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During yesterdays drive, I was retarding the timing as a means to see how far it needed to be backed off to stop full throttle detonation. I had no intention of keeping it at 20 degrees of total timing! I just wanted to get it home and get a timing light on it so I had a number to record.



I'm sure you didn't want to keep it this way but I wanted to point out the retarded timing was most likely causing the glowing headers so you didn't chase your tail on that one. :-)

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It idled and cruised fine at 30 degrees total. It was only at 1/2 throttle and more that it made audible detonation.



What was the base timing when it goes to 33* total? What we need to know is how far (and when) does the dizzy advance. Most distributors will advance 20* or so from the base. The issues you have with low vacuum could be timing related...as in still not enough base timing but you are advancing too far it seems. What is the calculated compression ratio? What kind of heads?

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Today I reset the timing to 33 degrees total and checked idle vacuum in gear....4 inches of vacuum? Really? This is with a 700 rpm in gear idle. I have not touched the idle mixture screws yet. Having the idle vacuum number lower than my power valve rating is obviously a problem.



So two suggestions on the low vacuum here: I still think you need to give us you base timing here (as opposed to total). This engine probably wants 15-18* of base timing and will generate more vacuum with it. Once this is done, you should adjust the idle mixture screws for maximum vacuum. And yes, the vacuum being lower that the PV rating will potentially cause an overly rich condition. I wouldn't worry about that yet until we can achieve a better vacuum reading at idle.

It sounds like if you do advance the base timing like I suggest, your total will go too far and you detonate. We really need to know how far your dizzy is advancing. If you need to rein in the total timing there is a plate you can buy from FBO here part J685 . Other than that you will have to weld the slots to limit the advance.