Vintage Dirt Bike Q & A

so I took on the complete rebuild and restore of my 75 cr125, completely rebuilt the bottom end and had the top bored 1.00 over. I got the thing back together and it was running perfect. shut it off and came back and had no compression and now it wont run, sounds like i blew the head gasket or broke a ring, so i disassembled and everything looked great and cant figure out what the deal is. i should note that i had a cooling fin welded back on vie tig but i wouldnt think it would get it hot enough to warp the cylinder surface, i put a straight edge on it and it looked good. any iodeas?

The fin (head? cylinder?) was welded on with the motor apart, or together?

If it were me I would carefully check everything from the piston crown to the spark plug cap - 19 items including the cap and washers.

I used MT125 head gaskets which seal much better than the OEM copper ones, but the head has to be milled to get the compression back up.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

i'll be sure to re check all of that again, the clylinder was off the bike. what do you thing about a leak down test to isolate the compression leakage either at top or bottom? obviously is we have leakage it will show up there right?

If you have very low regular (cylinder) compression I'd concentrate on that for now, especially if the bike ran well before. The primary (crankcase) compression won't really affect cylinder compression at cranking speeds, but it's still important, though. Do you have a crankcase leak-down test setup?

What's your regular compression reading now?

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

How long did you ride it? Does it have a properly oiled air filter in it?

I ran the bike for probably about 20minutes max with an oiled air filter.........my latest thought and please tell me if this makes sense is piston ring gap. to be honest I started this project about a year ago (no deployed to afghanistan)n and remember checking the gaps before installation but cant remeber if I had to file them or not (due to not being in tolerance), my initial feeling is I did slightly (big maybe). However, this was all done just prior to install and before the newly bored and honed cylinder I had done was bolted to the bike. Could i be that due to slight adjusts on the rings and the newly unworn cylinder and the initial break-in that during the maiden voyage the cylinder wore as well as the rings and no i am left with bad rings causing my low compression?

Oh man, I am so glad you had this bike to think of coming back home to. a half done restoration project in the home garage should be mandatory for any moto head going off to battle!!!

These had some issues with cylinder head studs loosening within the cylinder. If you had an air leak up at the cylinder head it could be blowing the compression right out there. Mine got steel insert put into the cylinder when one of the studs loosened up. That helps ensure that they won't loosen up again. Take a look at the gasket surface, if you see scorch marks blowing out in an hour glass shape, then you found the problem.

Good luck and welcome home.

Jon

yes i cant wait to get home and work on it, wanting to get parts ordered and the problem figured out before i get back. hard to do from here i know but my wife has been a huge help, huge learning curve wrench on things for me haha.
I do remember one of the head studs being loose on the cylinder and i never could get the thing overly tight like the rest, as i replaced them all during the engine rebuild. i'm familiar with what the blow by would look like (hour glass shape) you talk about and i didnt see any sign of that, not to say that it wasnt happening though as i replaced the head gasket about 3 times back to back, so it may sound like you're on to something: my questions are:

1.) if the loose stud felt tight but never fully was would it have been noticble? or would it lossen apon compression? Lastly, would the you recommend new steel locking helicoils in the head to remedy this?

2.) what did you think about my above reply about the increase in ring gap? plausible?

For what it's worth, I set the ring gaps on my MR to .008" - .010", and the bike will run (barely) with .200" gaps. Since you have the motor apart, what are your ring gaps now?

My MR cylinder studs were installed with some kind of mega-strong clear locking adhesive that was stronger than one of the studs (snap!). If your cylinder stud threads are shot it's Heli-Coil time, but if they're still OK you might try reinstalling the studs with red Loc-Tite and see if that holds.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

well since i am deployed i'll get my wife on it asap lol! i'll post reply as soon as i can walk her through what needs to happen to measure the gaps. Thanks for all the help!

ok so i convinced my wife to take the gap measurements, with my assistance via skype. The manual calls for ring replacement if it exceeds .0195.......mine measured .019 and you say .008-.010, so it looks like rings are one of my problems even though they were new rings to begin with which scares me a little, i cant remember but maybe i had to file them during the first rebuild and did it too much but i dont recell? anywho, i'm think the rings in conjunction with the head studs are bad...........

can anyone tell me what helicoil set i need to buy to repair these, i believe they are m8?, obviously need a bit and tap as well, but i am thinking of going with locking helicoils or maybe regular with red loctite? your opinions?

At my last MR175 top-end replacement the old-ring gaps were .023" top / .016" bottom, and the motor still had 175+ psi of compression. I relize your bore is smaller and the gaps should be a bit tighter than that, but those gaps alone shouldn't cause total loss of compression.

I'm pretty sure all CR125M head studs use M8x1.25 bottom (cylinder) threads. This kit seems quite reasonable:

http://www.amazon.com/Helicoil-5546-8-M ... B0002SREP4

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

I appreciate the link, I failed to mention mine measured .027 bottom/ .0195 top either way it sounds like we are on the right track. i'll get this helicoil kit ordered and install as soon as I get home, thanks again!

Ring end gap wont effect compression that much. A ring stuck in the piston groove, Broken ring, crankcase seal, head or base gasket will.

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jeff

I'd bet that stud is giving you trouble. I went through the same thing on my MT. Locktite wouldn't hold it. Finally had to heli-coil it. I could get it tight and even torque it, but once I rode it, it would loosen up.