YZ80 / CR125 Build

Well, not a build exactly. The bike has already been built in a backyard redneck way, but it was free on craigslist so I couldn't pass it up.

It has what I assume to be a YZ80 chassis (Can anyone check or direct me to somewhere I can check the vin number in the pic to see what year a model the frame is?) and a 1983CR124 motor grafted into it (going off of what is written in pain pen on the pipe lol, again, can anyone confirm this?). The motor doesn't run, but one of my buddies (who doesn't ride) offered to throw a few hundred at it to see if we can get it running to get him into the sport and see if he likes it enough to buy his own bike.

I just got the bike home tonight. I am going to start off by pulling the carb and giving it a thorough cleaning, checking (and most likely replacing) the piston and rings and flushing the motor to clean out debris and what not. For this would diesel work or is there something better? Then, if all looks to be in place put some fresh fluids in along with fresh gas and a new plug and cross my fingers.

This build/repair will be very very basic. Wheel bearings and steering stem bearings are shot, but will not be replaced, tires are worn and will most likely stay as is. The goal is to just get it running to give my friend some time putting on easy trails and to play with around the house.

Things that will probably be replaced/fixed are:

piston and rings

trans fluid

radiator fluid


spark plug

front brake master cylinder

I'll add to this list as it grows or until the parts list price surpasses my friends budget and we abandon it.

The build will be slow because it will most likely take place just one night a week when the both of us have a few hours to devote to it at a time in between parts arriving from the postman.











Edited by woods-rider

It'll be interesting whether or not you can keep it cool with only 1 radiator.

It'll be interesting whether or not you can keep it cool with only 1 radiator.

I have the same concern, but this bike will be ridden extremely easy by a beginner so that should help. I plan on getting some of those thermometer sticker things to put on the side of the cylinder to keep track of the temp. If necessary we will cross that bridge when we get there.

what about just go faster to avoid over heating?longer or looped hoses could provide extra capacity and a bit of room for expansion?

rad cap with a higher pressure rating?

isn't that old 125 motor equipt with the famous magniesium rotting water pump housing?

Where is the tank going to go?

Where is the tank going to go?

The tank seems to fit. It looks like they made some ghetto spacer piece to hold it up a bit higher. Haven't looked too closely at that part yet though.

what about just go faster to avoid over heating?longer or looped hoses could provide extra capacity and a bit of room for expansion?

rad cap with a higher pressure rating?

isn't that old 125 motor equipt with the famous magniesium rotting water pump housing?

Going faster doesn't work well for a beginner who has never ridden before on slimy muddy trails lol.

The coolant hoses are fairly long now, but that is a great idea. I can always add more and zip tie it along the frame in protected places to see if that helps.

I have no idea if this motor has any famous problems or not. To be honest, this is my first two stroke :jawdrop:. I have never worked on a two stroke either so this is a learning experience, but so far it has been a cake walk compared to the thumpers I have had. In less than an hour I was able to remove the exhaust, head, carb, ignition cover, and reed valve. On my WR it would probably take me an hour just to get the head off :banghead: .

Here's where i am at after removing all the above parts.

-No spark. I will test the stator tomorrow and work my way out to the plug. Hopefully it's something simple.

-About a cup of water in the lowest point in the pipe, most likely there is more in the cases also.

-The carb was a little gummed up, but not that bad. A thorough cleaning and it should be good to go.

-The top of the piston looks good to me, but as I said before, I have little knowledge of smokers so any insight is appreciated.

This stator looks OLD... it is older than I am!


Head looks to be in decent shape to me. What do you guys think?


I don't see any major damage on the cylinder walls. Will know more once I get the piston out.


Coolant still in the water passages which gives me hope that it's not totally corroded inside.


Intake port.


Reed valves seem to be in good shape too.


Main goals for tomorrow are to get spark and clean the carb. If we don't have to chase electrical gremlins for too long before getting spark we may flush the motor with diesel also.

Edited by woods-rider

So I just talked to the guy who i got it from (didn't see him when I picked it up, it was just outside and had the address on CL). He said that the CDI is bad which would explain why I am not getting spark.

Anyone know what years CR125 CDIs are compatible with the 1983? Or will one from another make/model work?

Worked on the bike for a bit tonight.

Determined that the stator is good.

Took the front brake caliper off because it was dragging badly making the bike hard to push. We will look into fixing that later.

Took the right side engine cover/clutch cover off. The clutch basket has lots of grooving going on so we will file those down. Plates and fibers look good. The oil in there (if you want to call it that) was so thick and dark grey it almost looks like a watery clay material. We will clean this out with diesel.

The cooling system/impeller is just caked in what I have to assume is old dried out coolant? Its super thick and dried up most of the way. It's gritty and slimy at the same time. We cleaned it up best we could with flat blade screw drivers, then our OOPS of the night happened. We were trying to get the impeller off, but to do that we had to find a way to keep the back side of the shaft from spinning. We tried holding the plastic impeller gear (number 5 in the diagram) in place while using an impact driver on the nut... that didn't work out so well and was a stupid move. The plastic gear broke and there doesn't seem to be any on ebay. I think that years 80-84 should fit, but I couldn't find any and they aren't available from Honda anymore. Anyone know where I could get one?


There also seems to be a bit too much play in the clutch hub. I think it may be the bearings in it, but they didn't look bad. I'll look into that more closely once we source a water impeller gear (if we can even find one).

Any help finding a water impeller gear for an 83 CR125 would be greatly appreciated. Would be willing to buy the whole right side case cover also if it came with the impeller since ours has a hole in it at the water pump.

Edited by woods-rider

Notched clutch basket.


Inside right half of motor.


Here's the water impeller shaft plastic gear before we broke it. This is the part we need and can't find.


Here's what the inside of the impeller housing looked like AFTER taking many spoonfulls of this junk out!


I dont really think the one rad is a big issue, as its longer than a standard rad and a 125 has larger capacity coolant passages. Good luck on any water pump/case parts. They corrode and are hard to find. Probably the biggest issue facing the rebuilders of these old Hondas is the water pump case isssue

Good luck on any water pump/case parts. They corrode and are hard to find. Probably the biggest issue facing the rebuilders of these old Hondas is the water pump case isssue

Damn, that's not what I want to hear. Everything looked fine (aside from being disgustingly dirty) until we broke that plastic gear. Right now all we need that we know of is this impeller gear and a CDI. The CDI seems to be relatively easy to find so we aren't going to spend any money on this project until we have sourced that impeller gear.

You'll hate yourself more now that you broke that impeller gear it's no longer avil and it's unique to that year you have to source a used one or try and make a new plastic one yourself. there was some guy outta the uk i think that was remaking the covers in aluminum but it would cost you more then that bike is worth.

You'll hate yourself more now that you broke that impeller gear it's no longer avil and it's unique to that year you have to source a used one or try and make a new plastic one yourself. there was some guy outta the uk i think that was remaking the covers in aluminum but it would cost you more then that bike is worth.

Yeah, I am finding it very hard to source. I found a guy in Idaho through craigslist that supposedly has a non-running 83 that he is parting out, but he has only returned one of my emails so far saying that he will check to see if he still has that part.

I do have access to a CNC milling machine so we may try our hand at making a sprocket out of some type of plastic or aluminum. I am a mechanical engineer and use all sorts of CAD software daily, but I am not sure what type of code my friends CNC machine needs. I'll give the Idaho guy a few more days to respond before I get serious about making my own.

If you have to make your own gear there is a reason why they are made from nylon not metal same with oil pump gears sorry i am forgetting why i think has to do with it if fails it won't break the shaft etc

If you have to make your own gear there is a reason why they are made from nylon not metal same with oil pump gears sorry i am forgetting why i think has to do with it if fails it won't break the shaft etc

Yeah, but we stripped off all the knurling from the shaft that kept that plastic gear from spinning. I was thinking aluminum so we could do a press fit and it would still be softer than the mating steel gear.

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