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Fuelling Problem Or Something Else?


nordy
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Hi all,

Have recently bought a 2012 290st, seemed to run ok when i looked at it. Got home and as you do gave it a service, i noticed that the fuel mixture screw was only half a turn out from stop,i adjusted out to 1 1/2 turns, bike would'nt idle properly revs would hang then drop and it would pink when under load. Thought it might be crankseals soo replaced them, still the same, stripped carb soaked it in carb cleaner, blew out all the channels set float height as splatshop , sealed inlet manifold reed block , bike still runs rough unless i turn mixture screw to 1/2 out, then it runs nice. The carb runs with 125main 45 pilot and second clip from bottom, am abit puzzled by this one anyone any ideas. Cheers

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Hi Nordy,

I'm no expert but will start the ball rolling for you. Your own explanation shows you have addressed everything likely to cause the problem other than jetting. If the bike is sweet in the middle and top of the rev range under load you can change your 45 pilot jet for a 48. The screw I believe is an air screw that richens the mixture when turned in and weakens it when turned anticlockwise. If you fit a 48 the screw will probably need to be weakened (turned back). I wouldn't worry about it being a half turn of the seat if the bike runs fine. The screw position has an inverse relationship with pilot jet size as I understand it. The jet sets 80% of the fuelling characteristics and the screw fine tunes the other 20% if that makes sense. the ratio of screw influence is determined by size of pilot jet.

Hope this helps.

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I was allways told the preferred airscrew settings on the Kiehin were between .5 and 1.5 turns out. As most of us are in the cooler time of year(higher air density), the .5 seting is not out of the question in my book if it runs fine, however you may experiment with the larger pilot jet.

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i don't want to sound like a jerk but if the bike worked ok i wouldn't have messed with it. if what i did made it worse i would have undone what i did that made it worse. most bikes run fine even when not jetted exactly perfect. most bikes run pretty good with stock settings. things that can affect jetting are dirty air filter. water / dirt in carb. air leaks. not sure of your location / altitude but it's always good to check with other local riders to see what they have done. what did the spark plug look like when removed? compression? all factors.

some of the guru's will be along soon i think to offer more help than i can give but dabster should get you moving forward.

good luck and i hope you get to enjoy the bike real soon look forward to hearing about your trials riding exploits vs your repair time.

rob

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Thanks for all the advice,

I know i should have left it as it was but i thought the mixture setting was masking a different problem, tried another carb off a 2013 his jetting was the same but his mixture screw was out 11/2 from stop, same bad running until i turned mix screw in, the sparkplug looked rich thats why i messed with the mixture in the first place, gonna leave it as is, and ride it and see what happens. Thanks again.

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You changed the crank seals based on an air screw setting alone?

I hate to say it but that's a terrible mis diagnosis of a simple thing

It's adjustable, not fixed for a reason, wherever the screw is when the bike runs well is correct

If it cannot be adjusted or requires a setting far from normal then the first thing is to change the pilot or clean the carb

Engine tear down is strikingly odd

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Hi 0007,

When i get a new bike i change piston and crankseals as part of its service, a good starting point i think when you dont know it past, been caught out before.i do know the mixture is adjustable for a reason, didnt think it would have to be set this far in, my fault i was comparing it to the settings on 2 other 290s, was alway.told it had to be between 1 and 1/34s, takin it for a good run at the weekend, Cheers...

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Hi 2 stroke 4 stroke ,

No roadracing background, just in the past i've been unlucky with 2 strokes, the first 2 secondhand 2strokes ended up needing pistons after a few months, so ok i change pistons on any bike i get , if that makes me a weirdo ,so what, i stand over any bike i sell on, what goes around ,comes around, nothin wrong with that, is there?

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It doesn't make you a wierdo, and there's nothing wrong with it I suppose but it does make you , for a trials rider, uncharacteristically spendthrift :-) Most of us, of a certain vintage at least, are traditionally as tight as that part of a duck's anatomy when it comes to anything less than essential.

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I'm old school, dont have a lot of money, but i'd rather spend money on a piston kit than bling, spendthrift, dont think so, just like me bike to be right, when your as bad as me.you dont be long putting the hours up on them, keep trying though.

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The 2011 to 2013 Sherco's had terrible fueling, Way too lean. With a 45 pilot about 1/4 turn is it's scope at sea level. Try a 48 pilot with around 1 3/4 turns for a base point going up to 2.5 as needed. Make sure needle is 2nd groove up from bottom.

Most impotant thing to check is fuel hose is not pinched under the airbox.

The Ideal fix is to set the floats slightly richer, fit a t-piece to the carb hose and return back to tank with an extra nipple from splat shop.

Then jet to 48, 128 and a 3.5 slide, Still use the jjh needle on the 2nd clip from bottom.

This will make the bike smoother, richer, give stronger torque and the revs will drop as normal to idle, Just like a Gas Gas.

Fit a bp6es plug it reduces the pinking.

I've been there with the sherco and been through everything, convinced it had a slight air leak.

Most people dont notice the weak fueling they just ride it as they dont know any different, I hear loads of shercos running lean and have to grit my teath and bear it.

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