HH160 Starting Issue

I have a Tecumseh HH160 engine that cranks very slowly. When engaging the starter with the throttle closed, the engine turns at a normal rate. If I open the throttle even very slightly, cranking speed reduces drastically. I have replaced the battery with one that has 365 CCA, replaced the starter, replaced the camshaft with one that has an automatic compression release, adjusted the valve lash to specs. The problem still exists. I can get the engine started after repeated attempts (with a closed throttle) and it will run smoothly and has good power.
I'm at wits end. Does anyone have a solution??

I'm probably way off here but I'd take a look at the timing. Maybe flywheel key is slightly sheared and this will alter the timing. Too much advance and the engine will be hard to start like this. Check the flywheel key, look real close.