From your description, it does sound like it's loading up and once cleared out, runs O.K. for awhile.

The plug should be chocolate brown, but that's if it is running good all the time, in all ranges. If you "clear the plug" by running WOT, you could be getting a false reading.

Also, does the mixture screw turn in for leaner, or out for leaner? Contact grubee for the answer, as there are both kinds.

Where is the screw set at now? Gently turn it in until it stops while counting how many turns it take to turn in.

DO NOT TIGHTEN IT LIKE A REGULAR SCREW.

Should be around 1 1/2 turns.

BTW- how much choke do you need to start it, and what throttle opening do you use when starting it cold?

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