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06 YZ 125 bog, whats causing it?

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So I recently rebuilt the entire engine on my 06 YZ 125, compression is strong. Bike is all stock except for carbon fiber reeds (reeds are used, didn't think to replace them during the rebuilt). I fully rebuilt the carb with all new jets and needle.

 

Here's the problem:

-When you snap the throttle it goes bwaaaah like it wants to die, Its always had this bog even before the rebuild

-It will also sometimes idle high and then shut off (just like when you run out of gas). It will then start after a few minutes or so of sitting. That only happens sometimes, I saved it from dying one time by pulling in the choke which tells me it must be starving for fuel. The plug looks black/golden and not white.  

 

It will scream once you get past the bog and onto the main jet, then it pulls. I tried moving the clip down one on the needle and making fuel mixture richer on the air screw but it didn't help.

 

Anyone have any idea what this could be? The only thing I can think of it the float or bad power reed?

 

Thanks for any help with this 

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Reed block area, center cases not sealed properly in that area, leaking left main seal,

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Reed block area, center cases not sealed properly in that area, leaking left main seal,

 

I checked the reeds and took out the block, reeds looked good. I just set the float height to 9mm and now the bike wont even start. Mabe the float or height is bad? Everything else in the carb is new.  

 

The main seals are new and the cases were just put together with Yamabond 4 after a bottom end rebuild. How often do new seals and a case not seal properly? I won't even wanna look at the bike if its gonna need a case split again after all that work haha, well mabe once winter comes and I have more time to kill.

 

Thanks 

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Well the case leaking would be from not using enough sealant to get a proper seal but not that common, stock reed block you'd check the o-ring on the block for any tears or nicks. Now left main seals are a more common cause during install it can get torn, most often caused when putting in the new seal they didn't use a light film of lithium grease on the seal surface. But sounds more like ya need to get your float height setup correctly.

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Well the case leaking would be from not using enough sealant to get a proper seal but not that common, stock reed block you'd check the o-ring on the block for any tears or nicks. Now left main seals are a more common cause during install it can get torn, most often caused when putting in the new seal they didn't use a light film of lithium grease on the seal surface. But sounds more like ya need to get your float height setup correctly.

 

Thanks, would you happen to know what the float height should be for an 06 yz 125? I read 9mm on a forum and the manual didn't say what height it should be at

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I took the carb off to change the float height. For some reason the carb was bone dry and had no gas in it? The petcock works has a clean filter and a full tank of fresh gas. I'm not sure that an air leak could cause the carb to stay bone dry when trying to start it so that makes me feel better that I won't have to look at crank seals or the case seal. 

 

After lowering the float height it started up, it seemed to run great for the first five minutes but then that 1/4 throttle lean bog came back after it fully warmed up.

 

Not sure where to go from here, mabe lower the float even more? Air screw is at two turns and Im running all stock jets

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Thanks, I set the float to the spec. 

 

It still has the bog and for some reason the idle is higher once its warmed up. Does anyone know if air leak symptoms are more noticeable when the engine is warm? I figured they would be more noticeable when cold.

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Being lean can cause the bike to bog and the  idle increase after it warms up would make me believe it is an air leak like a weeping main seal would do. it's more likely to show up once things warm up and expand before you get all crazy doing that double check your idle setting first that you have proper slack in the throttle cable. Ran it long enough to notice if the idle keeps increasing as it's running? since the choke enrichens it and when you shut it off the idle jumps tends to say air leak.


 


I forget how to check for air leaks using starter fluid spray it in the likely areas and if the idle drops you have a leak maybe help zero in on where it is happening at. It's been a long while since I've had to deal with one.  


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Being lean can cause the bike to bog and the  idle increase after it warms up would make me believe it is an air leak like a weeping main seal would do. it's more likely to show up once things warm up and expand before you get all crazy doing that double check your idle setting first that you have proper slack in the throttle cable. Ran it long enough to notice if the idle keeps increasing as it's running? since the choke enrichens it and when you shut it off the idle jumps tends to say air leak.

 

I forget how to check for air leaks using starter fluid spray it in the likely areas and if the idle drops you have a leak maybe help zero in on where it is happening at. It's been a long while since I've had to deal with one.  

 

I did run it long enough to fully warm up, it has to be running for atleast 10 mins before the idle increases and the bog starts. If it were a case air leak it most likely wouldn't get worse as it warmed up because the aluminium would expand and the gap would shrink right? Just trying to eliminate that I didn't seal up the case half's with enough Yamabond.

 

Crank seals on the other hand makes sense. Since they have a steel lining if they aren't sealed right, mabe the aluminium case is expanding more then the steel/rubber crank seal and allowing this gap to grow as the bike warms up. 

 

I will try messing with the idle and air screw and check for air leaks on the intake boot with carb cleaner. I can sometimes get it to idle down normally when its fully warmed up to by blipping the throttle quick. 

Edited by crmaverick

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With the left side cover off and the engine running, spray some carb cleaner into the stator area. If the RPM climbs like crazy, you have an air leak. Same check with the reed block.

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What you are describing sound like a simple lean condition caused by the carb needle, or pilot jet. It sounds like you need to raise the needle, by lowering the clip position, or you need a richer needle, and or pilot jet. Start with the simple one first and raise the needle. 

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What you are describing sound like a simple lean condition caused by the carb needle, or pilot jet. It sounds like you need to raise the needle, by lowering the clip position, or you need a richer needle, and or pilot jet. Start with the simple one first and raise the needle. 

 

I did try moving the clip down and it still had the bog. Bike is all stock and at sea level, should jetting be all stock in these conditions?

 

Will a lean pilot cause an increase in idle as it warms up? 

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With the left side cover off and the engine running, spray some carb cleaner into the stator area. If the RPM climbs like crazy, you have an air leak. Same check with the reed block.

I'll try that. Can bad reeds cause a lean condition and feel like an air leak? Can minor chips on the edge of a reed or something else that I missed when looking at the reeds cause something like this?

 

I sprayed carb cleaner over the read block/carb/intake area yesterday and had no change in rpms. 

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Chipped or bowed out reeds will cause all sorts of odd problems, but it won't cause the isle to rise (as far as i know). Worth pulling it out and checking it (hold the block up to a light, look thru the intake, see if you can see light...if yes, replace the reeds).

Remember, 125's have a huge carb, and not allot of bottom end, so quick throttle opening at low RPM will always give you a slight bog, and then recover and scream.

Try doing some plug chops at 1/4 throttle (ride at 1/4 throttle for a few hundred feet, pull in the clutch and kill the motor, pull the plug, and see what it shows), let us know what it looks like.

BTW, the needle doesn't affect until about 1/2 way up the RPM, you still have the mixture screw, pilot and slide before that.

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I'm having the same issues you're having actually... I have no good input sadly. I tried everything you have done, except a top end and my compression is only 135-140 so I will try that first. Hopefully you find a solution because I'm worried that the top end isn't the issue.... otherwise I'm just selling my bike and getting an EXC 400 :p

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Chipped or bowed out reeds will cause all sorts of odd problems, but it won't cause the isle to rise (as far as i know). Worth pulling it out and checking it (hold the block up to a light, look thru the intake, see if you can see light...if yes, replace the reeds).

Remember, 125's have a huge carb, and not allot of bottom end, so quick throttle opening at low RPM will always give you a slight bog, and then recover and scream.

Try doing some plug chops at 1/4 throttle (ride at 1/4 throttle for a few hundred feet, pull in the clutch and kill the motor, pull the plug, and see what it shows), let us know what it looks like.

BTW, the needle doesn't affect until about 1/2 way up the RPM, you still have the mixture screw, pilot and slide before that.

 

I didn't plug chop but I checked the plug and it was a charcoal color. Held light to behind reeds and didn't see any light come through. Im gonna build a pressure tester and check for air leaks.

Edited by crmaverick

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