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You're basically correct.

October 4 2017 at 6:34 AM
Gerry  (Login gerryproctor)

Response to Tuning the carb

It's best to tune to what the engine wants. Instruments and data are great to help you establish a baseline and to accumulate tuning data. The one mistake I see being repeated here and elsewhere is when someone gets an AFR meter and tries to tune the engine to reach a data point, like 14.7 idle AFR. Well, what if the engine just doesn't like that? Are you going to stick with that even though the engine is hard to start and has a lean misfire? Probably not. Yes, I have an AFR and yes, I tried to tune the idle circuit as lean as possible using the gauge. The 390 just does not like anything above 13.9 AFR at warm idle.

As far as the vacuum gauge; In my experience, leaning out the idle mixture to get highest vacuum usually leaves you a little lean on the idle circuit. I normally go to highest vacuum leaning it out, then go rich until the gauge drops around a quarter point. This seems to make the idle circuit rich enough to assure easy cold starts and a clean warm idle.

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