1997 Yamaha C40 will only run at half throttle.

[meanallis]

I was crusing along at full throttle when the rpm's dropped to about half of what they should be. Runs fine below half throttle, starts fine, fuel flow is good, throttle linkage does open up all the way. The motor sounds like it's trying to go but something is holding it back. Is the C.D.I. unit going bad? Any other ideas as to what the problem could be? Thanks for any help!!!

[Sequoiha]

could be, sounds like the break over point for the timing advance, check it with a timing light and see what the timing is. i think it should be 18 to 22 deg. at wot.

[99yam40]

Test compression to start out with, spark at problem RPM, timing at problem RPM, check fuel pump for leaking fuel into crankcase.
The findings of these test will direct you to what to test next