I've got an 02 youth 90. Had like 50 lbs of compression and a bad coil. Did a while new top end, coil and a carb. It idles great and takes off but full throttle it dies when I'm on it.

If I pick the back end up it revs out fine but with a load it just putters and dies. Spark plug is a flat grey color.

 

Make sure it's getting good fuel flow to the carb. If it is pull the aftermarket carb off and blow out all the jets and orifices and check the jets against the ones in the old carb.Sounds like it's lean either from trash or jetting.

 

I put the original 77.5 main from the old carb into the new one. Also swapped throttle needles.

Was going to consider moving the clip on the needle.

 

Any chance it could be the primary spring for the clutch?