04 yz450f valve job won't kick over

well i recently did my first valve job on my 04 yz450f and man was it a pain. the buckets didn't want to come off and we put it back together and took it apart about 20 times! finally got it all in spec and timed up but now i have it all back together and the damn thing won't even kick over. i finally got it to kick and it just let out one huge backfire. i weigh about 180 lbs and standing full weight on the kick start lever wont really kick it over. what did i do wrong? also have one piece left out. its a black plastic piece. probably something for the change. kind of in the shape of a crescent about 5-7 inches long? sorry i'm a newb to this and don't have much time to look it up. any help would be appreciated.

The part you have left over is a cam chain guide ... I'd go back to square one asap and if you're lucky you didn't push the piston past a valve that wasn't supposed to be open ...

Like the man said, that's the cam chain rear guide, and is the part that applies the pressure from the tensioner to the chain. Without that part, the cam chain will (will, not may) slip before the engine even starts, putting the cams out of time, and causing a collision with the piston.

You will have to take it apart again, and the head will need to be checked for bent valves.

alright the piece i left out was the chain guide/protector thing in the top of the valve cover. i think my chain tensionor does not work. i back the bolt out and it does not get loose. i checked the timing when i had the valve cover back off and it is still on just extremely hard to kick over still or even turn over with a socket (counter clockwise so i don't skip timing).

Check behind the flywheel to see if your cam chain is correctly seated on the crank sprocket. You could have a "looped" link hanging off it.

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