Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
Fine tuning Q-jet
Cliff Ruggles:
APT changes the metering area at cruise, or light engine load. You MUST use the vacuum advance to tune with the APT, otherwise your engine is missing a lot of timing and can not effectively burn a lean mixture.
Hook up the VA and start over. I would also replace the primary rods with 42B-44B's. Since you have APT you can raise them up and richen things up if/as needed, but will have more control leaning it up.
Running on typically means the throttle is too far open at idle. This can be caused by not enough timing, not enough idle fuel, bypass air or just the idle speed is too high.......
bob69:
Update. WOT was too lean with 68 jet on E10 gas. Changed jets to 69 with 41 rods. Better wot but lean in part throttle cruise. Turned apt 1/2 turn richer and pt cruise now running in mid to high 14s. Turned idle screws down to 2 1/2 turns to get best idle (was 3). Set drive Idle speed to 600 rpm and park idle dropped 100 rpm to 700 (was 800). Idle af in 13s. No longer have run on.
Still have one issue to figure out. The af goes very lean momentarily when throttle is opened. This happens at all engine speeds, from idle to steady cruise, when throttle is opened. This reflected in the af meter readings/recordings.
The accelerator pump was adjusted to stock measurements at 5/16 from air horn to top of plunger (measurement was 6/16 before adj). The af does NOT go rich. The check ball from the kit was used. I see fuel coming from the shooters with the engine off.
The choke pull off provided no delay at all (parts store replacement). This was causing a major stubble on fast acceleration. A brass restrictor from an old choke pull off was inserted into the hose to the pull off. The delay is now about 1 1/2 to 2 second.
The secondary air valve rod was adjusted to keep air valve closed when not needed. Air valve spring set to 1/2 turn.
What else should I look at to fix this monetary lean condition.
I think this is part of a larger issue as acceleration is not what I expected. I have another car with the same cam profile in a 396. Even with the displacement difference the acceleration (in a 350) is still too low.
old cars:
"Update. WOT was too lean with 68 jet on E10 gas. Changed jets to 69 with 41 rods. Better wot but lean in part throttle cruise"
Wot should be adjusted with secondary rods after the primary side is figured out.
"Still have one issue to figure out. The af goes very lean momentarily when throttle is opened. This happens at all engine speeds, from idle to steady cruise, when throttle is opened. This reflected in the af meter readings/recordings."
How much throttle opening ? What is very lean?
Cliff said
"Second I would quit chasing the A/F meter and tune for best results instead."
I believe Cliff is making this statement because some people do not understand what A/F ratio's should be at..... idle/cruise/light acceleration/wot...
bob69:
old cars, the afr I'm getting is in the mid 16 range. This is lean especially since the afr should be rich due to the extra fuel provided by the accelerator pump. I see this at all throttle opening rates, from idle to steady cruise.
I learned some time ago that trying to tune by afr does not give the best performance. This is based on first hand experience tuning my four classics. Three 1969s and a 1968, all chevys with different v8s (327, 350, 396 and 400). All have/had q-jets.
Lars has an excellent write up on how to tune a q-jet that I follow. He is a well respected Corvette tuner and has written several articles on tuning corvettes.
Back to my current situation. I use afr to help identify possible problems. I'm not seeing a momentary rich spike as expected when opening the throttle. I see the monetary rich spike on the other three q-jets. I don't see why this one does the opposite.
The car's acceleration does not come close to what I see for the other cars. I want to fix this before moving on to other tuning adjustments.
old cars:
AFR ratio going in to 16 range on light acceleration with a quadrajet is quite normal. A jab of the throttle may show the accelerator pump (richness ) momentarily. I tune with AFR all the time. I don't usually need AFR to identify possible problems.
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