Told bike had top end, was I lied too?

GeoffM24 · Jan 27, 2003

Ok,
I just bought a 99 RM250.

I wasn't sure when the last rebuild was done so I ran a compression test and got a reading of 185 when it should be 235 - 250 psi.

I pulled the head and there was a decent amount of carbon on there that was black in color.

I pulled the cylinder the piston and everything.

The cylinder looks clean and the cross hatching marks can still be seen.

The power valves are just gross and covered with spooge.

There is no carbon build up on the cylinder walls.

Well, I finally got an email back from the seller and he said that there was only one ride on the new top end!!!!!!!!!!

AGGHHHHH!!!!

Can this be true? Was I lied too?

I have never done a top end before but I had pulled the head on my 88 YZ125 that was NEVER rebuilt and it had much less carbon.

Is it possible to have carbon build up and a low PSI reading on a new top end?

Please advise.

Thanks

Sphinx · Jan 27, 2003

What did the piston look like and what was the sellers definition of a new "top end"? He may have only replaced the rings and might not have used the correct ones or reused the old gaskets etc. There are many reasons why even a new piston and rings etc. would still give you poor compression. Most of them being due to the wrong parts and poor assembly. When in doubt measure everything, bore, piston, rings etc. and make sure you have the right parts.

GeoffM24 · Jan 27, 2003

I'm trying to find out exactly what was replaced right now.

Do aftermarket pistons come with specific markings on them that I can check?

I have read that the rings have markings on them.

There shouldn't be carbon build up after one ride on a new piston should there?

-Geoff

Zenith · Jan 27, 2003

It depends on so many things - what oil and fuel he was running, what the jetting is like, whether just the rings were replaced etc etc. I had practically no carbon build up on my piston after 4-5 months riding, so it's impossible to say.
Surely 250psi is higher then can be expected from a 250?

KrackerRacin · Jan 27, 2003

Originally posted by GeoffM24
Ok,
I ran a compression test and got a reading of 185 when it should be 235 - 250 psi.

I have a cr250 with .20 shave off the head and bord .50 over and i just bairly hit thouse numbers with a fesh motor

GeoffM24 · Jan 27, 2003

I have searched the net for what the P.S.I. *should* be and believe it or not it is very hard to find.

Take this site for example, with all the tens of thousands of posts there is very little said about compression, honing, and the ins and outs of a top end rebuild.

Some places list a 250s reccomended PSI to be 170-230, another lists 235-250, another list 135-200, and another says a fresh 250 should atleast be over 200 PSI.

In this article http://www.tricmotorcycleclub.org/topend.htm the author says his compression was 177 PSI and he road it another season and end up needing to replate the cylinder.

Eric says that you should keep track of what it reads new and then when it gets down by about 20% it is time for a rebuild.

From what I have read your 80cc should be about 150 PSI, your 125cc should be about 190 PSI and your 250cc should be about 235 PSI.

These numbers may be incorrect.

Assuming that my bike should be 235 PSI new then at a 20% loss in compression the top end should be done which would be about 188 PSI which is higher then my bike is right now.

So the current reading of 185 is AT LEAST the low and leads me to suspect the top end was not recently done.

The other thing I'd like to add is some people say the readings are useless if they bike is not warmed up but other sources say nothing about warming the engine prior to testing.

Needless to say it is confusing.

MikeT · Jan 27, 2003

I did the compression on my 96 YZ125 when it was cold, got a reading of 130 psi. Dave and I did it when the bike was hot and we got 165psi. Heat does matter.

GeoffM24 · Jan 27, 2003

Originally posted by MikeT
I did the compression on my 96 YZ125 when it was cold, got a reading of 130 psi. Dave and I did it when the bike was hot and we got 165psi. Heat does matter.

That is very interesting!

I have also read that some people test the PSI and THEN add oil to see if it raises the psi. If it goes up by more than 15 PSI or so the rings are probably worn and the oil is sealing the worn rings to the cylinder wall.

Maybe in your situation you have worn rings (explaining the 130 PSI) and then warming it up led to oil in the cylinder and led to higher readings???

Any experts in the house?

bclapham · Jan 27, 2003

you are all pissing in the wind here, take the thing apart, measure everything and replace parts as neccassery. MX bikes like the RM250 are competition machinery that require constant maintainance, its not worth being cheap on $120 for a complete top end rebuild. I would leave the psi reading for car motor mechanics.

marcusgunby · Jan 27, 2003

Im with bruce-the psi reading is affected by too many variables.One of the biggest variables is the guages-most like me get a cheapo one from a car accessory shop-these are not accurate, mine reads 5psi at zero.Also if your top end is really carboned up it will make combustion chamber volume smaller and so affect psi readings.

Jaybird · Jan 27, 2003

I have to agree with our British friends. Get out your measuring instruments and take readings. Your manual will list what are acceptable ranges. If you don't have a manual, get one. First thing to measure is your ring end gap.
My 125sx reads at 160PSI max. with fresh top end, just after break-in w/20:1
I can manipulate that reading with different ratios of fuel.

GeoffM24 · Jan 27, 2003

Guys I need a little more help.

The piston I just pulled has a +2 on it as well as ART.

I just ordered a std wiseco piston for it.

Did I screw up??

marcusgunby · Jan 27, 2003

It maybe 2mm oversize-measure it roughly to see if its std size or 2mm oversize.

GeoffM24 · Jan 27, 2003

Well I measured it with my calipers which are naturally not metic. Doing a rough conversion it worked out to my 64.345 which is smaller than the stated 64.4

I called midwest action cycles where I ordered my new wiseco and they said 99 RM 250s are not borable and it have to be resleeved to go to +2???

bclapham · Jan 27, 2003

from the wiseco website, the standard bore for the 1999 rm250 is 66.4mm so i think your measurements maybe wrong (the only oversize piston they do is 68.5mm). the usual place to meaure the cylinder is around the bottom where it is widest.

just get a metric ruler and meaure accross the top of the piston, if its around 66.4mm then you should be good to go, since wiseco use 1 size fits all.

GeoffM24 · Feb 1, 2003

The Compression tester worked!!!!

So I get my new piston in and compare it to the ART piston installed in the bike and they used the WRONG piston.

They had installed a flat top piston instead of the required dome piston.

No wonder the compression tester said the compression was low for a new rebuild.

Told bike had top end, was I lied too?

GeoffM24

Sphinx

GeoffM24

Zenith

KrackerRacin

GeoffM24

MikeT

GeoffM24

bclapham

marcusgunby

Jaybird

Apprentice Goon

GeoffM24

marcusgunby

GeoffM24

bclapham

GeoffM24

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Told bike had top end, was I lied too?

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