OldTimer

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Feb 3, 2005
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Just bought a brand new 2006 model 250X and after 116 miles of pretty tame riding, I'm getting an uneven idle. The idle speed will periodically surge for a few seconds and then go back down. Seems like it happens every 20-30 seconds when idling. Also, after a couple of hours of stop and go group trail riding last Thursday, I had to cut out early and took the dirt road back to the parking area. After a few minutes of steady throttle down the road, when I reached the parking area, the bike would bog when I gagged the throttle. I tried to replicate the problem by going up and down the parking area and tweaking the throttle, but it did fine in the confined area. It's like it had gathered itself. This was the coldest day of riding I've done on it, but it was still above freezing and by the time I got back to the parking area, the temps had risen to about 50 degrees.
?????
 

Rich Rohrich

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Jul 27, 1999
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They come from the factory with very lean low speed jetting. Even on hot days it's lean, on a cold day it easily needs a full pilot step richer to be right.

At the very least adjust the pilot fuel screw farther out to richen it up. Keep in mind not to take it past 2 1/2 turns from the closed position. Past that point there is very little spring preload on the screw and it can easily fall out from normal engine vibration.
 

OldTimer

Member
Feb 3, 2005
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Well that's simple enough, but can you please help me understand how the pilot jet would cause this?
I thought the pilot jet only affected the idle circuit and that when I yank the throttle WFO it's on the main. Also, I'm confused as to how a lean pilot would cause the idle to surge? I figured it would be a steady state of lean?
Please to enlighten oh swami.... :cool:
 

luckymonkey

Member
Dec 9, 2007
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What are we dealing with. Have you checked the filter, have you checked the carb? Has it been setting? What type of fuel do you run? What is your elevation?
 

Rich Rohrich

Moderator / BioHazard
LIFETIME SPONSOR
Jul 27, 1999
22,839
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OldTimer said:
Well that's simple enough, but can you please help me understand how the pilot jet would cause this?
I thought the pilot jet only affected the idle circuit and that when I yank the throttle WFO it's on the main. Also, I'm confused as to how a lean pilot would cause the idle to surge? I figured it would be a steady state of lean?

When you yank the throttle wide open it has to transition from the pilot circuit, through the needle taper and eventually it will make it's way on to the main jet. As it's heading toward WOT, the carb spends most of it's time OFF the main and IN the transition. The circuits overlap and the pilot circuit has an impact that reaches beyond just idle to nearly 1/2 throttle.

Surging whether at idle or in steady state running, is a sure sign of lean jetting. Basically what's happening, is some of the cycles will fire and some won't. The cycles that don't fire, leave enough residual fuel behind that one or more of the cycles that follow will have a chance of enough fuel to light off. When everything burns, the cycles that follow again become too lean because no old fuel is left behind, they misfire, and the whole process starts again. The engine is basically firing and coasting and firing and coasting a few thousand times per minute. We feel this fire/coast pattern as a surge because power is not consistently applied to the crank, which means it won't be consistent to the drivetrain. This is oversimplified to a large degree, but I think you can see the point.

It's pretty simple when you look at it from the standpoint of one firing cycle at a time.;)
 
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