General Category > Quadrajet Carb Talk and Tips

APT adjustment

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jagness:
Yes, I think I’ll keep working on the curve in my HEI and keep it in the trunk just in case.

Cliff Ruggles:
You can achieve amazing results with a carb and distributor, plus the less complicated and fewer electronics that you put in place the more reliable the set-up will be in long term service.

When you are finished dialing in your Q-jet and HEI about the only difference between that set-up and any attempt at fuel injection will be that you will have to "pat" the throttle once or twice to set the choke and put a shot of fuel into the intake on a cold start.

I've literally spent countless hours tuning, testing, and evaluating the results on several vehicles that I own/owned over the years dialing these things in.  It's remarkable that the final numbers were very close for all of them.  This surprised me some at first, until one realizes that the requirements of a N/A automotive/truck engine are very close. 

Of course this assumes that you have a very well thought out engine build and didn't make too many poor choices for it along the way. 

Compression and cam selection are critical.  I see folks make poor choices in both areas and the engine never has a chance from the day it is fired up to be efficient and make optimum power on pump fuel and thrifty for "normal" driving at the same time.

I know I'm getting off topic just a tad but these things are important and good reading for those who are planning to build a strong running street engine for their vehicle.  I'll be brief here but the most common mistakes that I see are folks LOWERING compression to effectively manage pump gas.  That myth has been around for quite a while now and is one of the most inaccurate statements on the Forums.  Usually those that are making recommendations are regurgitating the proverbial brick wall of 9.5 to 1 static compression for pump gas.

The biggest reason that statement is inaccurate is that the tolerance for octane is based on many factors, static compression only being one of them.  I also see folks quickly ditching stock intakes to buy a high rise aftermarket part and most also buy an antiquated design inefficient aftermarket carburetor for the top of it. 

Many will also attempt an "upgrade" to their ignition system which typically brings nothing to the table anyplace other than most will prove to be unreliable in long term service.  With no exceptions every single aftermarket ignition "upgrade" I've tried here failed sooner or later, most sooner than I was happy with.

Anyhow, I'll sit down now before I end up writing a book on the subject.  For anyone reading this if you take anything away here it should be that distributor tuning is equally as important as carburetor tuning and changes should be made in both areas to find the ideal timing/fuel curves for what you are doing.  So there is really no "magic bullet" with these things and finding optimum settings can take many miles of driving and tanks of fuel, but once you are finished you can button things up and enjoy the vehicle for many thousands of miles........Cliff

jagness:
Thanks Cliff. That's exactly what I'm still working on. I think another problem I'm having is that not every tank of 91 octane gas I buy is equally good. But I want it to run well on what's actually available.

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