Can't get the bike to crank all of a sudden. All I get is a little backfire and nothing more.
Has enough spark to shock the ever lovin' **** out of me.
Fuel is fine, has no water in it. The carb is in great shape.
The spark plug will show spark when grounded to the engine, but I'm suspecting it's fouled when screwed in the engine.
I checked the compression with the engine cold and got 115 psi.
Any thoughts?

 

yes that is good enough compression for sure. you might check a new plug to see if that is the problem first beforee going any further. personally i would find another way of testing fire then grabbing it for sure.

 

I grabbed the end of my spark tester too quick after I let off the button. The engine was still spinning. When I put all the potential on myself rather than the cylinder head....

Woke my butt up real quick....

Gonna put a new spark plug in before I do anything else.

 

should run on 115psi but its not great. my 90 250 2stroke has 143psi.
I have heard of plugs not firing under compression for all it cost i`d start with a new plug
pull the air box out and put your hand over the carb opening see if its sucking, if it isnt you might be looking at crank seals.
just a thought.

 

The compression would probably be higher if the engine was warm.
It want's to suck your hand hard to the carb boot when you put it over the opening anc crank it.

 

thats good, sounds like the bottom is sealed. maybe try putting an ounce of gas in the cylinder, use your wifes turkey baster (dont tell her just replace it before she needs it again) a fresh plug and try it, if it starts and runs for a few seconds do it again then if it dont want to run on its own put the plug in and with the air box removed try starting it while spraying WD-40 into the carb. some use quick start but i don`t like it because it has no lubrication and its a violent start. if it runs like this its telling me there is a problem with the carb not letting gas in.

 

correct way to test compression is engine cold, wide open throttle, and crank a good 5 times(if you were doing it by hand) for the best reading.

 

Well, it's not the plug. Tried a brand new one and it's doing the same thing.
Took the carb off and sprayed some ether in the intake. It just backfires every so often.
Placed my hand over the opening and it seems like it's really not sucking my hand in that hard to the boot. Kinda feels like it's sucking and blowing out the intake boot.

Crank seal? That's about all I can figure. Good fire, good fuel, good plug. What's left?

 

Found it. Crank pin that holds the rotor's sheared off.

Now to see if I can did the remains out the crank.

 

Happy to hear you found the problem.