General Category > Quadrajet Carb Talk and Tips

APT adjustment question

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Kenth:
50C rods are for the earlier 4MV/MC style Qjet.
Most later M4MC Truck Qjets uses rods with M suffix.
.0575" idle needle holes are somewhat small for non-emission usage, i would enlarge to at least .085".
Float at 9/16" is rather low, i would aim for at least 13/32".

FWIW

Cliff Ruggles:
He's probably using my 50C rods, custom from .050" to .036" full taper.

Idle system is a little on the lean side for what you are doing.  Do you have full control from rich to lean with the idle mixture screws?

The swirl port heads are excellent for a truck or towing build, not so much for "high performance" work.

I think I have a 1987 Monte Carlo SS intake around here someplace that fits those heads......

Greasy Harley:

--- Quote from: Kenth on April 28, 2021, 11:04:32 AM ---50C rods are for the earlier 4MV/MC style Qjet.
Most later M4MC Truck Qjets uses rods with M suffix.
.0575" idle needle holes are somewhat small for non-emission usage, i would enlarge to at least .085".
Float at 9/16" is rather low, i would aim for at least 13/32".

FWIW

--- End quote ---

*EDIT**
Idle needle holes are actually .089
And I was having trouble with low float and raised the float just a touch. no problems since.
Sorry, it's been a while and I was looking up specs from my old posts.


--- Quote from: Cliff Ruggles on April 28, 2021, 04:59:29 PM ---He's probably using my 50C rods, custom from .050" to .036" full taper.

Idle system is a little on the lean side for what you are doing.  Do you have full control from rich to lean with the idle mixture screws?

The swirl port heads are excellent for a truck or towing build, not so much for "high performance" work.

I think I have a 1987 Monte Carlo SS intake around here someplace that fits those heads......

--- End quote ---
Yes, they are your rods, jets etc.
Yes, I have full control of the idle circuit.

Greasy Harley:
I think I got it. I turned the APT down 1/4 turn at a time 'til I got light detonation with the Vacuum adv, then I turned it up just over 1/4 turn. pulls hard, no detonation, no stumble.
I am still unclear what I was doing wrong during the tip-in procedure.
I am doing something wrong. The adjust and test method works, but it takes a lot more time.
Anybody have a link to a good tip-in procedure video?

Cliff Ruggles:
We tune for results when driving, so nothing really to worry about.  The "tip-in" procedure helps get you close and insures you don't have the rods too far out of the jets (rich).  You should see or hear a very slight engine speed increase if the engine is running around 1800-2000rpm's on the fast idle when you gently close the choke flap just a tad......

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