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bron yr aur

2012 290 difficulty with summer jetting

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Hi all

Now that summer temps have arrived, I've been having a bit of a problem with the pilot circuit on my 290. Presently...

Altitude: 2,500'/800m +/-.

Temps: 80-85°F. 

Pilot: Keihin 48

Needle: DDJ (if I remember correctly), clip 4

Main: 128 (aftermarket)

And I believe the original owner installed a titanium 125 pipe with an additional crush gasket stacked in cylinder.

In the winter-ish cold temps, I noticed a rich burble just off idle, but I was able to tune it out with just a tweak of the air screw. 

Now, I started to notice some issues a few weeks ago, but had no idea what was in my carb, so I waited until I could break it down. When I did, I found the needle in clip 5. So I dropped it back to stock 4 position. But it doesn't seem to be responding to turns of the air screw, all the way up to 3 turns out. I don't see spooge out the exhaust, nor do I hear surging idle at 3 turns.

As the problem seems to be a little worse on descents, my next thought was the float level. I opened up the carb again, but found the float at 22 mm(!) so the previous owner had set it even lower than the stock 19mm. Seems they had encountered the same problem? I know the float needle valve is fine, since I just realized today that I've been putting the petcock in the "res" position instead of "off" when it's parked. Doh! (Pointing backwards is off, right?)

Even on level ground, on slow approaches to a log, I have to clean it out just a couple seconds or so prior to the first blip. If I don't, it'll stutter and break up my rhythm. I will say, though, that at between 1 3/4 and 2 turns out, with the bike idling in neutral, a hard whack of the throttle sounds smooth, without hesitation. But when riding, the idle tends to want to keep declining until it nearly stalls. So I'm a bit stumped at this problem. 

I've just recently REALLY started applying myself to tuning my 2t KTM enduro bike, so I've been getting a little braver with making changes on my somewhat new-to-me Sherco. But I'm no pro, either. Im not sure if the characteristics of this trials motor, with such a heavy flywheel and straight pipe, are the same as the enduro bikes. The bike was originally owned by a local pro, so I somewhat trusted the settings  Yes, there was another owner between, but he didn't seem to even care enough to have ever gone into it. He was a young guy who planned on learning trials, but I guess just didn't take to it.

Any suggestions?

Edited by bron yr aur

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Wow I thought you all would've been a little braver than this, at least some suggestions lol. I took it onto Facebook and got some help. I'm going back to stock jetting, but replacing the jet block gasket with the 3 tiny O rings. Then, if it persists, at least I'll know that's right. 

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Hi, you should do a search in this sherco forum as there is much pertinent info already here that should help you sort your bikes issues. Not touching on your bikes condition/wear & tear, jetting changes from stock, possibly previous owner made timing changes, etc....this model uses a fuel pump with some funky fuel hose/pressure pulse hoses that can be problematic and I believe this model might have the problematic inlet manifold that leaks. Good luck and hope you get her sorted soon:)  

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