Vintage Dirt Bike Q & A

I have a '76 MT 125 that I'm working on.It has been stripped down for off road only so it won't ever have any real collector value.I bought it for $225 knowing that it had a seal leak, sucking gearbox oil into the cylinder.I'm splitting the cases this weekend and will know for sure if it's seals or worse,the cases.The top end is at .080" over, good shape but on it's last leg.
Soooooooo,here are my questions: Is it possible to drop a CR top end,carb,pipe on it for added performance? If so, what years would bolt onto my current lower end? Or, would a complete CR engine from that era bolt up to the current frame??
The rest of the bike is in good shape and I would really love to bring this cool bike back to life!!
Thanks,


[quote]Being too old for this crap is not an option..

The 74 and 75 CR top end will bolt right on. The 76-78 cylinder liner is too thick to fit into the MT cases. For a proper package use the CR carb and pipe too.
Use the CR CDI ignition if you can find one. It bolts right on.
I recommend a complete CR engine rather than just the top end so you get the narrow ratio 6 speed transmission. The MT's 5 speed gear ratios are too wide for the narrow CR powerband. You may have to modify the airbox to carb boot to get it to fit the CR carb. I think you will need a CR intake manifold too because of the bigger carb size. You will have to weld on a mounting bracket for the pipe behind the right footpeg for the CR down pipe to attach. Sounds like a big project to keep you busy all winter.

I finally have the CR pipe,jug and carb. How does the CR CDI ignition and stator fit with a MT crank and flywheel?

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

Remove the MT flywheel. Remove the MT stator. Remove the MT coil.
Bolt on the CR rotor. Bolt on the CR stator.
Install the CR coil and CR CDI unit. You will need to add some mounting tabs for these as they fit differently under the tank than the MT coil.
You will need a CR wiring diagram to get the connections right (ground, killswitch, etc.).
Post again if you need some of these parts or the wiring diagram. I'm sure that someone out in "Elsinore-land" can help out.

Check ebay items:
250319582577 CDI
380078409081 stator (with wires cut off)
250319584554 coil
260310336768 intake manifold

Thanks a ton for all the help, I have my eye on those ebay items. I'll post an update as things come together,

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

i would also swap in the 6 speed trans from the CR125.that is me though.

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(2) 1968 DT1 1970 RT1M,1970 DT1C 1971 RT1B Enduro 1974 MT250 1974 DT250A (2) 1975 DT400B 1974 DT100,1978 DT175
1975 CR250M1 1979 CR250R 1979 Kawasaki KZ400H1 Jesus is Lord!!

Well, as things go, it seems the rod bearing on the crank is bad so that's going to slow things up a bit.I'm finding out that a replacement rod is almost impossible to find and is BIG $$$$ if you do. Does anyone have specs on the ID for the rod or any input as to telling if the rod needs to be replaced?I'd love to use the old one if I can.
Elsinore.com has a replacement for a 73-78 cr125 at $89 but they say the journals need to be "jig bored" to accept the pin.
This sucks as I know the bike will never be close to being worth what I'm about to pour into it.

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

These old bikes are rebuilt for the love of it, not for profit. It takes a lot of money to keep them running, especially the little 125s. You are unlikely to get your money back but you get the enjoyment of working on it, the satisfaction of knowing you got it running yourself and the pleasure of riding on a bike that brings you back to the old days when bikes (and life) were simpler.

Check ebay item number 370108914942 for a used 75 CR crank. This will work with your engine.
Used cranks are hit and miss. Sometimes they are okay. Sometimes the big end bearing is shot.
Usually you would replace the main bearings (don't use Chinese) and the main seals when you do the crank.
The CR crank uses a high strength bolt to hold the CR rotor on. Your MT crank uses a nut. You will need the bolt.

I know, that's why I'm into them as much as I am.Will take a shot at the CR crank on Ebay, one question.My manuals talk about replacing the big end bearing when needed but re-using the old rod if it's okay.All the rebuild guys are telling me I need a new conrod.Can I tell if the old part will be usable??
Thanks again for the info and the much needed b!tch slap to keep me going.

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

I'm no expert but the way I think of it is to compare to a normal ball bearing. On a normal ball bearing when you replace it you are replacing 3 components, the balls, the inner race and the outer race. On needle bearings such as your rod bearing you are replacing only one of these 3 components; the needles. The "inner race" is the crank pin and the outer race is the rod. Normally all 3 are replaced at the same time when you buy a "rod kit". A rod kit consists of rod, needle bearing and crank pin (plus thrust washers).
If I were you and I won the used crank, before you start spinning the bearing by hand, I would flush out the bearing with solvent the best you can to clean it out, lube it with oil and check for excessive big end bearing play. If it seems okay just use it. A mechanic can check it for you or you can check it yourself. There is a technique you can use to check for excessive play by whacking it on top with your hand and listening.
Once you get your bike running always use a good quality pre-mix oil because that is what is lubricating this bearing.

Elsinore4ever;
I sent you a pm. Not sure if it got through. Let me know.

All ready for the rebuild, splitting the cases today.
Just one more question,I have my eye on a complete set of trans shafts and gears, (main and lay) along with a shift drum from a 74/75 CR125.
Is this all I'll need to convert to a 6sp elsie transmission?

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

One thing - the '74 CR125M mainshaft has a larger diameter slotted shaft section through the clutch hub, along with a different hub. '75 and later CR125M/all MT125/all MR175 mainshafts share the same smaller-diameter clutch section. You didn't mention the shift forks, I think you'll need the CR ones. My notes say the shift fork shafts, bearings, washers & circlips are the same for the CR125M, MT125, and MR175. I'm pretty sure the shifter shaft and detent setup is the same for all too.

I really don't know for sure, but I read in the December '75 issue of Cycle World that machining is required to put the CR125M transmission in an MR175, but since then I've corresponded with two people who've done it without any machining at all, and heard of at least two others online who've done it as well. The reason I mention this is that I believe the article was actually referring to putting the CR tranny into an MT125, which might well involve machining since the MT cases have provisions for an oil pump and a tachometer drive, both near the countershaft. A quick measurement of the cases should clear this right 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