In Reply to: Backfire on Overrun - Mixture or Timing? posted by Hamish on March 07, 2000 at 13:01:04:
That's interesting point that you raise about pulling extra fuel through the idle circuit because of the high vacuum. Unfortunately, if it's set properly at idle, there's not much you can do about it as far as adjusting the idle circuit.
The power valve is probably also going to figure into this equation. I believe the concept is that there is a vacuum circuit which feeds from the bottom of the carb and when the vacuum is high, the valve at the bottom of the float bowl is opened through a that rather elaborate diaphram/spring/shaft thing in the float bowl. This valve is in parallel with the primary main jet and when opened will inrich the mixture. Again I don't think that there is any way to adjust this to any great extent without messing up your high vacuum acceleration.
I'm betting that more advance on overrun is the way to go. I'm not sure how your going to achieve this. Perhaps simply advancing the dizzy a bit will help. If it runs rough at idle, then check the vac retard system.