Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

General tip in question.

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Pav8427:
When tuning APT using tip in procedure, would it be correct to say that when tipping in and you have little to no RPM change, you would raise APT? And on the opposite end, if you have more than the 50-100 rise in RPM, you would lower APT? Would that be the case as long as everything else was close and there are no obvious other issues?

Thanks. Doug

Cliff Ruggles:
Raising the APT will cause less change, lowering more change in RPMs as it makes things leaner.

The APT test only tells you that you are close and have control, any/all final tuning should be done based on engine performance, fuel economy, etc......Cliff

Pav8427:
If I undertand correctly. What you are saying is that when richening things up, a 1/4 turn will give you 'x' amount of RPM change, and leaning up, a 1/4 turn will give you 'x'+ RPM change. Thats good to know if that is the case. 
To clarify my question. Upon intitial start up, and to get to the 50-100 RPM rise for the base tip in tuning, if there is no RPM change during tip in, would you raise APT? And on the other end of that. If you had a lets say 200-300 RPM drop during tip in, would you lower it? As long as there are no other obvious issues rearing there ugly heads, would that apply?

Cliff Ruggles:
The APT is set after the engine will fully heat soaked and idle mixture screws have been set.

Using the fast idle adjustment set the engine rpms around 2000rpms for the test.  This will insure the carburetor is well on the primary main system.

If you see a 200-300rpm's drop the carb is pretty lean, so raise the APT some.  Raising the APT lifts the rods, richens up the A/F ratio and you'll see less RPM drop with "tip-in".  I like to see a very slight change, about 50-100rpms and use that as the baseline setting.

Also make sure the vacuum advance is hooked up and applied during the test.

The big myth in all of this is that a leaner setting will improve fuel economy.  For most set-ups it will not.  All engine need adequate fuel in the mix for complete and most efficient combustion.  The more efficient the engine the less fuel it will need, same for timing.

What I see here is that engines with very tight squish, optimum compression ratios and very well chosen camshafts are the happiest everyplace when it comes to how much fuel and timing they will need. 

Engines with "low" compression ratios, a lot of squish area, poor flowing heads, crappy combustion chambers, and poorly chosen cams are the worst in terms of power production, and how much fuel and timing it takes to make them happy.

I'll also add this while on the subject, the camshaft choice alone can hurt engine efficiency.  Cam companies push very tight LSA's because the customer base demands a some "attitude" at idle speed.  I've watched this trend for many years.  They will also tell you that the "whiz-bang" modern lobe profiles will make more power, more area under the curve, improved throttle response, blah, blah, blah.

I've tested modern camshafts on tight LSA's and not one single example has made the grade.  I will say they certainly produce an aggressive idle quality, but on the dyno and at the track we've made more power and ran quicker with camshafts that have longer seat timing, less aggressive seating velocity, and wider LSA's.

When you get time wonder over to the Lobby and read my thread on camshaft selection........Cliff

Pav8427:
Thank you Cliff. That explains alot. I have read many posts. Over and over. Lots of good info. My question actually applies to a recent build up of a 17058253 with one of you hp kits,44 rods and jets. Just didnt want to throw out a ' whats wrong with my build ' question. You learn by doing.
Thanks again for all the great help from you and others here. And good parts.

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