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Carb FCR 37 Surge problem on a 05 KTM 250ecxf-HELP!


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TT is always my last resort after what I think have tried everything. Any here is the lowdown.

The bike has a low to medium idle surge cold or hot, not fun on downhills.

Here is what I have done so far.

*rebuilt carb, cleaned carb twice

re-jetted several ways, new Tusk fuel/air screw and went back to stock to check if was problem.

checked for air leakage around inlet

new cables, lubed twice

 

Well I think thats it but still getting a surge often but not all the time

Any help is appreciated to keep me from going cookoo

 

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You need to note the EXACT throttle position (not rpm or speed).

 

Being you make no complaint about running on the flats at steady throttle or going up hills under load makes it clear your problem is you are just a little bit rich on the needle. If I were going to bet, I'd say it was the straight diameter of the needle which is only change by swapping out the needle. I'd go one or two sizes leaner (last letter if it is a Keihin needle) Up the alphabet, same clip position.

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Define 'cleaned carb twice"

 

If you did not do all this, you did not clean and service the carb:   http://www.dirtrider.com/features/protips/141_0707_keihin_fcr_carb_rebuild/

You can't ignore the hot start, choke, slide plate seal, fuel screw tip and oring, 

I would also go in there and just replace the jet needle, but also the needle jet, in case it is worn. 

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The surge or high idle is when i let off the gas. The needle jet was replaced in rebuild. Bike starts great, idles fine and basically runs very good aside from something that sounds like a throttle sticking but is not that

Edited by kma
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A 'surge' is not the same thing as a 'hanging idle'. A 'surge' (bike speeds and slows) occurs when riding, a 'hanging idle' is when happens when you let off the throttle and the roms 'hang' high and a blip might be needed to get them to drop (or even a little 'clutch drag') Different problems with different solutions.

 

So... is it a surge or hanging idle?

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Then you need to fine adjust your fuel screw  (and idle speed), assuming your pilot jet is spotless, the fuel screw is undamaged, the oring, washer and spring are there and the slide vacuum release plate seal is good.

OK but have messed with it quite a bit but have new understanding after some research. Also looking at carb schematics I dont see "slide vacuum release plate seal". Is there another name for this?

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Hanging idle is due to a lean condition

The most common causes are:

 

Partially open hot start plunger, do to corrosion (clean and polish and grease)

Leaking hot start nut (cracked/replace)

Leaking oring on fuel screw (torn, replace)

Loose carb boots (tighten)

Extremely worn out needle jet and jet needle (usually combined with a slide that has to be rasied to keep it running)

 

...and the number one answer is....

 

IMPROPERLY ADJUSTED FUEL SCREW!

 

You must start with a hot motor, set the fuel screw to 1 turn, THEN LOWER THE IDLE AS LOW AS IT WILL GO with the idle screw

NOW you adjust the fuel screw out for max idle.

If the idle goes to high, turn it way down again with the idle screw, and adjust the fuel screw again.

Your goal is a target idle of 1800-1900rpm using the FUEL SCREW to raise the idle to this point.

 

You can further adjust the fuel screw for improved throttle response by 'blipping' the throttle and listening to the idle drop.

Slow drop means lean, so you go out 1/8 turn, till it gets a fast drop.

 

Yup, sure was hard to find......

 

https://www.google.com/search?site=&source=hp&q=fcr+slide+plate+seal&oq=fcr+slide+plat&gs_l=hp.1.1.0i22i30l5.307.2844.0.4686.15.15.0.0.0.0.117.1250.13j2.15.0.msedr...0...1c.1.60.hp..1.14.1182.0.Ro6C8P7cmoo

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Hanging idle is due to a lean condition

The most common causes are:

 

Partially open hot start plunger, do to corrosion (clean and polish and grease)

Leaking hot start nut (cracked/replace)

Leaking oring on fuel screw (torn, replace)

Loose carb boots (tighten)

Extremely worn out needle jet and jet needle (usually combined with a slide that has to be rasied to keep it running)

 

...and the number one answer is....

 

IMPROPERLY ADJUSTED FUEL SCREW!

 

You must start with a hot motor, set the fuel screw to 1 turn, THEN LOWER THE IDLE AS LOW AS IT WILL GO with the idle screw

NOW you adjust the fuel screw out for max idle.

If the idle goes to high, turn it way down again with the idle screw, and adjust the fuel screw again.

Your goal is a target idle of 1800-1900rpm using the FUEL SCREW to raise the idle to this point.

 

You can further adjust the fuel screw for improved throttle response by 'blipping' the throttle and listening to the idle drop.

Slow drop means lean, so you go out 1/8 turn, till it gets a fast drop.

 

Yup, sure was hard to find......

 

https://www.google.com/search?site=&source=hp&q=fcr+slide+plate+seal&oq=fcr+slide+plat&gs_l=hp.1.1.0i22i30l5.307.2844.0.4686.15.15.0.0.0.0.117.1250.13j2.15.0.msedr...0...1c.1.60.hp..1.14.1182.0.Ro6C8P7cmoo

I really appreciate this info and link. Not sure if ur being sarcastic but I really have done a lot of searches and have found no info such as this, and I figured that was the vacuum release plate but not 100% sure. Even if you go to u tube and look at rocky mtn tutorial vid on adjusting fuel screw it is nothing like the method above. Again I really appreciate it as always guys, THANKS!! I will let you know how it goes

Edited by kma
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I really appreciate this info and link. Not sure if ur being sarcastic but I really have done a lot of searches and have found no info such as this, and I figured that was the vacuum release plate but not 100% sure. Even if you go to u tube and look at rocky mtn tutorial vid on adjusting fuel screw it is nothing like the method above. Again I really appreciate it as always guys, THANKS!! I will let you know how it goes

Well guys I got it running right, no hanging idle. Also like I said before some of the tutorials on u tube are different. For instance in the link below

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