Jump to content

Recommended Posts

Hey guys, I bought a new project bike off of my brother. I will give a bit of history before I explain what is wrong with it. It first blew up do to an air filter not being put on right. The dirt got in there and did its quick job. We rebuilt it and also changed the air boot going from the carb to the engine because it was starting to crack. 

On the second most recent blow up, he started it up one day and it sounded like ting ting ting ting. He shut it off. When I got there, I started it so I could listen and almost immediately it died. I ripped it apart, and the piston is scored. Surprisingly the head looks fine, cylinder looks surprisingly good (can still see the crosshatch from the last rebuild), the reeds are ok, and I just don't know about the crankshaft. I have rebuilt a CR before that had a bad crankshaft, and I found it very hard to feel any up and down play until I removed it. It defiantly has a LOT of side to side play. The only other thing I noticed was that there was no liquid gasket in-between the reed cage and the air boot. Also the 4 bolts were too loose for my liking. 

From what I can see, I narrowed it down to one of two things:

1. The crankshaft lost its bearings and scored the piston (that theory doesn't really go along with the fact that usually the top of the piston and head look mashed because of all the bearings)

2. It had an air leak in between the air boot and reeds

 

If anyone has any theory or questions please let me know. 

Share this post


Link to post
Share on other sites

What does your jetting look like? Fresh fuel? dirty tank? Lots of variables to explore. There shouldn't be any gasket sealant between the reed cage as it is soft and seals against the soft aluminum of the engine case, unless it is falling apart. Have you checked all the parts connected to the intake for flaking or deterioration?

Share this post


Link to post
Share on other sites

On the first failure (air filter not sealing), was the bottom end inspected/rebuilt as well ?

If this was a pre-owned bike with no past maintenance history, the 1st fail should of been the time for a complete top to bottom rebuild.

 

 

Share this post


Link to post
Share on other sites

Fuel is one thing I am still going to explore. As far as jetting is concerned, we never touched it. There is a gasket between the reeds and aluminum. That is a good idea to check the air filter boot!

On the first failure we looked at the bottom end and it looked fine. Now I don't want to get confusing, but the previous owner said that he did a compleat rebuild just before my brother bought it. I will poast pictures of the crank and piston asap.

-Maybe all that dirt ruined the bottom end. The only problem with that theory is that he rode it a year after the fist blow up.

Thanks for the fast responses guys. Much appreciated!

Share this post


Link to post
Share on other sites

How many hours are you getting on the motor between rebuilds? A 125 is made to run extremely high rpms. If you race one hard motocross or hare scramble and ride it like a 125 is made to run (screaming) you will be rebuilding atleast the whole top end every 10-15hrs or so. If your to heavy for a 125 it will be sooner than that. That's why I got rid of my sons when he got up to about 175lbs he could explode one about every 1-2 motocross races.

Share this post


Link to post
Share on other sites

You're whole problem is when your top end goes metal is in bottom end.  Without splitting and cleaning it will keep happening I bet. Plus no air filter.  Everything goes totge crank first including dirt from no filter.  Do a bottom end on it and then will last:thumbsup:

  • Like 3

Share this post


Link to post
Share on other sites

Its time for a complete rebuild, top and bottom end. Spend no more time on the Internet, get some wrenches and pull the motor, split the cases and replace the parts and seals.

  • Like 2

Share this post


Link to post
Share on other sites

I would ask, when you ride, do you start the bike and immediately take off, ripping it up? Or do you give the engine time to come up to temperature first.

Share this post


Link to post
Share on other sites

When my brother would ride, he would let it idle for a short time and just ride around slowly. Then after a while ride normal.
I've felt his cylinder after a ride and it wouldn't even be hot. Guess that's ok?

Share this post


Link to post
Share on other sites

 

1 hour ago, CR125B said:

When my brother would ride, he would let it idle for a short time and just ride around slowly. Then after a while ride normal.
I've felt his cylinder after a ride and it wouldn't even be hot. Guess that's ok?

Just want you to be aware that heat expansion of the piston and cylinder happen at different rates after starting. Immediately ripping it up as soon as you get the engine started can lead to cylinder and piston scoring. Piston expansion happens faster than cylinder, so with the little clearance it has initially, you can imagine it gets pretty tight in there. Some have even experienced seizure.

Edited by Trailryder42

Share this post


Link to post
Share on other sites

When you get new piston make sure exuast bridge lubrication holes are drilled in it. That piston was never drilled.  Wiesco may come pre drilled :thumbsup:

Share this post


Link to post
Share on other sites

Got her rebuilt! New everything. It runs mint now. I had a little bit of trouble in the transmission because I somehow had put an extra washer in there. It took me a total of 12 hours. Thanks for all the help and ideas!

 

  • Like 4

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:


×