My '05 SP400 is doing the same thing when climbing up a good hill hill high range. Not so much in low unless I'm pulling a heavy load. Will do it more with a heavier operator. I understand my main jet to be 172.5 but planning on dropping to 167.5 and see if that works. My plug looks to be OK. Not sure why yours suddenly started acting up. It might be dirt found its way in the carb.
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