2001 CR250 Fresh top end, no compression

[Anakin]

Hey everyone, I have just done a full rebuild on my 2001 CR250 and then I was in the middle of running it in when it just started clicking and then I pulled the head back off and found that the piston was put on backwards (thanks dad). So I took the head and cylinder out to the shop and the shop dude said that the cylinder was still usable but there was a few small gouges which he cleaned up the best he could, same with the head.

So I got my new top end kit today and I proceed to put it back together and I go to start it and there's no compression like whatsoever, like when I first started after the rebuild the compression was insane and now it's like there's no piston in there. I read that if you have the rings upside down then it can lower compression, same with the powervalve stuck open (i didn't check that) but it has spark, I tried bump starting it and it still didn't work. Do you need the expansion chamber on to start it?

Any input is welcome, Cheers - Anakin

[Akzle]

thats sad for you. How are you checking compression?
Id say you haven torqued down the head proper.

[Katman]

....and now it's like there's no piston in there.

Did you put the piston in?

[haydes55]

Put the expansion chamber on it if you start it. It won't magically get compression, but sucking in only cold air burns a hole in the piston in a hurry. My flatmates mate had a cr250, started it up with no expansion chamber and did a burnout, only took a 30 second burn out to blow a hole in the piston. No compression with a hole in the piston

[Anakin]

I don't know why or how but I had a friend that's a mechanic and I had some shoe fresh and it was flammable and he sprayed it inside of the exhaust port and it fired right up on the first kick but god damn it was loud, sounded like a rotary haha. The thing goes like a grape ape, wheelies in first gear and scared the living bejesus out of me