intermittent spark- weird

I bought my first real dirtbike tonight, Yamaha RT180, and it started
up easily enough at the sellers and ran fine. When I got it home I
couldn't get it to start. I replaced the gas, drained the float bowl,
etc.

On checking for spark by pulling the plug, grounding it, and cranking
the bike over I'm noticing that there won't be a spark for maybe 15-20
cranks and then it will come on for a dozen or so and then go off for
a while again. Really weird.

In the spirit of "don't overlook the obvious", try a new plug.
They do sometimes short out internally.

Check all the electrical connections from the pulser coil to the
ignitor box and the ignition coil. Also, make sure that the nut
holding the generator flywheel to the crankshaft is tight.

Can anyone shed some light on what's happening? I really need to get
this bike working reliably since it's elk season.

We're all hoping it never starts.

Sincerely,
The Elk