dr3906 posted a topic in DRZ400/E/S/SMHi folks! Long time lurker and first time poster here; love the site. I know alot of the topics i'll mention have been covered separately in other threads and I did my best to research those, but these are all sortof related to problems I'm having with fixing my DRZ... Very sorry if this has been covered before... I've got a '02 DRZ400S that blew an engine and I was able to track down a replacement from a local seller that took it out of a '05 400S model. The guy said it had less than 2K miles and ran great when he pulled it (cracked his frame offroading and was parting the bike). I dropped the new engine in, hooked it all up and added fluids. The engine started easily the first time and I ran it for 3 minutes to circulate the oil/coolant. A second after I stopped the motor, a "pop & hiss" sound came from the exhaust. Huh. Waited another 3 min and finished adding oil up to the right level, and rechecked coolant, so all that looked ok. Then second time I tried starting, engine was very slow to start and in fact had to give it help via jump starting from my truck. Ran the engine again for about 3 minutes (1/2 min with the choke out) and noticed that the base of the header was glowing red and there was steam/smoke coming off the radiator. Stopped it and again, a second after finishing rotation, there was an even bigger "POP & hiss" from the exhaust. After 20 min of research on the TT forums I return to check on the bike and there's a pool of gas underneath it. Looking closer, I see it's actually dripping from the airbox and the air filter is wet with gas. So perhaps it's running hot (lean?) and if so, I think maybe it might have something to do with the fact that I didn't have all the panels mounted on the bike (so the airbox was open) and the carb hasn't been cleaned for a couple months (clogged jets?). Of course the first thing i'll do is clean the carb and mount the panels, but what's the deal with the "pop" and gas seeming to go the other way than it's supposed to? Kind of afraid to start the engine again because of it; I really can't afford to blow another engine. Bike had/has all stock exhaust and carb, and new engine is all stock. It never had a problem with overheating or "popping" before. Sorry for the long post, but any ideas? Thank you!