marshrabbitt,

InitiallyI had the same problem you are having. It is not a starter problem but a user problem. Though these starters are high torque, they will not be able to turn over a primed or over primed engine from the point of compression.

1) Take the glow plug off as CGR said and turn over the engine with the starter. This will expell all excess fuel. Then install the glow plug carefully again being careful not to cross thread

2) Clean the rubber insert of the starter and the spinner

3) With the booster connected, turn the prop backward by hand (or tip of the starter motor) till it hits compression. Then apply the starter. This way some amount of momentum is builtup before the engine hits compression. This will surely start the engine.

If this doesnt turn over your starter battery is low. If it turns over but the engine doesnt start there is a tuning problem

Even your 90 starter will turn over the engine if you know what you are doing. The 120 size HD starter can turn over even a 20cc gas engine
Ameyam