Vintage Dirt Bike Q & A

So, I picked up a '76 MT125. The story is that it was a good running bike until the throttle cable broke. He let it sit a while, bought a new cable, but never got around to putting it on. So I ended up with a titled bike from the original owner, a brand new throttle cable, and he even went and bought a new battery for me. It has sat long enough that the tank has rusted a bit, but I'm thinking of having it boiled out and painted or otherwise cleaned out somehow.

Anyhow, I'd like to know if I should bag the injection system and pre- mix the gas or run the oil pump. Were the pumps dependable? What would it take to bypass?


Here are some pix





It fits nicely between my '69 CT90 and '88 XR600R...






Thanks,
Dave

yes leave the oil pump on. make sure the oil lines are not cracked and you'll be fine.

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jeff

I ported my mt250 and lost the oil pump in the process. I would rather have it than not but its not a huge deal to have some oil and a ratio right cup with you to refuel at the gas station if the pumps removed.

I'd leave the pump on to. A full tank of injector oil will last several fill up's of the gas tank.

As far as the tank is concerned,no need in having to repaint cause you dipped the whole thing. Most dealers sell rust removers you can put in the tank that will take it all out. I personally use KR002 by Kreme. It works excellent just be sure to follow the directions.

I'm super mega stoked!!!! I hooked up a fuel bottle to the inlet hose, gave her 2 kicks and she runs!!!!!! Now I gotta put the new throttle cable on. What is the oil pump cable adjustment procedure?

I don't have the manual in front of me, but pull the pump cover off and you should see a couple of marks on the face of the pump. Adjust the cable length till the 2 marks line up.

I got it running pretty well now. This is quickly becoming my favorite bike!!

Couple questions:

She bogs off the line. Where to look? Once the revs come up, she goes like hell. I have owned many 2 strokes ('78 RM100, '81 RM125, '84 RM250, '86 CR250, and a '88 CR500). I realize this is no race bike and has a 7k redline on the tach, but I don't think it should bog.

Every once in a while, I hear an air sound. Like a pft, pft, pft that is blowing little puffs of air from a vent on the bottom of the crankcase. What is that about?


Thanks,
Dave

Sounds like a leaky right crank seal. This can also result in reduced low-end power, and possibly bogging. Try removing the oil fill plug on the clutch cover and see if you smell gas.

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

Thanks!!!

No gas smell in the transmission oil. I only have a few miles on this oil change, though. It doesn't do it all the time and, the vent is on the bottom left of the case. It just goes "pft,pft,pft,pft" for a minute or so, then it quits the noise. It comes and goes. The idle does change when it is making the noise, and if I give it some gas when it is doing it, it bogs bad. Even in neutral.

Is that seal easy enough to change with the clutch out? Is there a seal on the left side too? I'm gathering the clutch parts together do put a new one in and would like do do anything like this at the same time. Is the Clymer manual good enough, or should I keep the search for a genuine Honda book?

Hate to be the one to tell you, you have to split the cases to change the right crank seal. a case splitter tool comes in real handy here. A impact driver will also come in handy to get the phillip screws loose.

there is a left crank seal but it is changeable without splitting the case. it's just under the stator. A flywheel puller will come in real handy here. if you try it with a 2 jaw puller there is a good chance you'll crack the flywheel.

Take lots of detailed pictures during the breakdown if your memory is like mine.

Oh, OK - you mean the condensation drain at the bottom of the LH (ignition) cover, not the actual crankcase vent at the rear of the motor with the hose on it. In that case it's most likely the LH crank seal, which as OkieRob mentioned is easier to change out (did mine last week) PROVIDED you can get the flywheel off. I set up a 1/8" drill bit with a tube over it to limit the drill depth, drilled into the side of the seal (I got lucky and didn't break the rubber skin inside), then used a hook puller to get the seal out. Any drill chips should vacuum right out of the bearing.

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

I might as well just split the case and change them both. That way I know they are done.