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On a 99 YZ 250 my friend and I are trying to figure out what is causing this fuel puddle at the rear of the carb.

What I know now is that the problem will happen running or fuel will accumulate here even if the bike is not running.

This situation is causing a rich running condition and poor engine performance.

The needle has been replaced but not the seat.

The reed pedels are new.

The float level has been check many many times.

No external leaks.

http://i217.photobucket.com/albums/cc230/claykyle/ATT00203.jpg

Has anyone had this problem and how was it fixed?

The fuel cut solenoid has been replaced, we are running out of ideas for a fix for this.

any help would be appreciated since we have a three day trip planned in a week.

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You said you didnt replace the float seat? I would gess that is your issue. There are only a few things in the carb that cause a fuel issue like that. Float height, needle and seat worn, or a small hole in the float.

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I will be looking close at that tonite but it does not run out the overflow. which leads me to believe there is an internal passage problem rather than a fuel level problem.

Anyone have an enrichment/choke problem like this? the fuel could be running down from the upper hole/passage and collecting at the lower part of the carb.

Is it possible for the choke valve to bypass fuel?

How about a crack in the main carb body? Fuel leaking from the air screw passage?

How about is this a normal condition and i should look elsewhere???

All suggestions will be considered!

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it could be a capalary effect and the fuel traveling up your jets. If it is really hot out i could see it happening.

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i had a similar problem on my KTM, gas was pooling up under the air fuel screw, the problem was a hair line crack in the carb body. I Jb welded it and its held up fine

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I checked for cracks last night and found none.

I did find a piece of the fuel cut solenoid tip in the FCS jet.

Can someone explain to me what the fuel cut solnoid does. when 12V is applied to the solenoid it extends or closes the fuel circut. the long jet rod is not uncovered by the slide until approx 90% throttle open. the fuel passage originated in the float bowl (floor) and passes up through a jet then past the Fuel cut solenoid and into the carb throat via that long brass tube.

Is the system normally open or closed?

What is the operational criteria for this FCS system. What is the functional goal of having this system or what is it designed to do?

what is the actual electronic control over the FCS

Can anyone fill me in on how this works???:worthy:

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I think the term usually used for the fuel cut solenoid is the "powerjet." The control for this is the throttle position sensor.. The ignition turns this "jet" or sol on in the midrange. It gives the bike improved throttle response as if it were rich, yet allows lean jetting on the main for improved top end .

I like this explanation in Eric Gorr's book far better than mine.

"The latest trend in two-stroke carbs features a pump that sprays fuel into the venturi from 1/4th to 3/4th throttles. In the past, carb manufacturers made jet needles that attempted to compensate for the natural lean condition of the mid-range but that compromised the jetting at full throttle. The auxiliary pumps are powered by electricity supplied by the alternator (about 5 watts) and controlled by either a throttle position or an rpm sensor."

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How about is this a normal condition and i should look elsewhere???

This gets my vote.

Did it start acting up one day out of the blue, or did something change that might have caused this...

RC is right though, you're talking about the 'PowerJet'.

jetting_chart.sized.jpg

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Thank You Guy's:worthy:

I believe that the Carb is now good to go if nothing else it is the cleanest thing you ever saw.

Some remaining question:

How do I test the electrical operation of the Powerjet? I can backprobe the jet circut? is it safe to assume that when the bike is running there is power supplied and the CDI box applies ground under certian operating condition (like 1/2 to 3/4 throttle)?

I think I am going to explore other possible problems outside the carburetor. Like the reeds. and see what we find.

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This gets my vote.

Did it start acting up one day out of the blue, or did something change that might have caused this...

RC is right though, you're talking about the 'PowerJet'.

jetting_chart.sized.jpg

The 00' YZ 250 and newer owners should take note of Faded's graph for the powerjet and consider changing the one in the manual to coincide.

RC, GCannon's powerjet carb is a PWM which is different than our PWK version. I don't think it has a TPS, and notice the powerjet outlet (see his pic) is at @ 3/4 throttle rather than ours which is closer to @ 1/4 throttle.

Faded do you have any ideas on how the powerjet carbs without a TPS are controlled and where in the throttle range they make a contribution? The differences in the outlet placements is interesting.

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How is the powerjet controlled without having a TPS to monitor it? This sounds interesting.

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I think it gets a signal off of the cdi box but not sure. Make sure the float is right follow the manual I learned the hard way. Make sure the the little spring plunger is not compressed when you measure it. If it is your float level will be way high.

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I think it gets a signal off of the cdi box but not sure. Make sure the float is right follow the manual I learned the hard way. Make sure the the little spring plunger is not compressed when you measure it. If it is your float level will be way high.

That makes sense. The TPS does not show up on the carb until 2000.

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I had been fighting with the carb on my 99yz250 ever since I got the bike last summer. I messed with everything that I could, jets, float height, etc. The bike still ran poorly, and got bad gas mileage. It was boggy, and off idle power was not great. I eventually ended up buying a pwk airstrikerII from the TT store for 229 dollars. It was the best money that I have ever spent on any bike mod or part. The thing runs great now. I put the carb in, adjusted the idle, rode the bike. I expected to have to do some playing around with jetting, and the needle clip settings, but the thing ran wonderfully right out of the box. The pwk also has an idle adjust screw which the stock carb on the 99 doesn't.

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That makes sense. The TPS does not show up on the carb until 2000.

Wouldn't that make the only input criteria for the power jet RPM?

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My yamaha manual says that the powerjet is on until 8500 rpm then shuts off. But it doesn't say if it's on at idle. It's not very big so it can't dump that much fuel. What is a normal pilot size for a non powerjet carb?

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Wouldn't that make the only input criteria for the power jet RPM?

Correct.

My yamaha manual says that the powerjet is on until 8500 rpm then shuts off. But it doesn't say if it's on at idle.

It's not functioning at idle. Earlier models used the RPM-only input to trigger the power jet but it was later refined to rely on TPS and RPM...since jetting is throttle position dependant. This allows for a stronger midrange and extended top-end pull through more precise jetting since 2-strokes naturally require richer jetting in the midrange and need to be leaned out on top to get back over-rev.

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Is the system normally open or closed?

What is the operational criteria for this FCS system. What is the functional goal of having this system or what is it designed to do?

what is the actual electronic control over the FCS

Can anyone fill me in on how this works???:worthy:

I can only speak on the 'newer' system like that found on my '06 YZ250 since that's all I've played with. This system is normally open...for liability reasons I'm sure since they can't have a part fail that would contribute to leaning out the jetting and possibly causing engine damage. Once the bike is started the soleniod receives a voltage that closes the system and disables the power jet until the CDI sees greater that 50% from the TPS and the RPM's are under 8500.

Faded do you have any ideas on how the powerjet carbs without a TPS are controlled and where in the throttle range they make a contribution? The differences in the outlet placements is interesting.

As said before it's triggered off of RPM only; probably in the 5.5k-8k range, plus or minus a few hundred RPM. My swag is that they must have figured that if you're getting into the RPM range that uses the power jet you must have the throttle twisted close to WFO.

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