Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Idle Fluctuations in Drive

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quadrajam:
It wouldnt hurt anything to go ahead and open up the idle mixture screw holes.

Mark H:
Hello

An update and a few questions.

I have a slight nozzle drip and exposed transfer slot beyond the desired square shape.  It takes ¾ of a turn of the idle screw to eliminate the drip, however this drops the rpm in gear and the motor stalls.

My idle system is .0375 and .055 dcrs with air bypass blocked  and I am using 71/49B combo with a 7-9 hg pp spring.

My AFR is showing low 12’s WOT, low 13’s during light cruise and high 13’s during cruise.
My idle out of gear is 720rpm 13.0AFR and 15.5 HG
In gear idle is 480-510 rpm 13.5-13.7AFR and 7-9HG

I have excellent idle screw responsiveness an can see changes in AFR with every full turn.  The screws are currently set at 2 ¾ out.

I’m very happy overall with the responsiveness, idle in gear and all around performance.

However,

I realize that I need to lower the throttle blade angles to eliminate the drip and likely need some bypass air to accomplish this.
Stock configuration of the carb had approximately 0.057 bypass holes and I realize by blocking them and going to .09375 in the throttle plates  was too much air and possibly caused the wider rpm fluctuation of over 100rpm in gear.

Questions
1.   If I leave the nozzle drip as is what is the downside?

2.   If I start adding bypass air I would like to start slow with drilling a single hole in only one throttle plate or do I need holes in each plate to balance our the air?

I would prefer to start slow with one 3/64 hole(0.046) then 3/64 x 2 if needed or a combo of 3/64 and say 1/16, trying to keep below the stock .057.

 Thank you

Kenth:
No need to drill the throttle blades for bypass air.
With your engine/cam i would open the original .059" bypass in throttle plate to .095"-.100".
I would open the idle tubes from .036" to .039"-.040", downchannels from .051" to .055" and idle needle holes from .085" to .095"-.100", keep the original main jetting.

Set initial timing to 10-12° and use the ported source for vacuum advance (you may try full manifold vacuum later).
Also, set the idle mixture screws for best vacuum/rpm in gear at 650 rpm´s, under 500 rpm´s are way too low.

This should make a strong steady idle in gear and a smooth transition to low speed driving.

HTH

Cliff Ruggles:
NEVER drill the throttle plates for idle bypass air.  The Q-jets idle bypass air system is EXCELLENT and a very precisely controlled vacuum leak.  Using it you can fine tune the position of the throttle plates for the correct amount of transfer slot coverage, or exposure, depending on your engines needs.

You are still rich at cruise and may be adding a bit too much timing with the VA as well.

Didn't see where you tried to remove the APT screw and install an adjustable one.  It is an excellent feature and provides the tuner with full control of A/F without taking the carb apart and changing parts......

Mark H:
Hi Kenth

My first attempt at dealing with the idle issue was using recipe #2.  38 tubes, 55 dcrs, and .09375 mixture holes with ported VA and stock jets/rods.  It did not run well.  I ended up starting over and going back to stock tubes and a new baseplate for stock mixture holes and now I am at .0375 tubes and 55dcrs.

Timing is set at 12 initial and 35 all in and it really likes manifold vacuum at idle.

I know i can get the idle higher to 600 with bypass air while eliminating the nozzle drip.  The question is how much bypass air.

Hi Cliff

I understand that it is preferred to use the carb's bypass air over drilling the plates.  As mentioned the bypass holes are plugged and before I drill them out I would like to get a rough idea how much bypass is needed and plan to get to that number slowly with holes in the throttle plates.  Once I'm happy with a number I will replace the plates and drill out the stock bypass holes to the size that's best.

I'm not comfortable at this stage trying to access the APT screw.  The 71/49B gets me close to the stoichiometric ratio.  I have read elsewhere that due to ethanol in today's fuels that the 14.7 number is closer to 14.1 and I am very close to that number and my spark plugs have very good color.    I would be interested in you thoughts on 14.1 and some input into my two previous questions will help me move forward.

Thank you to both of you.

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