I just bought an 05 400ex, it started when i got it and it started the day after. Well I tried to start it today and all it does is crank. Checked the plug and that still looked fine and it had spark. Checked the carb and its working like it should as far as I can see. After I put the plug back in it almost started once or twice but now all its doing is cranking again... It does have a rev eliminator (I think thats what its called)could that be causing it not to start? What else could it be??

Try a new Spark Plug!

You know he could be right, on mine I couldn't get started either for the first time. I had spark on the plug, gas going into the engine but it would not start. I decided to change the plug and on the first rotation it started up and running like a charm.

Do you have some gas in it BAHAHAHAHa .....hapened to me once

Go back to the stock CDI and see if that helps. Also, on my old ex, I use to feather the throttle when starting and it seemed to start better.

Did it happen to kick back or sorta back fire, my 400 we had out riding and when we came to restart it it kind of kicked back and back fire. After that happened I couldn't get it to detonate on starting fluid. Come to find my key way had sheared off and had a loose flywheel so to confirm it I took the inspection cover and took plug out , when I could see the timing marks on the fly wheel the piston wasn't even close to he top.
The bigger of the two problems though is when it was off the valves tapped the piston and colapsed the ring groove

Man that sucks the same thing happen the my brothers Quad.