1994 YZ250 help!

[Blue250X]

Hopefuly you guys can help me out here. I got a 1994 yz250 not running. Well after tinkering for a while I got it to run but it was just barely above idle and would not respond to any movement of the throttle. So I tear apart the engine, replaced piston, rings, wristpin, bearing, got new reeds, and threw it all together. Well I just got it to start up again, and it does the same thing! It has a little more authority this time, but same thing! It will fire up cold, but once it runs for about 5 seconds it dies and wont come back to life. Also, there are some mean backfires it has when you are kicking it over. The carb on the bike isn't stock but it has 1 size bigger pilot and main jets. Any help is appreciated!!

[John451]

if the carb's clean it sounds like an air leak... Is it fouling plugs? or leaving them all nasty and oily or puffing blue smoke? if it does any of those it blew out the crank seal on the clutch side. If it doesnt have any of those symptoms then it sounds like an air leak.. take a look at the rubber boot on the back of the reeds.. if its all cracked or w/e then it needs new rubber. Or it could be the seal on the magneto side which would let more air through than it needs.. I'd check in the carb first defiantly and make sure that the floats, float valve, and all of that stuff is okay, it may be the leak jet.. try taking a needle and put it in the main jet. and the jet thats up and to the left of that and make sure it's not clogged. then blow in where the gas line goes on the carb, and with the floats on move the floats up and down and see if its letting gas (air) through into the bowl.