slanter Posted June 29, 2007 Share Posted June 29, 2007 Me and a friend of mine have been working on a 1983 XS400 that she bought recently. It ran, but it ran a little rough, so we took off the carbs cleaned out the jets and ports (found something strange... for some reason one of the jets on both carbs has a rubber plug in it... figured it must be only used in higher displacement engines or something) reassembled it, and tried running it again. it runs a lot better now, it idles very nicely... but when you rev the engine (especially when riding it) the bike backfires loudly. seems to MOSTLY do it out of the left pipe. So i'm just looking for some input on this. I've also noticed that the pipes don't seal against the cylinder very well. If I put my hand in front of the cylinder where the pipes pit on I can feel exhaust coming out around the seal. I'm wondering if maybe this is part of the problem. I took the pipes off the cylinders, didn't see any cracks or damage to them.. but I noticed that there isn't any sort of "gasket" between the pipes and the cylinder. the metal pipe presses right against the opening in the cylinder, then there are little spacers and a metal coupling that bolts on. I havn't worked exhaust systems much so I'm not too familiar with this, but i'm pretty sure exhaust leaks are bad. Link to comment Share on other sites More sharing options...
Moderator drewpy Posted June 29, 2007 Moderator Share Posted June 29, 2007 Me and a friend of mine have been working on a 1983 XS400 that she bought recently. It ran, but it ran a little rough, so we took off the carbs cleaned out the jets and ports (found something strange... for some reason one of the jets on both carbs has a rubber plug in it... figured it must be only used in higher displacement engines or something) reassembled it, and tried running it again. it runs a lot better now, it idles very nicely... but when you rev the engine (especially when riding it) the bike backfires loudly. seems to MOSTLY do it out of the left pipe. So i'm just looking for some input on this. I've also noticed that the pipes don't seal against the cylinder very well. If I put my hand in front of the cylinder where the pipes pit on I can feel exhaust coming out around the seal. I'm wondering if maybe this is part of the problem. I took the pipes off the cylinders, didn't see any cracks or damage to them.. but I noticed that there isn't any sort of "gasket" between the pipes and the cylinder. the metal pipe presses right against the opening in the cylinder, then there are little spacers and a metal coupling that bolts on. I havn't worked exhaust systems much so I'm not too familiar with this, but i'm pretty sure exhaust leaks are bad. you're correct in assuming the pipe seals are to blame as that will produce the backfiring you are experiencing. You should have a gasket there, but it may be compressed and does look at first to be part of the cylinder head. try cleaning around the exaust port and check for a gasket (they can be metal copper or steel concertina shape) hope that helps, if you seal the exhaust and it still backfires, then timing or fuel mix is to blame. Link to comment Share on other sites More sharing options...
SiX Posted July 11, 2007 Share Posted July 11, 2007 Ok, so we got the new seals in. They are the correct seals, and as far as we can tell, have been installed properly. I've checked the repair manual and everything looks right. However, it still backfires and exhaust is still coming out. Is there something we may have missed? I really want to get on the road. Link to comment Share on other sites More sharing options...
armyofda12mnkeys Posted March 26, 2010 Share Posted March 26, 2010 Ok, so we got the new seals in. They are the correct seals, and as far as we can tell, have been installed properly. I've checked the repair manual and everything looks right. However, it still backfires and exhaust is still coming out. Is there something we may have missed? I really want to get on the road. My friends bike has same issue. I'll have to figure out whats wrong with it, and post back here on solution. Link to comment Share on other sites More sharing options...
HoughMade Posted April 5, 2010 Share Posted April 5, 2010 Couple of questions: 1) When you put the new seals on, were the muffler mounts loose? If they are tight, they can bind and impede tightening the head pipe enough. Loosen the muffler mount, then tighten the head pipe and then tighten the muffler mount back up. 2) Did you confirm whether or not there were seals in there? When I took mine apart, I originally thought there were no seals in there. Turns out there was, but they were compressed and sooted up and looked like part of the head. If there are old seals in there, they may prevent the pipes being tightened down properly. If you have figured this out, please share. It could really be a help for people. Good luck! Link to comment Share on other sites More sharing options...
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