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Has this engine ever run right, or is this something that was just put together, and you can't get it to idle?
It was recently put together, but ran good for break-in, and a couple short drives, then started acting up.

First thing I'd try is to rule out the carb. Either put a known good piece on the engine, and get running, or put the carb on a known good engine and see if it works.
Good Idea, but I don't have another engine that a 750 wouldn't choke out. I have another carb, but not sure if it's good, and it's a 650.

Obviously, once the carb is on an engine that you know runs well, you can get some approximate idle mixture and speed set. If the engine in question hasn't run before, it's a good idea to have the carb pretty well proven.

So, if the carburetor is the culprit...

You found the plastic in the carb, right? Where did you find it? Float bowls? Jammed into metering orifices?
In the float bowls. didn't see any in the metering orifices with a flashlight

The real question there is: why is the plastic there? What has broken, and what has been plugged up with plastic shrapnel.
probably from those crappy clear float bowl windows that have removed

It might be something from manufacturing. Or something from a needle/seat, or c) none of the above.

The 'won't idle below 2K' thing sounds like it could be one of two problems: a big air leak, or idle circuits not working. I'm going to jump out on a limb and guess the Proform carb doesn't have the old-style power valve circuitry that can blow out with a backfire...
Had found a leak around the base and replaced the gaskets. Come to think of it, I didn't check again after I replaced them, but I checked the carb base and it wasn't warped.

of course, if the engine spews black smoke, that might be a place to check.
no black smoke

If the idle circuits don't work, then you'll need to speed the engine up to pull out of the venturi boosters and get on the main circuit. This may be the cause of the problem. The cure is to disassemble the carb, taking notes (and pics if possible); note every gasket (is one torn/damaged/blocking?). Then sit down with a good book (like the "Holley" book sold at the speed shop, not a romance novel or anything) and chase all the passages with spray carb cleaner.

Check that the idle air bleeds are free, and blow cleaner thru every path in the idle circuit.

Reassemble the carb, and check that the gaskets you're putting in aren't going to leave anything unsealed or block any passages. Take more carb spray, and do one last set of checks for good luck that you still have connection from the idle mix holes to the circuits in the metering block, and the bleeds. Then put the bowls back on. Either pray or perform voodoo as your beliefs dictate.
Most of the above is way over my head

If you put a carb on the engine that's proven to be good, and you still have the high idle speed, I'd look for a vacuum leak. A pretty big one.

PCV? Brake booster line? All those vacuum plugs tight? Vacuum advance?
nothing vacuum operated, all 3 ports capped on the carb.

I always check hoses first; they're easy enough to pinch w/ pliers and see what happens. Faster than taking 'em off to see if the gizmo at the other end is leaking.

Then start squirting carb spray at places where you can have leaks. Carb base, plenum, manifold runners. If you get a response from spraying somewhere (other than at the top of the carb), bingo!

Now,as far as running backwards... beats me. If the thing didn't run real nice with a good carb on it, I'd try a compression test, and I'd TDC the engine, and check that the intake valve closing is where it is supposed to be. A skipped tooth on a timing chain will show up as being out a bunch (I don't have one here to count, but I'd expect 15-20 degrees per tooth; something like that... someone here wlil probably know).

Hope this helps.

-Bill

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