typically no more than about 2-3mph. 

When tuning for WOT with a Q-jet you will find it very difficult to swap around factory secondary metering rods and get the results you are looking for.  Too  many different rod diameters, tip length, upper section tapers and included angles leading to the tips.  You can actually go leaner in A/F going to rods with smaller tips depending on the tip length and included angle leading to them. 

We support quite a few dedicated racers who use Q-jets, and most if not all of the ones that are record holders or running at the head of the pack will have a full set of custom machine secondary rods, and a weather machine, and detailed notes to go with their documented runs........Cliff

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