gasgasssser Posted April 28, 2006 Report Share Posted April 28, 2006 Can anybody explain why when i put identical carbs off indentical bikes onto my troublesome sherco 290,i have to wind in the idle screw in about three to four turns to make the bike idle,even though they work normaly on the other bikes.My bike idles fine after adjusting the screw,but i still have a "splutter" on very low throttle openings,i would guess that this is linked to the fact that the idle screw is so far in. If i put the donor carb back on the bikes they came off,and cracked them up,the engine revs it a*** off.Why should my bike need this big adjustment of the idle screw to make it idle?.My jets are std,and the carb is clean as a whistle,and more puzzling,the donor bikes work properly with my carb on them!! What could be causing this.The bike runs spot on apart from the afore mentioned splutter.I think it could only be 2 things.First,the reed valve,which looks ok,no rips etc, If its not the reed,it cant be anything else other than the fact that the engine cant suck the fuel from the pilot. Could this be caused by a dodgy reed??Also,the plug is wet as if its running rich,but the mixture screw is the std 3.5 turns out. What do you guys think? Cheers Steve Quote Link to comment Share on other sites More sharing options...
lineaway Posted April 29, 2006 Report Share Posted April 29, 2006 Just remember no two bikes are alike, even when brand new. With this post you ruled out the carb? Remember the engine is just an air pump. Compression, Exhaust, or air leaks. Quote Link to comment Share on other sites More sharing options...
crashmctavish Posted April 29, 2006 Report Share Posted April 29, 2006 The dellorto PHBL has a complex pilot drilling which is notorious for blockage. see my previous posts. The last one I fixed required a new carb to put it right. #85 quid direct from dellorto in reading. Regards Quote Link to comment Share on other sites More sharing options...
copemech Posted April 29, 2006 Report Share Posted April 29, 2006 Try the reed block off the other bike then! Quote Link to comment Share on other sites More sharing options...
jaylael Posted April 29, 2006 Report Share Posted April 29, 2006 Just remember no two bikes are alike, even when brand new. With this post you ruled out the carb? Remember the engine is just an air pump. Compression, Exhaust, or air leaks. <{POST_SNAPBACK}> Also timing could affect the jetting at idle. If it were retarded or advanced it could require different fuel mixture. Quote Link to comment Share on other sites More sharing options...
gasgasssser Posted April 29, 2006 Author Report Share Posted April 29, 2006 Thanks for the replies so far. Mactavish,the carb works on the other bikes,so this rules out a carb fault. Copemech,the timing is ok.Although i havnt strobed it,the factory timing marks are in alighnment. Lineaway.I`m pretty sure there is no loss of compression.There is no air leaks at the carb manifold,and i have replaced crank seal bearing on ignition side.The exaust isnt blowing.Is there any other faults regarding the ekhaust that i should be looking for? I havnt swapped the reeds beacause they all look identical,plus i dont like keep stripping mates bikes down.(not sure if they like it either)Maybe i should try a new reed. Quote Link to comment Share on other sites More sharing options...
lineaway Posted April 29, 2006 Report Share Posted April 29, 2006 The exhaust would be some type of blockage so switch pipes. Have you had the top end off? Could just be the rings stuck in the grooves from oil build up. Quote Link to comment Share on other sites More sharing options...
copemech Posted April 29, 2006 Report Share Posted April 29, 2006 Would not take too much to strobe the timing on both bikes to compare. Seems to be some funny things going on lately with those stator coils! Quote Link to comment Share on other sites More sharing options...
lineaway Posted April 29, 2006 Report Share Posted April 29, 2006 Copemech, isn`t Lake Mackenzie this weekend? And you could be right weak spark at low rpm`s. Quote Link to comment Share on other sites More sharing options...
hemisavoy Posted April 30, 2006 Report Share Posted April 30, 2006 My 290 ran very poorly down low off idle, with a wet plug reading. I continued to lean the bike in an effort to clean it up. The problem was a cracked exhaust. It was cracked out along the top seam of the exspansion chamber and on the back side around the mounting flange. The pipe has to have no leaks for the engine to carburate properly. I had the cracks tig welded, which cured it, and ultimately put on a new pipe. It runs very nicely now, and the plug runs clean. Quote Link to comment Share on other sites More sharing options...
ham2 Posted May 2, 2006 Report Share Posted May 2, 2006 Have you tried the butane gas trick around the cylinder/crankcase to see if the idle improves (which confirms there is a leak)? Quote Link to comment Share on other sites More sharing options...
ham2 Posted May 2, 2006 Report Share Posted May 2, 2006 Butane gas trick....................?? Yes theres a bed vacant in Pinderfields Burns Unit, shall we provisionally check you in sir?Just a bit dangerous although I understand the theory. Dont try this at home folks.....but if you do, make sure the video cameras running, you might just get Quote Link to comment Share on other sites More sharing options...
gasgas Posted May 2, 2006 Report Share Posted May 2, 2006 (edited) M2c, since you told us that you had to dial in the mix screw that wld suggest a strong airflow tru the carb. So, what can create that? As is said earlier, the egine is just a pump. Yours is pumping better, but why? Did you had anything done to the engine? I just reread yr post, and are we really talking idle screw as in opening the slide higher? In that case forget my post..... and maybe i come back later Edited May 2, 2006 by gasgas Quote Link to comment Share on other sites More sharing options...
gasgas Posted May 2, 2006 Report Share Posted May 2, 2006 (edited) Oke, can't leave it alone I am now concentrating on the slide Your bike needs the extra fuel sucked up tru the accelarating hole/passage which is under the slide, in stead of the normally amount that goes via the idle passage. That wld tell us that you suffer airflow tru the carb. Your engine is sucking air from somewhere else also. So i am with Ferretflasher on this one. Do a compression test, you can make one yrself, and pump up yr engine. Look for leaks around the intake manifold, and/or cilinderbase. Btw for how long this problem exists? Edited May 2, 2006 by gasgas Quote Link to comment Share on other sites More sharing options...
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