CR125 Very Low Compression

J

JMOORE330

PR Addict
Ive got a 2000 CR125 with a brand new top end kit. Ive measured alll nessacary things and torqued everything to spec. BUT im only getting 40-45psi when i compression test it? Im thinking it has to be crank seals but i dont have all the stuff to do a proper leak down test on it. So should i go ahead and just replace them? OR should i looks somewhere else such as reed cage? I havent found any sort of info on about how much compression i would have if either the crank seals or reeds were bad. And personally ive never seen reeds get that bad. Any info would be great, i dont wanna just start throwing parts at it. Thanks guys
 
I'm not a certified moto-mechanic but I've learned a few things.

When you kick it over during the compression test are you holding the throttle wide open? You should be.

Your compression test only reveals the condition of cylinder, bore, piston, and head. Typical pressure (as listed our Honda Cr125 manual) should be 140-190psi at sea level.

When the top end was tore down was cylinder bore measurements taken with a bore gauge?
A new piston will not totally fix low compression on a worn out cylinder.
 
Did you check ring end gap on the new stuff? It is possible to get a brand new kit with a bad ring.
 
Yes i measured ring gap, it wasnt runnin real well then i turned it off and it wouldnt fire again. Im still thinkin crank seals. How much compression would a bike typically lose with the crank seals bad?
 
JMOORE330 said:
Yes i measured ring gap, it wasnt runnin real well then i turned it off and it wouldnt fire again. Im still thinkin crank seals. How much compression would a bike typically lose with the crank seals bad?
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This is going to be one of those, (BANG HEAD) moments... I have had plenty myself trust me.

**** The crank shaft seals will have no effect on the compression.***

**** COME on PPL *** slipping hear...

Bad seals will effect the aspiration of the engine and intake fuel mixture, similar to that of a bad intake gasket or leak on the intake side past the carburetor.

The compression is measuring the PSI of volume on the top of the piston between the head and piston. after the piston has passed the intake ports. bad seals will not effect the psi compressed up in the head, It may not be the correct air fuel mixture being compressed, but technically you could remove the crank seals completely and still have perfect compression in the top end.

NOTE: This doesn't mean you do not have bad seals it just means crank seals have nothing to do with compression.

Check your gauge and fittings (double check them on a known reading or good engine).

If all is good inspect every part for damage (bad gasket cracked - head, cylinder, piston)
 
Also just for consideration measure the horizontal width of the ring in several places around the ring. I once installed a new piston and ring. Bike ran great for a few hours and then started to run poorly. Took it apart and inspected everything except the horizontal ring width. I was banging my head for a few weeks till Parma Dave uncovered this. The ring was difference widths at difference locations. It took Dave about 15 minutes to figure it out what it took me 3 weeks to give up on lol.
 
MX724 said:
Also just for consideration measure the horizontal width of the ring in several places around the ring. I once installed a new piston and ring. Bike ran great for a few hours and then started to run poorly. Took it apart and inspected everything except the horizontal ring width. I was banging my head for a few weeks till Parma Dave uncovered this. The ring was difference widths at difference locations. It took Dave about 15 minutes to figure it out what it took me 3 weeks to give up on lol.
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Could your cylinder caused this by being out of round?
And a new ring corrected it?
 
In this case the cylinder was fine it was just the ring. It only had about 1 to 2 hrs on it tops when the problem was discovered. This was a few years ago so. We verified the cylinder was within spec. and ordered a new ring. However you raise a valid point. Its' always wise to rule out the cause, not just find the problem and fix it, but understand why it happened. If you don't you may end up right back where you started!
 
Noticed in your OP you said leak down test.
There really is no reason to perform a leak down test on a 2 stroke or a wet dry comparison on a 2 stroke really.
The reason I say this is a 2 stroke either has compression or it doesn't.
The purpose of a leak down test wet dry comparison is a way to determine if the low compression is from leaking valves or bad sealing rings (was going to explain this but just Google it).
2 strokes don't have valves (Excluding the power port that some interpret or describe as a power valve)... SOOOO! no reason to do all this dry / wet leak down testing yada yada stuff!!! With a 2 stroke...Compression test (pass / fail) and your done.
 
This bike is kickin my ass. Replaced reeds, crank seals, and still nothin. Im gonna take top end apart and meticulously measure everything. It has a namura? Piston in it.... Any idea which way the rings sposed to go? Writing upwards or downwards? No dot on it like most?
 
One of two things, the plating is at the point where new rings are just not sealing properly. Next it could be the cylinder head is warped or for whatever reason not sealing properly. You need to take it apart and take a good look at everything.
 
If your using the Honda manual as your guide for installation of a aftermarket piston that is going to confuse matters. I believe the Honda manual has you install the OEM Honda piston with the markings towards the intake. All other aftermarket pistons, the markings should go to the exhaust side. Also the top compression ring will have a silver finish on the outer diameter. I believe the numbers or markings on the ring should be facing up.​
 
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