Remove the spark plugs and try turning over the engine by hand.
Does it move freely?
I would look very hard at your ground between motor and battery.
Do you get any voltage drop between relay and battery? (should be zero)
If everything else checks out, I would probably pull the starter and see if it a) spins freely and b) if it spins once you connect 12V directly.
Maybe move the ground to the starter. The exhaust manifold SHOULD work, but there is a pretty thick gasket between manifold and cylinder. The bolt should take care of that, though.
I would look very hard at your ground between motor and battery.
Do you get any voltage drop between relay and battery? (should be zero)
If everything else checks out, I would probably pull the starter and see if it a) spins freely and b) if it spins once you connect 12V directly.
Also the starter is pretty much new. I picked it up from JetManiac
Just saw this, starter was from a running ski and it was tested again before it was shipped, so unless it locked up during the time of your build. I would remove it and test it directly. Bolt both of the cables to the starter and the ground to the battery, then touch the positive wire to the battery...
I would try new cables or check the ends of the cables.
Friend went to get his ski running few weeks ago and nothing but a click......... one of the ends at the battery was just about corroded and broke off