Join Date: Oct 2014
Location: Nashville, Tennessee
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That AFR is good at idle, 38 degrees advance some cars can take others not..
Back the base off 4 degrees just for the test, run it up and decelerations see about the backfire, if gone or better or no change then you can go from there. All carbs go lean when the throttle slams shut, but lean relative to the timing advance could be the culprit.
Of course as the guys said, exhaust leaks, carb jets and full throttle mixture. When you are going 5,000 Rpm all carb circuits are engaged,....idle and transfer slots a little they never shut off, primaries and secondary jets and PVCR for the power valve, like mini jets...
So your mixture should be like 12.2 neighborhood....you could be real lean there as well.
One final thought is that idle dashpots were designed to keep throttle from slamming shut....they worked off vac and electric and there were various kinds, it just eases throttle down in such a condition....some cars did not have others did, and they have aftermarket ones also...