Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
Idle Fluctuations in Drive
Mark H:
Hi Cliff, thank you for your feedback. Over the last couple of years I have enjoyed reading current and older posts as well as your book which has helped me understand these carbs a bit better.
When I started the tuning process after the rebuild, like many others I couldn't get the idle below a 1000 and it would stall in drive. I feel I have come a long way.
I was also surprised how much timing the motor liked, with a relatively mild cam upgrade. The initial 18 degrees is where the timing landed after setting the total at 36. The motor likes what it likes and I have had no issues. The engine at idle also liked manifold over ported.
However I will now turn my attention to checking for any changes in timing in and out of gear as well as comparing ported vs manifold in drive.
Cliff Ruggles:
18 degrees initial timing is a BUTTLOAD at idle speed.
As a general rule of thumb anytime you find yourself having to crank initial timing past 14-15 degrees or so to make an engine happy you just missed the cam choice vs the CID and compression ratio. In other words cranking compression is low and overlap is bleeding off too much compression at low RPM's.
The real fix is less overlap and or higher static compression.
Not enough fuel to the mixture screws will also require a LOT of timing at idle speed.
I consider butt loads of timing and tons of idle fuel "crutch" fixes for fundamental issues, but sometimes that's what needs to be done with these things. Besides who wants your new "high performance" engine to idle as smooth as the 305 in a 1987 Chevy half ton truck?
I've made a very good living because of the choices folks made with the compression ratio and cam events and very quickly find their new engine build doesn't idle or run for chit with "normal" timing and fuel curves, so it's all good from where I'm sitting.......
Mark H:
Hello again
I reviewed some of my notes from before adding bypass air and didn’t note the rpm fluctuations in drive.
So I temporarily blocked the throttle plate holes with aluminum pop rivets. Air bypass holes in the main body are also blocked.
The idle in drive now only fluctuates between 480-510rpm. Vacuum is 7-9mg and A/F is high 15’s although while cruising the A/F seems good at mid 13’s.
Total timing is 35.5 which results in 13.5 degrees at idle at 600rpm with VA plugged. Adding VA from the manifold bumps the rpm to 750 with 23 degrees of timing and 15 mgs of vacuum.
Overall I’m happy with these numbers except the high A/F ratio at idle unless this is normal? and a lack of responsiveness from the mixture screws which I settled on 3 ¼ turns out. Are these two issues connected and what steps could I take to improve?
As mentioned earlier my idle system has 0.0375 tubes and .055 DCRs with no changes to the size of the stock mixture holes. My primary side is 72/49 with 9-7 pp spring which is staying seated at idle in drive.
Cliff Ruggles:
Lack of response from the mixture screws may simply be the throttle angle is too high exposing too much transfer slot or a little nozzle drip.
13.5 is actually pretty rich at cruise and you are rich at full throttle as well. I'd put the .070" main jets back in it and see if you can get the APT screw out of the baseplate. I make one here in the lathe with a spring on it. This will provide full control of the part throttle A/F using .070" main jets and 49B primary rods.....
Mark H:
Hi Cliff
Thank you for your quick response.
I don't believe the transfer slot is open too far but I will confirm next time the carb is off likely when I change the jets back to 70's and I will also check for nozzle drip.
I read some other posts on removing the APT cover/screw. There seems to be some issues from us DIYers in breaking the screw and or damaging the baseplate when drilling the broken screw out. Although the turnability advantage is intriguing, as my part throttle is not an issue I think I will leave that alone for the time being.
I do have a 0.038 drill bit. Do you think enlarging the tubes from .0375 or even going to .039 would have any change on richening up the A/F at idle(currently sitting in the high 15's)?
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