98 kx250 compression @ 150 psi

Just checked the compression on my newly acquired 98 kx250. read 150psi after 6 kicks.

the owner said the top and bottom were done 15 hours ago. dose the compression seem too low for that amount of hours? I figured roughly -10% for 20 hours?

I read the compression should be around 175-200 on a fresh rebuild.

should i do the top end? what should be done to get the compression in the prime operating range?

Just want to make sure ----

run the engine, warm it up, get oil on rings, etc.

then check compression. Be sure and hold throttle wide open while kicking. Make sure reeds are good. no cracks or air gaps around the edges. No light should be able to get through.

Just want to make sure ----

run the engine, warm it up, get oil on rings, etc.

then check compression. Be sure and hold throttle wide open while kicking. Make sure reeds are good. no cracks or air gaps around the edges. No light should be able to get through.

hmm. yeah it was cold as hell with the air box off and no air filter. and i didnt even have the exhaust on? would that make a diff?

but i did test @ WOT

if the exhaust is off take a light and look into the cylinder and see if there are any scratches in the wall.

if the exhaust is off take a light and look into the cylinder and see if there are any scratches in the wall.

I did, everything looked smooth and clean. :thumbsup:

Your method of testing is not the same as what is mentioned in the service manual, so your reading may not be the same.

I have never measured with a cold engine, so not quite sure how much difference it makes. Maybe we can all learn from your experience! Put that exhaust back on, get it warmed up and running, then recheck the compression. Please report back here.

Comparing readings between different bikes and different gauges is useless. Production tolerances and wear will cause different bikes to read differently, and every gauge will give a different reading. The only way a compression test is really useful is to test a new or freshly rebuilt engine, and using that reading as a baseline reference, retest every few hours until you get a measurable loss of compression from the original baseline.

Now with that said, I would consider 150 psi to be quite low for a KX250. Assuming an accurate gauge, I would expect a reading of no less than 175 psi and as high as 220 psi for a healthy engine.

Just because the former owner put a new piston and rings in does not mean that everything was in spec. The piston-to-cylinder clearance could be close to or out of spec, as could the ring end-gap. The rings could already be worn. The lower rod bearing could be worn. Even a faulty power valve that does not close completely can give a low compression reading.

And guessing at run-time hours is very often wildly inaccurate.

Unless the engine has mechanical issues, hot or cold testing will make very little difference.

Comparing readings between different bikes and different gauges is useless. Production tolerances and wear will cause different bikes to read differently, and every gauge will give a different reading. The only way a compression test is really useful is to test a new or freshly rebuilt engine, and using that reading as a baseline reference, retest every few hours until you get a measurable loss of compression from the original baseline.

Now with that said, I would consider 150 psi to be quite low for a KX250. Assuming an accurate gauge, I would expect a reading of no less than 175 psi and as high as 220 psi for a healthy engine.

Just because the former owner put a new piston and rings in does not mean that everything was in spec. The piston-to-cylinder clearance could be close to or out of spec, as could the ring end-gap. The rings could already be worn. The lower rod bearing could be worn. Even a faulty power valve that does not close completely can give a low compression reading.

And guessing at run-time hours is very often wildly inaccurate.

Unless the engine has mechanical issues, hot or cold testing will make very little difference.

good info. i was planning on a top end rebuild for the winter anyways so ill test after everything is back together and has had a full look over.

I have never rebuilt a two stroke so it will be a good learning period for me.

thanks for the info guys.

service manual is already ordered.

Well I did another compression test tonight. had everything together... exhaust on, air box/filter on and warmed bike up for a few min beforehand.

compression after 6 kicks @ WOT gave a reading of 190psi. well over the 150psi reading from the cold test. plus i think I got some more solid kicks in during this test. my buddy kicked it during last test and said he thought I was getting more complete full kicks than he was, so that could cause a difference in testing.

took it out for an extended spin and she seems stout and full of power.

Now you have a good baseline cold and hot! :excuseme:

I have a CLymer service manual and it says the compression reading must be between 114-176 PSI

I have a CLymer service manual and it says the compression reading must be between 114-176 PSI
The low number is simply the minimum compression that will allow the engine to still run. If your 250 two-stroke MX bike only has 114 psi of compression, it's WAY beyond worn out.

The high number is, in my opinion, just stupid to even list. There are just way too many variables to say that a healthy engine will read "X" amount. Just as an example, my KX 250, at sea level, tests at 195-205 psi when fresh.

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