Engine stumble

[469 runner]

I am having trouble sorting out the tune on an engine.  This is a 440, Holley HP series 750 carb, Mopar perf. electronic ignition, timing set at 36 degrees, 52 degrees with vacuum advance connected. It has an Isky cam about 274 degrees hydraulic. This is the issue.  It idles great at 950 rpm in gear, picks up nicely from idle then at about 1500-1800 runs rough, engine misses or surges, above that speed it smooths out and runs very nicely.  I tried adjusting idle mixture and it didn't help, adjusted the timing and that didn't help. There appears to be no vacuum leaks, pulls 18 at idle.  Any help would be greatly appreciated.

[c00nhunterjoe]

Based on limjted info, sounds like a lean squirt. What nozzles in the carb? Part and full throttle bog?

[redgum78]

Are you saying 36 degrees initial timing or total (not counting vacuum)?

[469 runner]

36 degrees total, not counting vacuum.  Not sure on squirter size, has never been messed with. This only happens at part throttle.

[crj1968]

Ran OK and this happened, or never ran right?  Any recent changes?

[469 runner]

I swapped this engine from another car recently.  A fresh engine. Only thing was removed was the carburetor to use a lift plate.  So a new baseplate gasket was used. The carb. sat unused of course this swap took place.  Maybe some dirt?  I have disassembled and sprayed some Gumout through the air bleeds and passages.

[c00nhunterjoe]

Try running with the vaccum advance disconnected and see if the problem changes.

[469 runner]

OK, I tried messing with the vacuum advance some.  I removed some timing via the 3/32" allen screw in the nipple.  Removed about 5 degrees, now the engine is smooth throughout the RPM range.  Doesn't make sense to me as I was not hearing any detonation earlier, I usually try to give an engine all the timing it can take.  I do have small chamber heads, Brodix BS-B1s, that might make a difference. Thanks for the help.

[c00nhunterjoe]

You cant always hear it ping. The beginning stages of detonation is not even audible to us. Plug readings taken while the engine surges then immediately shut off will confirm your findings but if pulling vacuum advance fixed it, then odds are it was too lean with too much timing. Good work!

[crj1968]

Try running with the vaccum advance disconnected and see if the problem changes.

Good call on that one- interesting problem. I will remember this one