General Category > Quadrajet Carb Talk and Tips

APT adjustment

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jagness:
Hopefully, here are all the necessary specs for my question: Olds 350, 9.8:1 CR, solid flat tappet cam 239/241 on a 110 LSA. Quadrajet is a 17057553, rebuilt with one of Cliff's kits and his advice on idle circuit and air bleeds: main jets 74, Cliff's fully tapered 44 primary rods. I'll be happy to provide any more info if needed. Initial timing is currently at 15 with 19 degrees mechanical advance all in at 3500 rpm.

Here's my question. I had trouble with it pinging on 91 octane gas. It mainly happened going up steep hills around 1900-2100 rpm (running a 200-4R with 3.90 reared gears). So I started adjusting the APT out. Started at 3 turns out. Didn't stop pinging up steep hills until I got to 5.75 turns out. Is it unusual to need to adjust the APT that far?

Other than this issue, the engine starts quickly, cold or hot, idles crisply with 14" of vacuum at 800 rpm, and literally rips when I nail it from any speed or rpm. Overall, I'm really happy with it.

Cliff Ruggles:
Are you using vacuum advance?

Climbing a "steep" hill most likely the load is enough and throttle angle high enough the PP isn't down anyhow.  It is only down during light load scenarios.  Any heavy load or when the vacuum drops off the APT isn't a player anyhow.....

jagness:
Yes, I have a Proformance adjustable vacuum advance connected to full manifold vacuum. Right now, I have it adjusted to add just a couple degrees advance at idle. I already backed it off so much, I figured it wasn’t causing the pinging.

Cliff Ruggles:
Vacuum advance adds timing at light engine load to make the engine more efficient, burn a leaner mixture, use less fuel, etc.  You can choose to add timing at idle using manifold vacuum and doing so often requires an adjustable VA if/when the engine doesn't produce a lot of vacuum at idle speed.

This would require setting the spring inside the VA according to how much vacuum the engine makes in and out of gear at idle speed so it keeps the timing added for both scenarios. 

When you are driving the vehicle the engine will make plenty of vacuum at light engine load to employ the VA and it will drop out the timing anytime you load the engine and vacuum falls off.

If you are "pinging" with heavier load on the engine I would hook up a vacuum gauge to see what vacuum reading you see when this happens and if it is high enough to apply any timing from the VA unit.  Most likely the pinging at heavier load you are seeing is just too much timing from the mechanical advance and you just need to retard the timing till it stops. 

The carburetors APT system only adjusts the height of the metering rods in the jets at very light engine load.  In all driving scenarios when engine load is light engine vacuum is high so the Power Piston will be in the down position.

My guess here is that you are just running too much timing on the engine for the octane being used........

jagness:
Okay, thanks a lot Cliff. I’ll check the vacuum under load as soon as I get the chance.

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