[72sbc427]

If it is being caused by running lean or hot engine temps, a couple other things can cause it. Worn out parts in the dist causing the timing to fluctuate improperly. Also too hot of a spark plug. Idle mixture screws misadjusted. Cruise a/f ratio off for the rpm that you run. Cooling system not up to snuff.

I get it with 87 octane. It likes 89 and doesn't have an issue. Same with 91. I run my setup rather lean with hotter plugs for the throttle response.

 

[72sbc427]

Also have noticed it with undersized stock style mufflers. That one is easy to test. Just disconnect the exhaust at the end of the headers. Put in a couple washers at each bolt to create a big exhaust leak. Will be noisy, but if back pressure was causing an issue this will get rid of it for testing.

 

[72sbc427]

what is your initial timing set at? can you adjust your initial timing so that you can use less idle screw on the carb?