Vintage Dirt Bike Q & A

I just started my MT250. I've rebuilt the top end and rebuilt the carb. It started up easy. Was idling a bit high, but running pretty well. Decided to run it around the yard a bit. All of the sudden it reved to 10,000 rpm and about threw me. I adjusted the carb a bit and restarted it. Ran fine initially, I opened the throttle and it was running fine, then suddenly went up to 10,000 rpm again and I shut it off. Any suggestions? Hopefully the high revs didn't hurt it.

It could be a number of things, but good places to start looking would be the throttle cable, slide return spring, and checking for air leaks. If the motor is running fine (i.e., not super rich) and then suddenly revs to the moon, the throttle slide must have moved somehow; if it was running super rich, it could be an air leak (all just guesses). Is the carb cap sealing tight, with no leaks to the carb body or throttle cable?

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

Not sure Ray. I wish you could access the throttle cables on these things without taking the tank off! Throttle slide seems to move smoothly. This is my first two-stroke bike and it wasn't running when I got it, so I'm a bit in the dark. How do you know if it is running too rich? I don't have the oil injection hooked up, so I but a bolt in place of the injector. I did a mix of 32:1 with 93 octane.

The easiest test for richness at low throttle settings is the fuel-starvation test I've mentioned before; just turn off the petcock with the motor idling and wait. If it's too rich, the revs will start to climb, and you may have to use the kill switch to stop it from overrevving.

You could also mix up a small batch of 16:1 premix, drain the tank, and put it in. This will make the bike run slightly leaner at all throttle settings, and if the bike was too rich it will run better.

The MT250 dual throttle cables work fine when new, but they don't age well. If you're sure you'll never use the oil injection, you might want to replace the OEM throttle cable with a conventional single cable. If you're using the OEM throttle setup I'd think one that fits an early CR125M or MR175 might work, but I've never done the swap (both ends should fit, I just don't know about length).

It's pretty easy to tell if a motor's way too rich (blubbering, smoking, fouled plugs, etc.), but it's harder to tell when a motor's just slightly rich. Usually it will run better when cold, and will respond more slowly to the throttle but will otherwise seem fine (hard to tell without something to comapre it to, though). Slightly too lean will respond very quickly to the throttle and can sometimes have a sharper, raspier exhaust note, and often will run unevenly at continuous rpm under load.

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 Ray. Appreciate the guidance. I'll give that a try. I am using the dual cable, though it is in a really good shape. Apparently the cable wasn't used much.

How much smoke is normal? I'm getting some white smoke, but then it's newly rebuilt and only run maybe 10 to 15 minutes, so I figure that could be running rich or oil used in assembly.

Pure white smoke (not the normal bluish-white premix smoke) in an air-cooled engine may be burning gearbox oil leaking through the right crank seal, which I assume was replaced? This seal is notorious for snagging on the crank during assembly (I greased my crank and put the seal on several times, then left it on the crank overnight to stretch it before assembly). If air was intermittently leaking through it and the carb was very rich, it could cause the surging you described. You shouldn't get a lot of smoke no matter what. so if it's not the seal it's quite probably the carb.

Again, these are all just shots in the dark - I'm sure the last thing you want to do is split the cases again. I've read that if the (running) bike is leaned way over to the left and the smoking stops, that's a pretty good indicator of RH seal leakage, but I've never done it myself.

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

Confession time. I only rebuilt the top end, didn't split the cases. I didn't have the tools and wanted it just as a casual rider. Plus, I didn't even see a source at the time for that seal. Once I get the throttle straightened out, I'll decide if I need to pull the motor and get someone to split it. Should I be running a lower oil concentration than 32:1? The oil I'm using is Honda branded from the dealer. Thanks again for all the advice. As you can tell this is all new to me. I've done a lot of car wrenching in the past and this two wheel two stroke stuff is a bit foreign.

might check your main bearings and main seals.worn sloppy main bearings will allow the ends of the crank to whip around and the seals will then leak and cause a run away.if the bearings are ok then if the crankshaft seals may seal well at a idle then start leaking when the engine is revved then this causes the mixture to lean out and cause a run away.just a thought anyway.

you will have to split the cases to replace the primary side crank shaft seal.

73-75 CR250 crankshaft bearings and seals will fit the 74-76 MT250.they are fairly easy to find.

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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!!

here is a seal/bearing kit for the mains.
http://www.ebay.com/itm/Main-Crank-Shaf ... 48&vxp=mtr

complete seals kit,they are listed for the CR250 but they will also fit the MT250.
http://www.ebay.com/itm/Honda-Elsinore- ... #vi-ilComp

main seals kit,these also will fit the MT250.
http://www.ebay.com/itm/Honda-CR250-197 ... c9&vxp=mtr

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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!!

My bad - you mentioned this in your first post, and I missed it. If you don't have to split the cases, my advice is don't do it. :>)

The Honda oil should be fine, and 32:1 is a very popular ratio. I imagine the 2-stroke does seem really smoky if you're mainly used to 4-stroke motors, which only smoke when something's wrong. For what it's worth, my MR produces very little smoke, running Bel-Ray H1R at 36:1

The sudden high revs are definitely a concern, especially when they occur randomly under load and are high power events (as opposed to an uneven idle).

Is your confidence high in your top end rebuild? Did anything occur that concerned you at the time? I still think it's a carb/throttle issue or an air leak, and there are plenty of places that can leak after a top end job.

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

Confidence pretty high in the top end rebuild. I've rebuilt auto engines before. I took the jug to a respected motorcycle machine shop to be bored and used a Wiseco piston and ring set with new bearing. Torqued to spec. Not much to go wrong there. I'll work a bit on it as I get time in the next few days.