CCC Qjet rich idle problem

[slicer87]

Hello, I have been having trouble with my rebuilt CCC Qjet for a 1986 Monte Carlo SS. Last year I had it rebuilt by a local carb shop that has done good work for me in the past. The problem I have been having is it is way to rich at idle and nether the idle  mixture screws or the IAB adjustments will keep the M/C dwell below 50 deg. The carb also has nozzle dripping at idle, if I turn the IAB down enough the dripping will stop but it doesn't change the dwell. I unplugged a vacuum hose to cause a vac leak to see if this changed the dwell which it did by lowering it 10 deg and the engine seemed to idle better with a vac leak. I did check the M/C plunger travel and it is 3/32 inch which according to my repair book is what the travel should be. 

 The engine is a SBC L69 305 which has an Crane "2040" cam (computer friendly emission legal cam), emission legal Edelbrock headers, and a GM ZZ4 intake manifold. I figured these  modifications might affect the carb which is why I posted this info. Thanks for any help with this problem. 

[mikex]

In my experience, stock CCC Q-jets don't respond much to the idle mixture screws.  I suspect they won't close off fuel entirely, maybe due to binding on the springs.

Do you know how the rich stop and lean stop screws were set?  On most CCC Q-jets these are separate adjustments and if both are set too far CCW then the travel range of the MC solenoid could be too high, and the ECM may not be able to set the MC solenoid duty cycle - what the dwell meter actually indicates - low enough to compensate.  Fixed dwell over 50 degrees means the ECM is going as far as lean as it can.  You might try turning both the rich and lean stop screws CW a turn or two to see if the dwell comes down.  Turn them the same amount and direction to keep the travel the same (my information says total travel should be 4/32).  If you can set the MC dwell this way (it should vary slightly, not be absolutely fixed) this may correct the problem.  You will probably need to adjust the IAB again, as the MC solenoid operates that and moving the rich/lean stops will change its adjustment.  The best way to set the IAB is with a tool designed for the purpose, after the rich/lean stops are set.

[Cliff Ruggles]

In almost all cases, a camshaft change will require idle system modifications.

If it likes some idle bypass air, add some to the baseplate.  For that cam in a 305, about .086" on each side should be enough.

I'd also open up the holes under the mixture screws just a tad, no more than about .089".

Also, pull the idle tubes and clean in and under them, and open them up about .002".

Measure the idle down channel restrictions as well, they should be at least .046".

Computer controlled carburetors have a very similiar idle fuel system as non computerized carburetors, with the upper adj airbleed set-up which can be used to fine tune if/as needed.  It still helps to have enough idle fuel available, and enough bypass air to keep the throttle plate angle low at idle speed and no fuel dribbling from the boosters.

I'd also check the float level, and size of the fuel inlet seat.  If aftermarket parts were installed, the fuel seat is probably too small and the float level may be too low as well........Cliff

[slicer87]

  I just discovered one more thing, no matter how much I lower the idle speed stop screw the motor will not idle lower than 1K RPM. 

[Cliff Ruggles]

Vacuum leak someplace?  Check the secondary throttle plates making sure they are fully seated at idle.  The additional air has to come in somewhere to have no control of the idle speed.....Cliff

[slicer87]

 The engine is pulling 18-19 in of vacuum. When I unhooked the brake booster line to cause a leak, the engine idled better. 

[slicer87]

Here are a few more things I have found. When I back the idle stop screw out all the way it looks like the throttle plates do close. But the throttle shaft lever does not come in contact with the ear the idle stop screw threads into. Also the airhorn gasket has become wet looking. 

[Cliff Ruggles]

The carb would have to be removed and the throttle body taken off, and held up to a light to know if it's closing completely.  Many do not, for one reason or the other.  Make sure all 4 throttle plates are correctly indexed, and fully seated when closed, then put it back on and continue tuning.....Cliff