unusual rpm spikes, mixture control valve gone bad?

[pszabo]

when im cruising along at 55 mph 3500 rpms for my 1983 yamaha virago 920 my rpms will spike if i give it any gas past 4000 rpms. the rpms spike about 500 and the power drops significantly. it feels exactly like someone reached over and pulled my clutch as i was riding. if i continue to give it gas after the spike the rpms keep going up but i dont go any faster just keep going the same speed. if im going uphill and give it gas it does the same thing and the bike slows down if i slowly roll off the throtlle after it has spiked you can feel the bike almost re-engage and the rpms go down about 500 and you feel the power come back but if you give it any more gas it jumps right back into this high rpm powerless stage. it has more effect in the 4th and 5th gears.

i might also add that the rear cylinder coughs when it idles and i havent looked into its carb yet. i also just put on straight pipes instead of the baffled ones but it didnt affect the front cylinder. the book also speaks of a mixture control valve that may go bad.

thanks

[BuzZz]

At a guess....

Your clutch is slipping. Higher gears and increased drag at hiway speeds put the most torque through the clutch, and it sounds like that's when the plates start to slip. When you back off, the plates grab again until the load over powers the plates again.

But that's just a inter-guess through a computer screen......