Wr 400 won't start but a strange problem

Hello I am new to four stroke single cylinders, i have a strange problem with a 1999 wr 400f. I bought the bike it had dropped a valve on to the piston so i have had new valves and seats put in the head and new piston and rings. I built the engine back up checked the valve clearances and the cam timing over and over but it still won't start every now and again it back fires through the carb but not all the time. I took the air box off and the exhaust tail pipe off when you kick it over both the intake and exhaust side suck and then blow every turn of the crank. I don't think this is right but I have never worked on a single cylinder only fours, any help or advice would be of great help, I am not letting an engine beat me.

there was a topic somewhere on the yz forums. the guy described all the thing just like you did. finally he came to the conclusion that his head was warped. I'm not a specialist, don't treat my post as conviction. 

 

ps: maybe your valves weren't installed properly?

Edited by sosycz

Did you also have the valve guides checked for wear?  Just thinking that if the guides are worn out and the valves have any wobble, it may not seat properly and therefore not start.  

 

Backfire through the carb, you are not giving gas while trying to start are you?  Remember that the "procedure" must be followed without error to reliably start the bike.

Hello the head was sent to an engine rebuild firm they said that the guides were fine it needed 1inlet and 1 exhaust valve i forgot to mention that the carb is blowing and then sucking when you put your hand over it and the exhaust is blowing then sucking when you put your hand over it. It is as though the valves are timed wrong but i have done it to the letter of the manual and of the forums. All the marks line up. I am going to check the valve clearances again tonight and make sure they are ok

Took the head back off tonight and found two valves that are burnt out they were supposed to have checked them at engine place but obviously not so new valves and try again

They did not cut the seats correctly

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