Vintage Dirt Bike Q & A

Hi guys,
Looking for a little help on a rebuild that I'm doing for the 2nd time.What was a 1976 MT125 is now a CR125 for the most part.Replacement parts are all from a 1975 CR125. Cylinder, head, crank, clutch, CDI,coil,stator, flywheel, expansion chamber.Went with a NOS Wiseco single ring piston. Replaced all bearings and seals,did not rebuild the crank as it seemed OK and the seller said it was “good to go”. Oh, I should mention the carburetor is a spare I had from a 1980 CR125, Keihin PE06 model.
Upon completion, the bike started fairly easy,idled fine with some adj. And revved up smoothly.The only problem was running rich from ½ to full throttle.So, I started leaning it out on the main jet but never really got it dialed in. I was down to a #138 main when the trouble started.It was bogging out and I noticed a ton of air bubbles running through the gas line,the bike then died and the cylinder filled up with fuel to the point of Hydraulic lock.I pulled the plug and was able to free it up by rolling the bike back and forth in gear.At this point I could hear a bad noise coming from the top end, a broken ring.Damages include a cylinder that needs honing,lower end rod bearing is toast (turned blue from heat), and the main seals and bearings will need to be replaced.
I have since purchased a new single ring piston, seals and bearings and have a new crank rebuild.My question is this, what caused the initial destruction and how do I avoid it this time around?? I don't want to duplicate the first rebuild.
(I am concerned about the ring gap location, it lines up right in the middle of a small opening right above the intake ports).
Thanks,
Gregg

_________________
1974 CR250m
1975 MT125,CR125
1978 CR125
1980 CR125
1982 CR250
1979 RM400
1979 IT175
1973 XR75

Well I had to do mine three times before I got it right. Everyone on ebay says there cranks are good to go but they never test them for tru just up and down play in the bottom bearing and if thats good they say its good to go. An untru crank will eat seals every time. I dont know why you would need to worry about the ring gap location arnt you using a cr125 crank and cylinder and piston if so the ring gap should be where its supposed to be. Also I would check and see if your carbs working right it shouldnt dump that much fuel into the engine.

Gregg,

The bike looks good! There's a few small frame differences that a CR owner might spot, but otherwise it looks like a '74 CR125M.

It could be the single ring piston was meant for the '74 cylinder, which has no boost port in the rear of the cylinder. Rather than take the chance of destroying another rare single-ring piston, I would use an OEM Honda CR125M 2-ring piston; I've never had a problem with them.

Just for what it's worth... if I'm using an OEM-jetted carb for an application similar to the one it was jetted for, I start to get very nervous when I have to drop any jet more than 3 sizes. From what I can tell from the 1980 CR125R parts fiche, you dropped your main jet over 12 sizes. If you were getting rich operation from 1/2 to full throttle, the first thing I would have done would have been to drop the needle a notch or two. Running the bike this lean would almost certainly have caused it to overheat.

The only thing I could think of that would cause the air bubbles and massive fuel leakage would be a pressurization of the intake tract, which could have forced air into the fuel line through the float valve, damaging it in the process. Other than a broken piston skirt or cracked cylinder, I don't know how this could have happened.

I would do a very thorough visual inspection of the cylinder, crank, cases, and every part involved in the rebuild. If it were me I would also have the cylinder Magnafluxed for cracks, but that may not be feasible for you.

If you're worried about it happening again, you might consider having a shop with a good rep do the work; that way, if anything goes wrong, at least you'll have the possibility of a remedy.

Ray

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

Ray,
Thanks for the excellent info,
I'm looking for a different piston and have the carb back to book specs. I did pull the float valve (pressed in O-ring type) and found the o-ring was dry rotted. The o-ring has been replaced.I suspect the fuel level was surging too high causing the flooding. I did try running the bike with the fuel shut off at the petcock, and it came too life as the float level ran down.That should have been an obvious area to correct instead of messing with the jets.I'm learning.
Hopefully the fuel level and incorrect ring location were the causes of the problems. I can't see any obvious cracks or damage causing anything to leak,wouldn't a case or cylinder crack cause an air leak which would create a lean situation?
If you have any tips for finding a piston and rings that would be a bonus,
Thanks again,
Gregg

_________________
1974 CR250m
1975 MT125,CR125
1978 CR125
1980 CR125
1982 CR250
1979 RM400
1979 IT175
1973 XR75

I had a similar problem with a Keihin PE06 model on a 1979 CR125R. This carb was a 32mm where the 1980 has a 34mm. It turned out to be the check valve was not working properly and it was flooding the bowl. I eventually gave up on it and used a Mikuni 32MM round slide carb after trying to use a Mikuni 34MM round slide. One of these carbs will most likely work for you, I would think the 32MM. The 34MM was a little too much for a stock motor.

note: I had to shave down the air intake bell of the 32MM carb to make it fit inbetween the air filter and insulator boot. This was not the case with the 34MM mikuni. The 32mm carb is longer than the 34mm for some reason. It was easy, just used a hack saw and then filed it down smooth.

This is a good starting point for jetting on the 32mm Mikuni:
300 or 290 main jet (I settled with the 290 at sea level)
35 pilot jet
droped the needle one knotch from center to lean out the first 1/4 throttle.

good luck
nola_fiddy

Gregg,

OK, that makes sense - with a high float level and a way-too-lean main jet, the bike might still run a little rich below full throttle (masking the lean main jet) and then really lean wide open. I never could understand why every jetting-related thing I ever read always recommended setting the float level before anything else, but these days I do. I would also check to make sure that the needle and seat are sealing well, as these are famous for wearing out (replacements can be hard to find, but they're out there).

The bubbles in the fuel line may have been caused by the O-ring disintegrating and the float bowl/carb/motor filling with fuel. But yes - case cracks, case gaskets, main bearing seals, and leaks of all stripes can cause leanness. Unbeknownst to me, my CR's left main bearing cage fell apart, the sloppy crank destroyed the seal, and the bike ran leaner and leaner - I compensated by jetting richer, but I never could get back the power lost from low primary (crankcase) compression. Luckily I discovered it before the cage bits did any damage.

Not only does Wiseco make replacement pistons up to 2.0mm oversize (339M05600, etc.), but apparently Honda still makes the standard and .25mm oversize pistons, .25/.5/1.0mm oversize rings, piston pin, bearing, etc. I don't know about standard ring sets - you might have to check eBay. If you go the Wiseco route, you might want to call them to double-check first (ask them if they provide full piston sets w/pin/bearing/etc., or just single pistons as the Web site seems to indicate).

Believe it or not, .25mm OS pistons and rings fit perfectly and ran great in my CR, which I bought new and never bored out. I guess it was just bored too big from the factory.

Ray

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

nf,

Thanks for the CR125 carb size info! I've got 28, 30, 34, and 36mm Keihins, but the size I thought would work best with my modded MR175 motor would be a 32mm. I was sure some year of CR125 used a 32mm and tried hard to find out which, but never managed to do so.

Ray

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

Well, I have found the main source of my trouble. The "old style" single ring Wiseco piston all the ebay guys are claiming work on a 1975 CR125 head are WRONG! This piston (330 p4) has the ring gap at the 6 O'clock position which puts it right in line with a small port right above the main intakes. Thus the lock up, ring breakage etc.
SOOOOOO, I bought a 57mm NOS Honda piston and rings. I should have it running by this weekend.
Wish me luck!!!! I'll post the carb results then.
(Here's a long shot, but I have a Mikuni 34mm flat slide carb from a "87 YZ125 that fits PERFECT, maybe that would be a carb option???????

_________________
1974 CR250m
1975 MT125,CR125
1978 CR125
1980 CR125
1982 CR250
1979 RM400
1979 IT175
1973 XR75

If you are running rich from 1/2 to full, spend more time on experimenting with needles and needle setting. You aren't going to be full on the main jet until after 3/4 throttle.

Like mentioned by others - I'd be very careful about the jetting - go back to the stock suggested main - and try it again now that you have the correct piston.

It takes a lot to totally fry the big end bearing. Usually you'll stick a piston first (seize). I'm thinking going so lean caused the heat that caused the crank seizure.

With that said... try stock jetting - try doing a compression test if you can before-hand... that way you can check it after riding it the first couple times and make sure the compression is staying up. Plus as long as it stays together, this will give you a good idea of when to change your rings in the future.

But be careful and make sure you're running decent premix - not lawnboy 100:1 oil or anything like that. People have varying beliefs on oil. Many top tuners like higher numbers like 40:1 and 50:1 numbers. Some dyno's show that 20:1 causes more horsepower... me, I stick with the 32:1 or 40:1 with quality oil such as Maxima or Bel Ray. I've never had a problem with Maxima Super M.

Good luck and make sure you're not running too hot of a spark plug either.

Mikeee P

_________________
The Race Isn't Over Until YOU Cross The Finish Line!

NOS piston installed, still ran rich with the 1980 CR Keihin carb. I finally popped for a stock 1975 CR125 carb and things got way better.Stock jetting,but now it seemed to have a lean condition and I had a hard time dialing in the idle circuit.I finally found the culprit, a bad air seal on the intake boot.Replaced the intake and BINGO!!
Were back in the races.
Lesson learned, start from stock and work from there. It was a long expensive lesson, but I did come away from it with a better overall understanding of how these vintage strokers work and how fussy they can be.

Thanks guys for all the info,
rock ON!!

_________________
1974 CR250m
1975 MT125,CR125
1978 CR125
1980 CR125
1982 CR250
1979 RM400
1979 IT175
1973 XR75

Remember also that Keihin and Mikuni don't use the same numbers on their jets.

So a 320 in a Mikuni might end up being a 165 in a Keihin... I forget (off the top of my head) how the two companies do it. One is how many liters per hour the hole (jet) will flow. I think the other does it by hole size possibly? Nope, I think that is dynojet that does hole size...

Regardless they aren't even close to the same numbers so don't ever consider that.

Glad you got it fixed an air leak can cause the engine to suck air, run wide open and VOILA~ stick the piston or fry the crank!

Post some photos of that thing on the track!

Mikeee P

_________________
The Race Isn't Over Until YOU Cross The Finish Line!

Yeah, good point on the jet numbers. I"ve been with Keihin all along, just not sure why the newer 1980 carb was not a good thing. Maybe because that carb. was not set up for a port intake but for the newer reed intake on the "80 elsinore??Might be worth a new thread in itself...................???
The final tweak will be the sprockets. I have the stock enduro 44t on the back and have dropped from a 14t to a 13t and then a 12t on the front. Real snappy but I'm not grabbin' gears like a CR, and no big wheelies and hole shot speed is not quite there.Once it hits 3rd gear on the pipe 4th and 5th are friggin' flying.The 5sp MT tranny is geared a bit higher than a 6sp CR so some ratio adjustment is needed. Thinking on going to a 49T on the back to gear it down a bit.Or a 51T on the back and and work with the 13T and 14t front sprocket.
Any math geeks in the crowd??
More pics on the way!!

_________________
1974 CR250m
1975 MT125,CR125
1978 CR125
1980 CR125
1982 CR250
1979 RM400
1979 IT175
1973 XR75

If you won't be using your bike on the road and don't need a high top speed, I'd suggest a 15T/50T combination. I'll be putting a 15T/52T on my MR at the next sprocket change, but the MR gearbox has a higher 5th than the MT125.

I changed my MR from the stock 14T/43T to a 13T/46T and it made a big difference in acceleration, but if you're buying new sprockets I wouldn't consider using anything smaller than a 15T at the front. Smaller ones wear out quicker and destroy the swingarm protector (found out the hard way). Sprocket Specialists make front sprockets up to 17T.

Here's my quick analysis of the gearsets that will fit in the MT/CR125 and MR175 cases, for what it's worth:

CR125M: 6-speed: close ratio, very high 1st, low 6th
MT125: 5-speed, semi-close-ratio, "normal" 1st and 5th
MR175 ('77): 5-speed, medium ratio, "normal" 1st, high 5th
MR175 ('75-'76): 5-speed, wide ratio, super-low 1st, high 5th

Ray

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