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I have a 1999 yz125 and it has been nothing but problems. Its been running horrible at lower rpms, bogging and running really ruff with no power, the mid range misfires and the powerband doesn't start until almost full throttle. It smokes a lot too.

I rebuilt the top end because it did not have great compression and after that the compression was good. It still ran horrible. Then after that I rebuilt the carb and tried different jets and settings with my JD jetting kit. It still ran horrible. Then I made a leak down tester and the engine wouldn't hold any pressure, so I changed the crank seals. The engine held pressure really well but still ran horrible. I don't know what to do next! I've went through and inspected the whole engine so far from the air filter to reeds to power valve to the exhaust system. I have repacked the muffler. The plug fouls very often. The spark is strong so I dont think its an electrical problem.

What else could it be? Any help would be appreciated.

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Well since you have compression and spark it must be a fuel problem. You mentioned the bogging out, this could be any kind of carb problem (wrong float level, clogged passages , etc) but you say that you rebuilt the carb... Hmmm... From the information you have gave, the carb is prob the problem. Adjust pilot screw and idle screw see if that helps. Check out power valve maybe somethin wrong with the spring or it's all gummed up could be stuck open or not opening. Those are what I would check

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Could there have been oil that got into the crank area when the seals were bad and the oil is still in their and that is causing the problem?

I will check over the carb and try different jets and check the float level. I blew out all of the passages so I dont think any are clogged.

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After you make changes/repairs to the engine (replacing bad crank seals, refreshing the top end) you need to double check the jetting and re-jet as necessary.  Since it sounds like you have a fairly fresh engine, I would reset the carb (is it the original carb?) back to factory jetting and double check the float level, and that the needle and seat are in good condition.  When checking and jetting, make certain you are using the proper type plug and that it is new and gapped.

 

If that doesn't at least get you in the ballpark, then I would start at the top and check these in the following order:

 

1)  Compression and do another leak down test

2)  Clean carb, clean and oil air filter, verify carb settings

3)  New plug and double check your plug boot, wire and coil

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Don't be afraid to try some new fuel. I've gotten a bad batch before from a well used station, and my bike basically wouldn't run well enough to be ridden. New fuel, problem solved!

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I dont think the fuel is the problem because it has went through 2 tanks of different fuel. I will reset the jetting to stock and check the float level. But could oil have gotten in the bottom of the crankcase and this could be the problem? because it is still smoking a lot and the smoke is sorta white.

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You could have excess oil in the bottom of the crank area, but that should burn out pretty quick once the bike is running, and it should be BLACK, not white.  Black smoke is an indication of a failed wet side seal that is allowing your transmission oil into the bottom of the crank area.

 

If you are seeing white exhaust, I would start to think of a coolant leak into the cylinder (head o-ring is a very common culprit).  Are you losing coolant?  Does the while exhaust smell sweet?  If is does, that is a dead give away of burning antifreeze.

 

Normal premix smoke will have a blue tint to it, and should almost completely clear up once the bike is up to operating temps.

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Okay so probably not transmission oil. I went for a ride and it is burning 4x as much gas as it should. My brothers kx100 burned almost no gas and I burned at least 3/4 of my whole tank on a 25 min ride

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I dont even understand how that much gas can be getting used and the bike still runs. It is not leaking out of the carb or anywhere else.

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Carb problem, find it quick And stop running it. 4x normal would be it pouring fuel into the motor, washing the oil off of the bearings

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If that much gas is getting into the engine, you would fuel foul a plug quick and the bike would simply stop.

 

If the floats are set that far off, it would be running out the overflow tubes on the carb.

 

I am starting to think that you have a severely over rich jetting situation on your hands.

 

In the OP, you stated that it bogs down low, and has poor midrange, and only runs well at WOT.  I am thinking that you have the pilot jet so rich that your literally flooding your engine with fuel at partial throttle.  Your engine just cannot get enough air to use the excess fuel at the lower RPMs.

 

Try this, in this order:

 

1)  Clean and properly oil your air filter.

2)  Change all jets back to stock sizes.

3)  Change needle clip to middle position.

4)  Set your air screw to 1 1/2 turns out.

5)  Replace the spark plug with a new one of the stock heat range.

 

Then try running the bike.  Your jetting probably won't be correct with the above settings, but it will be in the ballpark and you can start fine tuning from there.

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Okay im gonna rebuild and clean the carb tonight and change the settings back to stock and also buy a new plug and air filter. The air filter I have right now is pretty old could an old filter restrict air and make the jetting rich even though it is clean and well oiled?

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could an old filter restrict air and make the jetting rich even though it is clean and well oiled?

 

Not likely.  If the air filter is clean and properly oiled, and otherwise in good condition, its age won't matter when it comes to how the engine runs.

 

What can happen if it is really, really dirty is restrict airflow and cause the fuel mixture to be overly rich.  However, if it is dirty enough to cause you to foul plugs, then it would also affect how the engine runs, ESPECIALLY at the high rpms.  You would probably be experiencing a high RPM bog or it would be way down on power.  But in your OP you stated that it ran well at the top.

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Today I took apart the bike and disassembled the carb.  I have a few questions.

 

The stock jetting for the carb is:

Main 350

Pilot 45

Clip Position #3

 

The jetting that was in the carb and recommended by JD Jetting for my alitude and temp is:

Main 360

Pilot 40

Clip Position #4

 

I am going the change the jetting to:

Main 350

Pilot 35

Clip Position #3

 

Is this jetting change enough to solve the problem?  The manual does not give a float height spec but rather a fuel level that you need a special tool to measure.  I searched for the float height online and a few people said 8mm so I set it to that, the float height was a little low.  Can anyone confirm this float height?  Should I leave it at that or order the special tool and wait for that?  The spring was not in the throttle slide right and it was binding when I twisted the throttle, I fixed it, could this of caused the problem?  Also, when the needle is installed in the throttle slide is there supposed to be back and forth movement/free play?  I am not talking about up and down just back and forth or side to side.

I also checked the needle valve and seat and it is not leaking at all, it seals perfectly. One more question,  when I slowly pushed the kickstarter while looking at the reeds I could see bubbles and a tiny bit a liquid that I am assuming is gas, is this a problem or does it indicate a problem?

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