Technical ideas required... Sym Joyride 180 - that's a single-cylinder, four-stroke scooter with cvt ( "twist & go" ) transmission. Runs fine : starts, stops, accelerates as it should... But every now and again it back-fires on the over-run. i.e. run along the main road at 65-70mph, then shut the throttle for junction or roundabout and it goes "bang". No missfire, no loss of power, and it accelerates away again afterwards no problem... I have no idea why, as I can't find anything obviously wrong with it. All suggestions gratefully accepted...
I had a Triumph Thruxton that used to pop and bang on the over-run, after I fitted Triumph TOR`s. A thread on a Triumph forum said it was quite common with these race pipes and was caused by a small air leak on the exhaust, the header pipe/silencer joint. Never bothered to fix it, along with the off road race pipes it made the bike sound "gnarly"
That sounds like the most likely cause... I did check that the exhaust bolts were all done up tight, but maybe I'll pull the exhaust off and put a new gasket in... Cheers
I think I've cured this one... Took the exhaust off, found the exhaust gasket was a bit too thin for the space it fits in to. Easter Sunday, so no chance of getting a new one, so I filed a bit off the end of the pipe where it fits up to the head - just enough so that the gasket id compressed when it's reassembled. Refitted the pipe, starting with the header bolts. Couldn't believe how far out of alignment the other bolt holes were ! ( I suspect it may have had a pattern exhaust fitted at some time ). "Modified" the bolt holes to line up and bolted it all back together. Quick test run and it seems fine... Half an hour well spent !
Air leak likely the cure in this case, but some bikes have an exhaust gas recirculation system that bypasses air into the exhaust port on a closed throttle, to help burn off unburnt fuel in the exhaust - reduces emissions and helps get the catalytic converter heat up quicker (which also helps emissions). Yamaha seem to like this setup, even on sportsbikes (I think R1s have it?). My Sons YBR 125 had it and it was constantly popping on the overun, I had to replace the exhaust due to corrosion so I removed the EGR crap from it at the same time - runs smoother without, lost some weight and no more popping.
The Tiawanese aren't quite as clever as Yamaha... No recirc air on the Sym. Having done a longer test I have found that I have not completely cured the problem - new gasket required I think...
might also be the mixture is off. Can't remember which way, but my old 'moto used to pop and bang quite spectacularly - also used to chuck sheets of flame out of the exhaust on occasions.
I don't think it's running rich - it pulls fine, doesn't use too much fuel, and doesn't soot up the plug... Definitely not running lean - never gets hot... Maybe I'll just chuck it in to Cornwall kawasaki for a service and see what they come up with... Thanks for all the ideas though guys
