Originally Posted by
abelard
-Engine has never been opened up to check valve timing.
Why not? Do it. This symptom can be many things, leave adjustments like this ruled out by checking them. Troubleshooting is methodically eliminating each system.
When you state the idle mixture screw has no effect but idle, that's not true. It handles the transition. What starting point did you use for that screw? IF you do change the position of the engine, hold it upright and vertical, does the engine behave differently?