Right on Jim ! the only other possible other answer is plugs braking down at a certain RPM.... and that is very unlikely.... I know that, that's why I questioned the carbs on mine too ! but it could also be setup somewhere on mine !
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I think he should clean his carbs again, this time take a small spring and straighten it out and go through each passage way as far as you can and then blow it out real good.... when I did my carbs the first time I found that the choke passage way was still dirty after 2 cleanings ...i found that with the spring wire..... it will go around a corner where probes will not and that is what you need.... I bent a tiny loop on the end so it wouldn't get stuck and that helped allot. I sprayed carb cleaner in that choke passage way before that and got cleaner out 3 holes... or at least I thought I did ...but after that cleaning it ran fairly well... still not right though it was very cold blooded and it shouldn't be that cold blooded ! LOL....
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it's STILL snowing here ...crazy weather !
...this picture helped me alot ! the RED passageways MUST BE CLEAN !
Bob......