Vintage Dirt Bike Q & A

Hi all i had some trouble with my cr125. i went riding recently and i was holding the throttle steadily and all of the sudden, my engine dies. it was like someone hit the kill button. i tried kicking it and the compression was gone. I later extracted the engine from the bike and opened the head, then proceeded with taking off the cylinder. The top of the piston looks like a cheese grater. Do you think its possible that one of the rings partly chipped off and got into the cylinder? How could this have instantly turned my dirt bike off?
Thanks !
_tim

well when you pulled the cylinder off were the rings all in one piece or were they broke. there are several things that can go on here. one the piston and bore could be wore out this lets the piston slap around in the cylinder and break parts of the skirt off and or rings. allso you could have got'n into a lean spot and burned the piston. some times it will burn a hole usualy next to the exhaust port. all of these things will cause no compresion. no compression no engine runny. sounds like its time for a bore job and new piston then look at your jetting

Well, when i took the cylinder off, i did not notice anything fall out or anything.The rings were in one piece. I just got the cylinder bored out about .5 mm around 4 rides ago. Would an in-proper break in procedure cause this you think? After i put it all together and started it up for the first time, the throttle cable was stuck in a spot where it kept the needle lifted up half way. it took me a while to figure out that that was the problem. Im not sure if that led to this blow out.

when you first started your engine and the throttle was open half way how long did you let it run that way ? allso when your engine quit, you said you were holding the throttle steady. how long and was it wfo? another thing that happins is when you had the bore job done was the old piston broke? if so if the person that put the engine together didn't clean the bottom end area good. a small piece of the broken old piston got up on the new piston and beat it to death, then shot out the exhaust. could you post a picture of the top of the piston that should tell it all. i'm thinking you just leaned it out that could be caused by a number of things

it ran like that total about 2 minutes all in all. It was among like 9 or 10 different trys though. I had just gotten back on the trail after resting. It was about 1-2 min into the trail.(luckily) "Wfo"? im not sure what you mean by that. And the old piston was just wore out, nothing had broke or fallen loose. One observation i made today when i removed the piston; The connecting rod has play.it can wiggle left and right. Im thinking the lower bearing on the crank to connecting rod is shot. or is that standard to have free play like that? I think one of the rollers on the bearing is the thing that caused this. i think its an old bearing too. i tried uploading pictures but they were too large

WFO= wide f_cking open/ full throttle

ohh hahaha...thats a good one..it wasnt wfo at the time

or Wound Full Out if you need a family friendly explanation.

Taking the bottom end apart may find a failed lower rod bearing. Those bits up top are likely the shattered needle bearing cage. You ought to be able to feel roughness in the lower end bearing as turn spin the crankshaft around by hand. The bearings could be so crammed together inside the rod journal that you don't feel up and down play until you monkey with it a bit. Just be sure so you don't destroy that brand new top end.

and thats a good one too! But anyhow, I am still in the process of taking the lower end apart. I got as far as the clutch basket. I cant sem to get that nut loose. Theres no way to grasp it! Since theres no clutch plates in there anymore, i cant solve it with a penny in between the gears. A buddy of mine said he has a tool that will solve it i suppose. But my 250 dollar top end is destroyed anyhow....=(....Whatever it was that exploded got sucked up into the cylinder and just demolished it...If you would really want to see the details i can send some to your email. This forum website doesnt allow pictures over a certain size and i dont know how to reduce them.

The tool is based on Vise Grips, but with long, thick steel fingers that fold over to and lightly clamp down into the outer grooves of the inner clutch hub. You can make one out of the long fingered vise grips used to hold sheet metal - aka welders clamps. The ones at Harbor Freight work fine. Do some searches on clutch hub tool and you'll see what they look like. I think Motion Pro makes them, and I think Flanders cycle supply at least used to sell them. Or, just hit that nut with a big old air wrench!!!!

There are other special tools you'll need to do the cases and crankshaft right. You might wan to find a repair manual and study it before you go at it if you are unsure of what comes next.

I made a clutch tool. You need an old steel clutch plate for your clutch (not the fiber friction plate). Weld a bar to it so that the bar will hit the kickstarter shaft when the clutch plate is installed on the inner clutch. This will hold the inner part of the clutch while you undo the big nut.

hahah yeah well im going to save money and use homemade tools..

Sounds like you lost the lower rod bearing. 2 of my sons used to work at Crank Works in Tempe, AZ and this is what happens when this happens.

As for other things to do... I use a steering wheel puller to pull the side cases apart. 2 or 3 long bolts into the left side and pressing against the crank pulls the left side case off the right side. Always do this with the right side down. I set it on some wood blocks to keep it more stable. When it seperates put a couple hose clamps on the gear shafts to hold all the gears in place until you are ready to go back together. Otherwise...
Makes the job a breeze and the puller is pretty cheap to purchase at an auto parts store.

Rick