FWIW. I've run though some long troubles with my .61NT Evo.

When it runs backward the engines running too rich or the plug is too hot causing the charge to pre-ignite and forces the piston back down on the first stroke..... From there that's all that's needed to keep running that way.

I can run an OS#8 in mine, but it runs a little better with the Hanger 9 "Super Plug" which is (as I'm told) a four stroke plug.

One more thing..... I think that if you do the pinch test and your engine SPEEDS UP, it's running too rich. When you block off the fuel by pinching the line, the engine runs off the fuel ahead of your fingers.... and leans out....and leans out..... and leans until all the fuel is used and it dies.


P.S. there may be more reasons than that why an engine would run backward.