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Discussion Starter #1
i have a kx-125 and i had it re-sleved, put a new piston in it, cleaned the carb. when i start it up it runs but it bogs out bf reving out,it starts in 1-2 kicks, also after re-sleeving it the compression is still at 60psi any advice? please help
 

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60 psi compression? um... I doubt it would start easily.

Often after rebuild the motor will draw more fuel and run rich.
Sounds like a fuel mixture issue, but it's always hard to say over the internet.
 

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Hey 90-kx-rider,

Good lookin' 90. I am glad to see somebody is workin to keep another one running. I have a 90 and a 91 KX125 and have run them for years. They are great bikes. I got a wealth of info on mods and what fits.

It sounds like you might want to dig a little deeper in the bottom end with the bad compression and bogging. The crank seals may be toast. Also, There is an o-ring on the clutch/waterpump side of the crankshaft that goes on the crank before the primary drive gear. If the o-ring is missing or gone bad, oil from the transmission will seep into the crankcase and make it bog. Tell-tale signs of this are black goo leaking from the silencer and pipe coupling. Its easy for a shadetree mechanic to forget to put the o-ring on before installing the primary drive gear.
Likewise, a leak-down test will tell if the crank seals and crankcase bonding are toast. You can have a shop do this or buy/rent a hand squeezed compression/vacuum pump (like for bleeding brakes) and modify a spark plug into an adapter. Call your local Kaw shop and the mechanic should be able to tell you how many psi to pump it up to and how long it should hold the pressure/drop.

However, the 90 is known for zero bottom end and killer midrange/overrev. So it will bog off the line if you don't ride it the way it was meant (Mid Throttle and feather the clutch). Do it right and the bike is hard to beat to the first turn. Boyesen Rad Valve helps alot on this bike. Google Pro Circuit Jetting for recommendations on jetting. And for the best bottom end contact Eric Gorr and have the cylider turned down and ported. A 91 ignitor box helps too. But all this turns it into a small-track/supercross bike versus the outdoor/big track killer that it is stock.

Good luck,
Jim
 

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Discussion Starter #5
thanks alot all this info wil help we changed all the seals and we are thinking it might be the crank is out of whack and vibrating thus causeing an air intake causing the bad compression, one of my dads buddies is a mechianic on dirt bikes and told us that could be a problem, but until i get the money to buy a crank it will have to rest for winter
 

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thanks alot all this info wil help we changed all the seals and we are thinking it might be the crank is out of whack and vibrating thus causeing an air intake causing the bad compression, one of my dads buddies is a mechianic on dirt bikes and told us that could be a problem, but until i get the money to buy a crank it will have to rest for winter
You definately have an issue soemwhere if you have a fresh sleeve job and only 60 psi of compression. Who did the re-sleeve? Did they hone match the bore to the piston ? You should have about .004" piston to wall clearence. Your crank will have nothing to do with low compression. Bad crank seals will cause lean, inconsistant engine running, and/or excessive transmission oil consumption. A leakdown test performed by a dealer will tell you a lot about the true condition of your engine's seals. I had an '84 KX125 with a slightly bent crank, and the bike actually ran great, it just vibrated, and burned a LOT of tranny oil until I figured out what was wrong...
 

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Hey 90-kx-rider,

Good lookin' 90. I am glad to see somebody is workin to keep another one running. I have a 90 and a 91 KX125 and have run them for years. They are great bikes. I got a wealth of info on mods and what fits.

It sounds like you might want to dig a little deeper in the bottom end with the bad compression and bogging. The crank seals may be toast. Also, There is an o-ring on the clutch/waterpump side of the crankshaft that goes on the crank before the primary drive gear. If the o-ring is missing or gone bad, oil from the transmission will seep into the crankcase and make it bog. Tell-tale signs of this are black goo leaking from the silencer and pipe coupling. Its easy for a shadetree mechanic to forget to put the o-ring on before installing the primary drive gear.
Likewise, a leak-down test will tell if the crank seals and crankcase bonding are toast. You can have a shop do this or buy/rent a hand squeezed compression/vacuum pump (like for bleeding brakes) and modify a spark plug into an adapter. Call your local Kaw shop and the mechanic should be able to tell you how many psi to pump it up to and how long it should hold the pressure/drop.

However, the 90 is known for zero bottom end and killer midrange/overrev. So it will bog off the line if you don't ride it the way it was meant (Mid Throttle and feather the clutch). Do it right and the bike is hard to beat to the first turn. Boyesen Rad Valve helps alot on this bike. Google Pro Circuit Jetting for recommendations on jetting. And for the best bottom end contact Eric Gorr and have the cylider turned down and ported. A 91 ignitor box helps too. But all this turns it into a small-track/supercross bike versus the outdoor/big track killer that it is stock.

Good luck,
Jim
Where did you find parts
 
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