RE: Engine tuning question
Chris,
The amount of smoke emitted by the engine cannot (despite what some modelers think) be used to judge if an engine is running too lean, or too rich.
When running too rich, an engine would discharge a relatively large amount of droplets from the exhaust.
This 'vapor trail' rarely looks very much like smoke... It will rapidly wet and oil your hand, if you put it into its path.
Smoke would not...
An engine running too lean will emit smoke, from the actual burning of the oil in the mixture.
What you are describing appears to be fuel-starvation, since a four-stroke engine would not stall from a little too much fuel.
And as to muffler pressure increasing at high throttle; it does not double the fuel pressure and would at most increase it by about 0.05 psi, over the ¾ throttle setting...
This will only have a minor effect on the mixture strength.
...Not such an effect that would dowse the engine to death...
If you have an experienced modeler living in your vicinity, that you can turn to, this person may be able to notice something that you did not.
From what you did tell us, we can only assume that the problem is fuel-starvation at high throttle...