[Vincent]

I have a 1998 yz80, it has no power and bogs out when it is in gear. but when it’s in neutral it will rev as high as you want. All the jets are clean the floats work. Can someone help.

 

[jaguar]

things to check:
1) crank seals. Check left one by spraying soapy water onto it while turning the engine over (with spark plug removed) You know the right seal is bad if the bike smokes excessively due to sucking in gearbox oil.
2) reed petals. If they are frayed then they dont seal and too much crankcase pressure is lost.
3) any intake tract air leak. Spray WD40 all over it while running. If no leaks then RPM will stay the same.
4) head gasket leak. With the head off you can see any trail of compression leak because it makes a trail.