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02 RM250 PWK38TPS Choke Knob . . . Adjustable?

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'Bought this bike a few weeks back. Fine bike but I noticed that the choke knob rotates and "clicks". So I look in the manual and lo-and-behold it's something the manual briefly describes. Apparently you can adjust it . . . huh?

The manual states that the "bypass (starter) system affects air/fuel mixture ratio in the throttle opening range of full close to 3/8 open position. The mixture will become leaner as the dial is turned out counterclockwise to open the system . . . "

I just looked at the throttle valve opening chart, you know, the one where it shows what tuning part (jets, etc) does what over the throttle range, and there it is . . . affects things much like the "Slow jet and air screw" does.

For those with this carb - do you use this?

How is this different than the airscrew?

Thanks!

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Well . . . NO ONE has a carb like this or knows about this feature on these carbs? Where are all the intake/jetting gurus?

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Per microfiche so far I've found out that only the '01 and '02 RM250 had this particular choke assy.

How 'bout you YZ, KX, or CR owners? I know the early 2000 CRs don't have them.

'Tried to talk to a contact at Keihin to answer the question but they say to contact Suzuki.

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I talked to a carb parts distributor and here's what I've learned. Indeed it's another way to adjust air/fuel mixture at lower throttle openings. Effectively you can adjust (turn) this knob and get a change similar to the airscrew adjustment. Note I state similar, not the same because apparently it impacts a slightly larger throttle range opening from zero to just under half-throttle. Overall, it works WITH the pilot or slow circuit.

The parts guy said the idea with the thing was in concept a great idea. The problem was that too many people were confused by it and didn't use it correctly so on newer models they discontinued it's use.

So I 'sperimented. After I warmed the bike up pretty well I changed it by three clicks at a time (BTW 45 pilot jet and at these temps A/S is a little less than 1.5 turns out so slow jetting is fairly good). My test was to get it going only fast enough to keep from stalling in 2nd then wick it open fairly quickly. I started out at what I'll call the baseline at 12 clicks out from completely clockwise (two full turns out).

From baseline I tested after three clicks out (2.5 turns out total), six clicks out (3 turns out total), nine clicks out (3.5 turns out total). I noticed improved throttle response at six clicks out from baseline but at nine clicks out it was starting to develop a little bit of what I'll call a lean bog, not bad but noticeable when I wicked it open quickly. Snap was best at six clicks out from baseline.

Then back from baseline three clicks in (1.5 turns out total), six clicks in (1 turn out total). No need to go any further because throttle response was breaking up.

In effect by turning it out (CCW) I was leaning out the pilot circuitry and by turning it in (CW) I was richening things.

Tried one more test. I got enough momentum to motor along in 3rd gear on a slight incline then tried wicking the throttle. This helped to gauge the difference without having to fight to keep the front wheel down. Best response about 6 clicks out.

Please note that there may very well be older models or other bikes that use the choke knob in a different way or it may be a different design - built in idle adjust or something . . .

but if you have the stock carb on an '01 or '02 RM250 and your pilot is jetted well this knob ought to give you some easy tuning - give it a try!! I'll be fiddling with it for sure, beats fooling with a little tweeker on the A/S.

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