Thumper600 Posted February 28, 2012 Share Posted February 28, 2012 Hey, I've got an '81 XT550 and it's backfiring alot, mainly when I come off the gas. It also does it when I kick it over whilst holding the decompression lever, usually just after an unsucessful attempt to start - it always starts on the second or third kick both hot and cold. The bike has always backfired, I'm sure most of the older big singles always have but these far more aggressive sounding, super loud buggers! Any kind of help is appreciated, cheers. Link to comment Share on other sites More sharing options...
slice Posted February 28, 2012 Share Posted February 28, 2012 Check your timing first, usually the cause of backfire misfire, then check that the carb is not running to rich, can't think of anything else that it might be other than really obscure faults. Backfiring is the result of to much fuel in the system when the exhaust stroke happens usually due to incomplete burning of the fuel in the compression stroke or to far advanced ignition, so there you have it unless someone knows better. Link to comment Share on other sites More sharing options...
YPVS TONE Posted February 28, 2012 Share Posted February 28, 2012 I’ve got an old Yamaha Technical Service Bulletin about this problem on the XT550 that was sent out to Yamaha dealers by Mitsui who were the UK Yamaha Importers at the time. " POPPING AND BACKFIRE ON THE OVER-RUN" This is a normal characteristic of this type of large capacity single cylinder machine and we regret that there is no practical way of altering this. In this case you need not worry that the backfire is caused by incorrect ignition timing or faulty C.D.I but if the backfiring seems more than normal double check the pilot screw setting is suitable for the particular bike. To check this warm the bike up and turn in the pilot screw till the engine runs roughly ( eight strokes ) and then turn the screw out again till you get maximum R.P.M. Test drve the bike and see if it is better. If it isn’t screw the screw in half a turn and test it again. You will find that by doing this you can find the best setting that allows least backfiring but still allows the bike to idle properly. Hope this helps. Tony P.S Also check you have no leaks in the exhaust joints as this can also cause popping on the over run. Link to comment Share on other sites More sharing options...
Thumper600 Posted February 29, 2012 Author Share Posted February 29, 2012 Alright cheers guys I'll test it out this weekend and get back to ya. Hopefully I can sort it out. Link to comment Share on other sites More sharing options...
cookie Posted February 29, 2012 Share Posted February 29, 2012 I've got an XT550 and had a similar problem. First thing to check is for an exhaust leak, particularly around where the downpipe fits into the exhaust. You can use high temp silicon to make the joint. For info mine had an aftermarket exhaust and the cutouts in the exhaust to put it over the downpipe where too long. When assembled there was a gap when the downpipe was pushed in. I made up a thin copper collar from sheet and wrapped that around then used the silicon. Second thing is to check for an air leak between the carbs into the inlet manifolds. They perish and then leak. Finally check if your carb has what is called a coasting enricher (looks like a cap screwed onto the left hand carb by, from memory, 2 screws). Under this is a diaphram, if this is damaged the valve does not work and allows fuel to be pulled through when you shut the throttle quickly, and this burns in the exhaust, hence backfire. Be careful if you remove the cap as it has a spring behind it! When I bought my bike it had carbs without this enricher on it, and even after fixing the leaks in the exhaust it still back fired badly. After changing them for a set with this on the problem has almost gone away. All big singles will back fire a little when shutting the throttle from high speed by the nature of the engine. If you cannot find the cover on the carb, or not sure drop me a note and I will take some pictures of mine to show you. Good luck! Nick Link to comment Share on other sites More sharing options...
Thumper600 Posted March 2, 2012 Author Share Posted March 2, 2012 Thanks guys, I'm pretty sure I've found the problem, when I was warming the bike up today I saw a bit of smoke blow from the joint where the pipes meet. I'm going to silicon it up and put a new clamp on, hopefully that sorts it out. Link to comment Share on other sites More sharing options...
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