I actually don't think this has anything to do with the actual engine;
my guess is carb or timing. Reason being is that the engine is a
brand new crate engine put into my 83 Amer skier. Just about
everything is brand new - the carb is about 6 mos older than the
engine, but still relatively new... here's the issue:

When the engine is cold, it runs great - full power, no hesitation.
Even after running it for 20-30 mins, it is still good. Once I stop
and idle for a few minutes and then try to get back up to power, I've
only got about 1/4 throttle. After that, it starts back-firing and
"starving". If I keep running, it eventually gets to the point where
I can't even idle up without it starving and dies. I try to start it
and it acts like it is not getting any gas at all. I remove the air
cleaner/spark arrestor and confirm that gas is coming from the jets
when the throttle is pushed. I let it sit for a few mins and then it
will start again. But, still no power. I let it sit for 10 or 15
mins and then it starts up, has 90% power (still a slight hesitation
at full throttle). Again, after a while of running and idle, it does
the same thing.

I'm rebuilding the carb today to ensure there is no left over junk -
I've already put a new fuel filter on and new water separator on.
Plugs, wires, distributor - all new. Someone has suggested timing,
but why wouldn't it act bad consistently? Anyway, I'm going to check
that today as well.

Anybody else have any suggestions as to what else to look at?