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post #1 of 6 (permalink) Old 09-06-2011, 03:27 PM Thread Starter
 
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Carb Tuning

My 68 GTO 400, 4 barrel has a new thumper cam 475 lift. I am having trouble with the carb idle mixture. First of all my vaccuum reads 8 at idle and 18 when driving correctly, then 0 when I stand on it. It read 12 at idle before cam change. I have searched for a vaccuum leak and have not found one. It smells very rich and surges at off idle (1600 rpm). I tried to adjust my idle mixtures and they did not respond, I took them all the way out and no change in idle. ANY IDEAS? I'm lost.
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post #2 of 6 (permalink) Old 09-06-2011, 03:46 PM
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Do they effect the idle if you turn them all the way in?
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post #3 of 6 (permalink) Old 09-06-2011, 05:40 PM
 
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When a 268 Extreme Energy with about the same lift was installed in my engine I had the same issues. With that cam I knew I would only have about 8-10 inched of vacuum at idle. Some of the things I had to do was install a lighter spring for the power piston, put on a vacuum advance that would start at 5" and be full out at 13" and took out about 8 degrees of mechanical advance and added it to the initial to still keep 36-38 total. Seems to be very livable and still have that thumpy idle I was looking for.
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post #4 of 6 (permalink) Old 09-06-2011, 06:19 PM Thread Starter
 
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carb

I can unscrew them all the way in and out and no change. I am told to check my float
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post #5 of 6 (permalink) Old 09-07-2011, 09:24 AM
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First off, the idle screws will not effect the running @ 1600rpms, only at an idle. And if it's idling too high, they may not effect the idle.
Bigger cams do make less vacuum across the board, and nearly every engine will have 0 @ WOT. I would double check the timing as that is probably cause the Serge at 1600.
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post #6 of 6 (permalink) Old 09-07-2011, 11:26 AM
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Originally Posted by Rukee View Post
First off, the idle screws will not effect the running @ 1600rpms, only at an idle. And if it's idling too high, they may not effect the idle.
Bigger cams do make less vacuum across the board, and nearly every engine will have 0 @ WOT. I would double check the timing as that is probably cause the Serge at 1600.
Big time. Luimpy cams that make relatively little idle vacuum do ugly things to a carb's calibration. It's not impossible to correct, but it does require a little understanding of the carb's various systems and how they work. Fr'instance - there's an off-idle transfer circuit that helps during the transition from idle to low cruise. There are slots in the carb throat just above the edges of the throttle plates. At idle these slots are normally blocked by the plates and begin to come into play as the throttle begins to open. Your idle screws having no effect is a classic symptom of a motor that's running on the transfer slots all the time. Because of the cam you have to open the throttle more to keep it running, and it's open enough to expose the transfer slots. The fix is to make sure the carb has enough "idle bypass air" capacity and also large enough idle fuel ports so that it can supply the motor the air and fuel it needs to run with that cam without having to expose the transfer slots. Another problem induced by low vacuum is the power circuit that controls the main jet metering rods. That circuit is designed to gradually "come in" as manifold vacuum drops, adding more fuel as the load on the motor increases. With a low-vacuum cam the spring in that circuit might be too strong, so that it never really "turns off" --- dumping additional fuel into the system when it's not really needed. Let me recommend the Cliff Ruggles book, "How to Rebuild and Modify Rochester Quadrajet Carburetors" - he goes into a lot of detail about these problems and how to deal with them.

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