94 SPX Won't start PLEASE HELP!

So I drop in the 657x motor into my 94 SPX. I have done just about everything to this engine except the bottom end (top end kit, swapped out magneto and rotary valve since it is from a 95 SPX, retimed rotary) and I finally get things buttoned up. I hit the starter and ... it lives!!! But I quickly shut it down because it is still in my garage (maybe 2 seconds of run time).

So I decide to try again and... the starter is turning, but not the engine. So I think... bad starter... good thing I have another! 2 hours later I got that in, same problem. So I take out the plugs, look in side.. things look good, still see the permant marker writing on the Mag piston. So I turn it over without the plugs, and it will turn over great. Even get the puffs of vapor on every stroke. But as soon as you put in the plugs the engine will not crank. Its like the compression is too much to turn the engine with the plugs in. What in the world could this be?

Hey Man What Do You Mean By The Starter Will Spin, But Not The Motor. Is The Starter Trying To Go But Wont Spin, Or Is Your Starter Spinning But Not Turning The Motor? Can You Turn The Fly Wheel With Your Hand? It's Alot Easier With The Plugs Out. Did You Pull The Rotary Valve Shaft That Goes Into The Engine? There Is A Brass Gear That Spins The Rotary Valve, Mine Was Chewed Up, And The Motor Would Not Spin. Just Check To See If You Can Turn The Motor Counter Clockwise By Hand.