neo Posted April 26, 2009 Report Share Posted April 26, 2009 I think my two Sherco's are competing for attention. I problem on my 250 that I previously thought I had fixed has has come back to haunt me ...just after I fitted a new air filter. The air mixture is continually varying too rich, too lean, too rich, too lean, too rich, too lean!!!!...but is getting leaner and leaner and the airmix screw (on the PWK28mm) is hitting the stop now. I've been down this road many many times now and I think I done just about EVERYTHING I can to find this problem. But today I had a brain wave... If a 28mm venturi is getting a little large for a 250 (border line a the best of times) then maybe if the piston rings are starting to wear with age and the compression is fading. Then the the air speed through the carb would be dropping and give a leaner mix. To support this theory I've noticed that the bike has been easier to kick over these past few months. A possibility....or am I deluding myself? Best of balance. Neo Quote Link to comment Share on other sites More sharing options...
nzralphy Posted April 27, 2009 Report Share Posted April 27, 2009 I don't think that sounds sound - the carb is unlikly to be too big is it?.... after all a 250cc endro bike will have a much larger carb and they run fine. You could have dirt in the carb blocking the pilot jet....Have you got a fuel filter on the bike? I've had bikes with poor compression and they still jetted correctly - ie no need to change carb settings from when the bike was fresh Quote Link to comment Share on other sites More sharing options...
tryalsryder Posted April 27, 2009 Report Share Posted April 27, 2009 I suggest getting your hands on another carb, and see if the bike has the same issues with that one on. If not, then you know its the carb. I had a similar problem with my 07. I think it was the needle and seat (seed?) that was bad. I put on a carb from another Sherco and it ran fine. I got lucky and got another carb, so I never was 100% sure the needle and seat combo was the problem, but I think that was it. Quote Link to comment Share on other sites More sharing options...
copemech Posted April 27, 2009 Report Share Posted April 27, 2009 Sounds like a water or venting problem to me, or float/needle? Did I suggest you to empty your fuel can in a clear clean plastic jug to see what comes off the bottom? As a matter of fact, empty that carb in a clear vessle while you are checking it. Those carbs run on a 125, so I doubt your theory holds true. Quote Link to comment Share on other sites More sharing options...
neo Posted April 27, 2009 Author Report Share Posted April 27, 2009 Thanks tryalsryder and NZRalphy, But the the carb on this bike has been changed three times and all 3 were cleaned out many times since this problem originally came about ( maybe a year ago). Since then it's always disappeared after trying something new. But it's always come back again after some time. This time was specific to fitting a new Jitse Airfilter and the old Sherco one was getting crummy. Oddly enough the past few weeks has been the best it's every run. I'm really referring to past post with Copemech as he might remember this situation from the past. Cope...it might (or might not) be a good time to try out that 24mm OKO?...what do you think? Best of balance. Neo Quote Link to comment Share on other sites More sharing options...
copemech Posted April 27, 2009 Report Share Posted April 27, 2009 Go for it! Quote Link to comment Share on other sites More sharing options...
stork955 Posted April 27, 2009 Report Share Posted April 27, 2009 Gday Neo, In no particular order - have you checked the floats are not punctured? Drop 'em into warm water and see if bubbles come out of it, - if OK put them in fuel and if they float just below the surface of the liquid they are knackered. Worth a try before fitting a totally new carb... Also, do a compression test -its better to diagnose properly than to feel around in the dark and hope you get it by accident... Clean exhaust system? Filter clean and oiled correctly? I like to blast carb cleaner (from a can) through the carb passage ways to really make sure they are clean. Jets too... Wear on the needle and/or in the needle jet? With a drill bit that just fits into the needle jet look for ovality. Any wear here means a new one and a needle (this will cause rich running off idle), blocked air vent hoses? Umm, cant think of anything else at the moment. - Oh - float level and needle and seat? No wear on the needle valve, adjust with the carb tilted sideways to around 45 degrees. This makes sure that the float level is correct, and no float weight error is included in the adjustment. Cheers, Stork Quote Link to comment Share on other sites More sharing options...
neil king Posted April 28, 2009 Report Share Posted April 28, 2009 I think my two Sherco's are competing for attention.I problem on my 250 that I previously thought I had fixed has has come back to haunt me ...just after I fitted a new air filter. The air mixture is continually varying too rich, too lean, too rich, too lean, too rich, too lean!!!!...but is getting leaner and leaner and the airmix screw (on the PWK28mm) is hitting the stop now. I've been down this road many many times now and I think I done just about EVERYTHING I can to find this problem. But today I had a brain wave... If a 28mm venturi is getting a little large for a 250 (border line a the best of times) then maybe if the piston rings are starting to wear with age and the compression is fading. Then the the air speed through the carb would be dropping and give a leaner mix. To support this theory I've noticed that the bike has been easier to kick over these past few months. A possibility....or am I deluding myself? Best of balance. Neo Neo, you are correct in that a low compression engine will move less air through the venturi and in fact run lean. You will see the most effect of this at start up same as bad reeds( like a need to choke it even when hot). Low compressiom on a 2 stroke has a unique sound at higher revs. However, it will not come and go Quote Link to comment Share on other sites More sharing options...
copemech Posted April 28, 2009 Report Share Posted April 28, 2009 If I understand correctly, Neo rides the crap out of this thing almost every day! I am starting to worry about those main seals if a carby does not change it! ! Quote Link to comment Share on other sites More sharing options...
neo Posted April 28, 2009 Author Report Share Posted April 28, 2009 Neo rides the crap out of this thing almost every day! Well I ride almost every day.....but I never ride the crap out of my bikes....I love em too much for that Thanks for all the replies gents....if I do a compression test does anyone know how much pressure I should get on the gauge? Cope...main seals....hmmm got me thinking....but if the main seals are going wouldn't I see some smoke and would blocking off the engine breather tube make the engine stabilise after a while? Neil....it's worse when the engine is warm but a bit more stable when it's really hot.....having said this.....taking a break can sometimes cure it for 10mins or so. So I'm rolling the selves up tonight...simple things like Compression test....Then clean out the carb (AGAIN!) and check for water....how do I see water in petrol? I seem to remember it was raining, time before last, when I rode....maybe I've failed to make this connection in the past Best of balance. Neo Quote Link to comment Share on other sites More sharing options...
copemech Posted April 28, 2009 Report Share Posted April 28, 2009 Well I ride almost every day.....but I never ride the crap out of my bikes....I love em too much for that Thanks for all the replies gents....if I do a compression test does anyone know how much pressure I should get on the gauge? Cope...main seals....hmmm got me thinking....but if the main seals are going wouldn't I see some smoke and would blocking off the engine breather tube make the engine stabilise after a while? Neil....it's worse when the engine is warm but a bit more stable when it's really hot.....having said this.....taking a break can sometimes cure it for 10mins or so. So I'm rolling the selves up tonight...simple things like Compression test....Then clean out the carb (AGAIN!) and check for water....how do I see water in petrol? I seem to remember it was raining, time before last, when I rode....maybe I've failed to make this connection in the past Best of balance. Neo Bowl venting is critical if this is a K or OKO , as I have never found the right vent only to be sufficient! Well, all I can suggest is to break the carb over a clean and dry 2L clear plastic jug that has been opened , cleaned and dried, then wait for what settles in the bottom. Small grit can also be seen as a reflection as things are poured off. If your fuel is alcohol induced, water will be absorbed up to the limits of absorbtion, then the water/ alcohol mix will stratify in the bowl, leaving you with a water/alcohol mix that you are running on, which can be quite lean. Further transmission from your collection tub into a 100ml or so vial may be more revealing. You need to look closely at this stuff. If you want to test for the alcohol content of your fuel, simply take a calibrated vial, add 50% fuel, then 50% water, shake it, if it is all gas, then the % should not change as things settle, if the ratio changes, it will tell the alcohol volume. Compression is probably around 10:1 or better. So with a good kick and throttle open, one would suggest a reading that was near 10 times your barometric pressure, or around 140-150 psi if you are within norm. Quote Link to comment Share on other sites More sharing options...
nzralphy Posted April 28, 2009 Report Share Posted April 28, 2009 Compression is probably around 10:1 or better. So with a good kick and throttle open, one would suggest a reading that was near 10 times your barometric pressure, or around 140-150 psi if you are within norm. Nice little calculation there! Quote Link to comment Share on other sites More sharing options...
neo Posted April 28, 2009 Author Report Share Posted April 28, 2009 Hi Cope, I started by cleaning the carb on the outside (with a toothbrush) before opening anything. Then I pored every drop out through the vents into a glass jar. Then again just as the float bowl was opening....But a long hard stare into the jar revealed nothing. Then I removed the bowl and looked inside and bingo!!!...Shiny black and red partials inside the big nut....Now guess what colours that Jitse airfilter has on it....Yep black and red. The partials were also a soft rubber so no real harm I guess. Also the compression test was a real ripper (ozzie expression) and hit 215 PSI!!....so there can't be too much wrong in that department. Anyway it was too dark to test the bike tonight.....shame there wasn't a full moon ...so I'll see how she runs tomorrow afternoon. But it does look promising Best of balance to all. Neo Quote Link to comment Share on other sites More sharing options...
stork955 Posted April 28, 2009 Report Share Posted April 28, 2009 Gday Neo, 215 seems a bit high - its probably due for a decoke. Save that till after you sort out whatever is going on now. Little bits of rubber in the carb bowl is generally rotten fuel hose. Definately an issue -it'll cause the float to stay open and flood the bike. Go down to your local chainsaw shop and get some 5mm chainsaw fuel hose for it. Take your big wallet though - its really expensive but worth it as it will not fail in 2 minutes like the stuff you get from repco. Cheers, Stork Quote Link to comment Share on other sites More sharing options...
neo Posted April 28, 2009 Author Report Share Posted April 28, 2009 Wow Stork, That's really interesting. But you'll have to explain to me how the high reading indicates a decoke? I'll check out the chain saw hose...what's it made of? Best of balance. Neo Quote Link to comment Share on other sites More sharing options...
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