Hey guys, new here. Quick question for you if anyone doesn't mind giving me some pointers. I have an 81 K20 pickup and I just put a brand new 396 sbc stroker in it. Compression on the engine is 10.4:1, And I have been running 91 octane gas in it. Initial timing is set to 14 and total is at 34. I believe the carb is tuned properly If not on the leaner side. It's a Holley 750cfm with vacuum driven secondarys. I live at 6200 ft elevation and have number 69 jets in the primaries, the metering plate for the secondaries does not have removable jets. Previous to this carb I was running an 850cfm double pumper and had the same issue. The issue is run on after shutoff. I know the problem is the primaries are not closing all the way due to my idle screw. I've set the idle mixture with a vacuum gauge. I can eliminate this issue by advancing the initial timing to 18 which in turn puts me at about 38 total. Forgot to mention, I do have an HEI distributor, I do not have vacuum advance hooked up. Engine builder really doesn't want to see the timing beyond 34 total, however it just seems to run way better at 38. I'm afraid to run it there due to detonation. Any thoughts? I've also tried running octane boost in the 91 octane fuel to rule out a low octane problem and it doesn't make a difference.

 

One thing I can suggest to try is back the idle air screws out 1/2 turn, and see if that makes a difference.