My MVVS 1.6 IFS, equipped with the tuned pipe is hard to start and runs unreliably. The first start of the day is fairly routine but on subsequent flights it gets harder or impossible to start. When starting hot it will "start" but dies like it is running out of gas. When it does continue to run it sputters and won't accelerate as though it is too lean but is in fact too rich. If I slowly open the throttle , after about a minute of running, it clears out and will run OK. If I readjust the mixture lean to make it run, it will go lean in flight after a time.
The engine seems to restart and run OK when the air temperature is in the 70s (F). In the 80s it's a little balky. In the high 90s it may be impossible to start.

I think the problem is that the carburetor heat soaks between flights and is vapor locking or boiling and cools enough to work after a few minutes of running. It is exacerbated by air temperature plus the extra heat generated by using a tuned pipe. What are your thoughts on that?

I've been able to get it running by closing the spark plug gap to about .015" from .025" when the OAT is near 100 degrees. (EXCEL ignition).
Why would that be?
It looks like changing the gap, changes the mixture requirement.
Is that possible?

Would 50:1 oil rather than 40:1, cause the heat problem? I'm using Stihl Ultra synthetic at 50:1.

The engine is out in the open. Prop is 18X8. Premium gas.
Once cleared out, the engine usually runs fine in flight, it is just the restarts and getting it to run prior to flight is a problem.

Thanks
Jerry