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1988 KX125 problems...


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Disclaimer: This will be a long post 👍

So this winter I picked up a "basket case" 1988 KX125. It was completely disassembled by the PO and then he lost confidence and sold it for next to nothing. I was in the mood for a project so I decided to pick it up, especially because it came with some new engine parts. Plus I love 80's MX bikes.

Here are the details off the top of my head:

1) Fresh lower end (bearings and seals included in sale)

2) New Crank (included in sale)

3) New Rings (included in sale)

4) New wristpin, bearings, and circlips

5) Piston and cylinder within spec according to my manual and yes it is installed in the correct direction, lol

6) New top end gaskets

7) New Air Filter

8) New clutch (fibers, steels, springs, and cable)

9) Carb thoroughly cleaned. PWK 39: 52 Pilot (48 is stock and it had a 68 in it!), 155 main, needle in middle position, air screw 1-3/4 turns out.

10) Stator timing on stock middle mark.

11) New B9EG Plug with new coil wire and cap.

12) KIPS disassembled and cleaned. Re-installed according to manual.

13) 32:1 Maxima SuperM pre-mix

So the bike starts first kick every time. Sounds great at idle, when you crack the throttle it seems to have a slight miss but overall sounds pretty good. Click it into gear and try to pull out and that's when the problem starts. The bike bogs and doesn't even have the power to pull out on its own, I have to push off and then when I give it gas it just bogs and either stalls or I have to grab the clutch and then it sits and idles clean, until I try to ride off again anyway.

32 degrees outside, engine slightly warmed up, WOT, kill switch on, compression 3 test average is 135psi. Not ideal I know (150-190 is) but for a 22 year old 125cc MX bike it is not "blown up" or un-ridablely low. I have owned and seen bikes with much lower run much better than this one, LOL.

The coolant and tranny fluid levels are unchanged so I am saying it is not burning either of those.

I used the old reeds because they looked ok, no chips or cracks, and they are tight to the block. Would worn reeds cause this drastic of an effect?

I do notice a fuel smell, especially when I was performing the compression tests.

So I am looking for ideas. What am I missing? Weak spark under load maybe? Really messed up carb?

Edited by AdvCoop
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hows the plug look, any indication of improper jetting, spooge etc?

bad reeds can really screw things up, but ive never seen it do what you descibe, they will cause a rich condition.

check your ignition timing.

do a leak down test, could be the flywheel side leaking.

could be weak spark i guess, im not sure. gap the plug, check the electrical system. bad thing about spark issues is that sometimes you just have to start replacing shit to figure it out.

you mentioned nothing of the exhaust, if it hasnt been looked at, check for obstructions

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The plug is wet with pre-mix but I haven't really been able to ride it long enough to even turn the plug color, it is still pretty much silver. I have had it idling for long periods and have tried to ride it twice when the problems happened both times.

The crank seals are new, and there is no oil leaking out the stator side so I was thinking they weren't leaking, but they still could be obviously. I had that cover off the day I posted this because I checked to make sure it wasn't leaking and that the timing was on the stock mark and not moved. Someone suggested I pull the flywheel to make sure it didn't shear the key-way so I plan to do that tomorrow.

I have checked the silencer for obstructions, but I didn't repack it. I have looked in with a light and you can see all the way through. I should check the pipe again though just to rule that out for sure.

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Mystery solved thanks to hi_im_sean, the pipe was clogged. I originally checked the silencer and looked into each end of the pipe. Today I took a piece of wire and tried to slide it through the pipe (the silencer is clear) and hit a blockage. It was all inside where the light didn't reach. An hour later, use of a wire coat hanger, my air compressor, a huge mouses nest, and one dead mouse later the pipe was cleared and the bike is running great! Much better than I expected a 22 year old 125 to run.

Thanks for the suggestion, I was stumped on this diagnosis.

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Mystery solved thanks to hi_im_sean, the pipe was clogged. I originally checked the silencer and looked into each end of the pipe. Today I took a piece of wire and tried to slide it through the pipe (the silencer is clear) and hit a blockage. It was all inside where the light didn't reach. An hour later, use of a wire coat hanger, my air compressor, a huge mouses nest, and one dead mouse later the pipe was cleared and the bike is running great! Much better than I expected a 22 year old 125 to run.

Thanks for the suggestion, I was stumped on this diagnosis.

awesome, i was totally betting exhaust blockage

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