Hmm, sounds like you need to clean out the carbs or it could be a clutch problem but it really sounds like the Main Jets are clogged. Does it idle alright and then when you hit the throttle it just boggs right away?
secondary clutch not shifting out? thats what it sounds like to me pull the driven clutch off and apart and clean and replace any worn parts. How high does the belt ride above the sheeves of the secondary clutch?
ideas of how to make it faster? porting, pipes, clutch work, gearing but those are all expensive...the cheapest way is to replace all the worn drive pieces making sure everything turns freely and get the carbs tuned correctly
I recently sold a 87 400 and my sled did the same thing, these are some things I replaced. New Clutches , New Carbs, rebuilt engine, and still had the problem rev it up and it would run fine , start off and it would fall flat on its face. The only thing I didnt replace was the (CDI Box) so check this first. good luck .
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