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The Diagnosis Project
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The Diagnosis Project - March 27, 2009, 07:44 AM

This is a thread for everyone to test their diagnostic skills.

I've been having a few issues with my pitbikes, which I'm pretty sure I know the solution to, but I'll start anyway.


Bike: SSR125SB-A
Carb: Mikuni 22
Modifications: Stocker
Issue: RPM-hang. Throttle action is very smooth, snaps back perfectly, about 1/8th" play. Issue occured before carb disassembly/cleaning, and still occuring afterwards. RPMs increase appropriately with throttle response, but hang when rolled off and gradually settles back down after about 30 seconds.
Additional Info: Occurs hot or cold. Stock jetting. Pilot 3 turns out, idles low and fine. I do not have a Mikuni22 diagram, but I'm 90% sure it's reassembled properly as my other bike is working fine.

If anyone has a carb diagram for this model, I'd love to get a copy.




Here's how this works: Please quote only the above info, and provide your input. I will perform each task suggested until the problem is resolved.

Feel free to post your own stuff as well.


I will have all whoring deleted.

Let the project begin.


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March 27, 2009, 07:46 AM

Sounds like a lean condition.


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March 27, 2009, 07:48 AM

Quote:
Originally Posted by boomboom929 View Post
Sounds like a lean condition.

At what point? Pilot, main, needle? All?


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March 27, 2009, 07:53 AM

Probably midrange...pilot, needle. Try to go full rich on the air mixture screw.


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March 27, 2009, 07:53 AM

If just sitting at idle and it doesn't rise I would say it isn't the pilot jet.
If you only turn the throttle a bit and it does it would the main jet not even come into play yet and it be the needle.
Just my thinking outloud.
An air leak is always possible too.


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March 27, 2009, 07:55 AM

Will try that this evening. Thanks gents.


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March 27, 2009, 07:56 AM

Quote:
Originally Posted by n2sport View Post
If just sitting at idle and it doesn't rise I would say it isn't the pilot jet.
If you only turn the throttle a bit and it does it would the main jet not even come into play yet and it be the needle.
Just my thinking outloud.
An air leak is always possible too.
+1 on the air leak. It would cause the same thing. Check out the intake boot. See if any vacuum lines are off.


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March 27, 2009, 07:57 AM

Not all Chinese "Mikuni's" are made equal - let me know if you need another to experiment with... I'll ship it up there.


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March 27, 2009, 08:02 AM

Quote:
Originally Posted by Nudist View Post
Not all Chinese "Mikuni's" are made equal - let me know if you need another to experiment with... I'll ship it up there.
It's the black one, which worked fine for Dan, so I'm guess the carb itself is ok.

Any diagrams around?


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March 27, 2009, 08:03 AM

Quote:
Originally Posted by boomboom929 View Post
+1 on the air leak. It would cause the same thing. Check out the intake boot. See if any vacuum lines are off.
Been a while since i messed with carbs. On my FI on the harley I have an auto tune and an air leak will cause it to just keep dumping more fuel and create a rich condition(I think I am there now) Bad wide band O2 sensors could do the same.

Kind of miss carbs...


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I'll also thank them for proving that you don't need the force to get a woman to make you a sammich.
"They who would give up an essential liberty for temporary security, deserve neither liberty or security."
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March 27, 2009, 08:03 AM

Im with boom, factory pro use to have a link to a jetting tuning page, they mentioned lean hang just as your symptoms describe. There's was needle based, trying to find the link but failing.


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March 27, 2009, 08:04 AM

You guys rule.

Now somebody else post up, even if you know the answer.


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March 27, 2009, 01:22 PM

Does the stock needle have variable positions?

Is the stock main adjustable like the a/f screw or just need to be replaced?


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March 27, 2009, 01:31 PM

Do an idle drop. Set the idle at factory spec and adjust the air/fuel mixture screw out till you reach peak rpm's then turn it back in an 1/8th or so and then readjust the idle. A hanging idle is usually the result of a lean pilot circute or air leak. You can test for an air leak by spraying a flammable substance such as carb cleaner around the intake boot and if rpm's rise then you have an air leak. Also, if the needle is not adjustable then you can just add shims under it to raise the needle and richen up the circute. I have tons of them from dyno jet kits if you need them and main jets have to be replaced to acheive a richer or leaner mixture.
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March 27, 2009, 02:22 PM

STUD.


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