radiohead Posted March 7, 2008 Report Share Posted March 7, 2008 Hi All My '05 Sherco 290 has been running sweetly for a while, until the other day when halfway through a ride it felt like the throttle/carb was "hunting" (as if I was gently rolling the throttle on and off, but while the throttle was steady or while idling). The next ride seemed like the problem had disappeared (although the bike was backfiring more than usual when going downhill in gear). At the end of my hour long-ride I was coming along a fast section of gravel road in top gear at about 3/4 throttle. When I slowed down to turn off the road, the bike was making huge clouds of bluish/brown smoke (not normal blue colour). Have I blown the rings?? There are no strange noises coming from the engine?!? Help please?!! Quote Link to comment Share on other sites More sharing options...
feetupfun Posted March 7, 2008 Report Share Posted March 7, 2008 (edited) You have just got the exhaust system hot enough to start cleaning itself out. Thats what makes the smoke. You can't "blow" rings on a 2 stroke. You can sieze 2 stroke rings or wear them out but neither causes smoking. You probably need to do a few more fast runs on you bike to clean it out fully or have the engine and exhaust decoked. Anything that requires the throttle well open for a minute or so will usually do the trick. Very long steep climbs are what I use. The strange uneven running could be anything from using old fuel to running out of fuel to a crankcase air leak to a partially blocked pilot jet. More info on the "backfiring" needed. Very rare for such a bike to backfire in use. I think you need a better description of what it does and how you are riding when it happens. It's not uncommon for 2 stroke trials bikes to only fire every 5 to 10 strokes on throttle-closed overrun if the revs are up a bit. Is that what you are getting? Edited March 7, 2008 by feetupfun Quote Link to comment Share on other sites More sharing options...
copemech Posted March 8, 2008 Report Share Posted March 8, 2008 Clean the carby to inspect for water in the bowl! Quote Link to comment Share on other sites More sharing options...
jse Posted March 8, 2008 Report Share Posted March 8, 2008 "The next ride seemed like the problem had disappeared (although the bike was backfiring more than usual when going downhill in gear). At the end of my hour long-ride I was coming along a fast section of gravel road in top gear at about 3/4 throttle. When I slowed down to turn off the road, the bike was making huge clouds of bluish/brown smoke (not normal blue colour)." Generally backfiring under off or trailing throttle (high intake vacuum) is a sign of a lean mixture. I'd pull the carb and check/clean everything (float level also) to start with. It wouldn't hurt to reseal the reedcage and manifold with some high grade sealer. It's common for riders to overtighten the manifold, which often bows the manifold area between the bolt holes, allowing air to pass. The smoke after a long top gear run is generally not too unusual (ask the SSDT guys) as these bikes are jetted for fast response section work and will spike high combustion chamber temps fairly quickly at prolonged high speed. It's usually just oil in the exhaust system (mid-muffler especially) being burned off but can also be carbon deposits loosened from the headpipe. Carbon can collect in the first part of the pipe out about 2" (depending on your jetting), and can be bad enough to significantly reduce exhaust port/time area to the point of causing overheating. The engine will usually run "soft" with the headpipe blocked, and since you didn't mention that, it's less likely to be a part of the problem. The "bluish/brown smoke" is a little puzzling as darker exhaust points to the possibility of an overly rich mixture. Jon Quote Link to comment Share on other sites More sharing options...
shercoman2k8 Posted March 26, 2008 Report Share Posted March 26, 2008 (edited) errrm. did the exhaust keep smoking gently after you killed the engine?? jamie Edited March 30, 2008 by shercoman2k8 Quote Link to comment Share on other sites More sharing options...
radiohead Posted April 21, 2008 Author Report Share Posted April 21, 2008 errrm.did the exhaust keep smoking gently after you killed the engine?? jamie Yup, it did. After a couple more rides experimenting with long throttle openings I have been reassured that the smoke was indeed the residual oil in the exhaust burning off! Thanks guys! PS: WRT the other issue, I whipped the carb off and gave it a good flush and clean out and haven't had any trouble since! Quote Link to comment Share on other sites More sharing options...
copemech Posted April 22, 2008 Report Share Posted April 22, 2008 Yup, it did. After a couple more rides experimenting with long throttle openings I have been reassured that the smoke was indeed the residual oil in the exhaust burning off! Thanks guys!PS: WRT the other issue, I whipped the carb off and gave it a good flush and clean out and haven't had any trouble since! Good going, as you do need to give it a good run from time to time to keep things cleaned out. Running the high quality synythetic oil in the fuel at 80:1 or greater ratio will help, and keeping fresh fuel will help prevent moisture(condensation) entering the fuel system. A periodic cleaning of the carby is still best! Quote Link to comment Share on other sites More sharing options...
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