irfgt wrote: ↑Tue Apr 16, 2024 9:51 am
You probably have a lean surge and not an actual misfire. Try increasing the main jet size by a couple of thousandths and I'll bet it goes away.
On a 65/66 140 the primary carburetors had a power enrichment circuit to deal with a lean condition at certain throttle openings aggravated with the use of secondary carburetors partially opened during acceleration (or high cruise speeds).
Having said that lets not start changing things that should NOT need to be changed. I have never encountered a 65/66 primary carburetor that needed bigger jets.
The poster/owner needs to let us know WHAT carburetors are on his engine. Check for vacuum leaks. Make sure the ignition is working properly (make sure the vacuum advance is working correctly). If everything looks good, then you get into fuel pressures and float levels, stuck PES valve, etc.
Hopefully the owner will not get overwhelmed with suggestions. A methodical, one thing at a time, approach is needed. Start with the easiest stuff first.