90 2 stroke has no power

[JKWHITFIELD]

It will idle and rev up but I put a load on it, barely runs 5 MPH. Changed the spark plugs, any oth er suggestions?

[greasyshaft]

Does it rev high and go nowhere when in gear?
Could be prop bush spun if that's the case.
Otherwise its just a matter of checking basics and going from there.
Confirm engine is running on all cylinders.
Compression test.
Check spark using gap tester.
Check synchronization of throttle & advance timing, is the screw touching the full throttle stopper when at full throttle?
Check condition of fuel, any water/debris/restrictions or stale gas? The fuel strainer on the engine does bugger all.
Was there something changed before the problem arose?
What did the old plugs look like?

Yamaha electronics are pretty tough so likely fuel related
if doing carbs be sure to check the reeds,thermoelectric en-richer diaphragms(prime start models) and fuel pump diaphragms

[99yam40]

not enough fuel getting to the cylinders to run with a load, clean the carbs

But making sure compression, spark, and timing are good is what smart people do first