slanter

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.