Vintage Dirt Bike Q & A

I've been trying to get this thing sorted out for awhile now and I keep getting stumped.

The last time I went out on it I would get some wet fouled plugs. I adjusted the float level and it seemed to clear that up a bit. Then I got it to where it would idle pretty well then as soon as I took it up and down the road after warming up, it wouldn't idle and I'd have to keep blipping the throttle to keep it running. Then I got into the woods and dropped it while trying to get over a fallen tree and couldn't get it started again. Plug was wet again. Tried drying it out with what I had with me and couldn't get it going. Then I got it home, changed the plug and couldn't get much spark. I did go through a creek and wasn't sure if the water damaged some of the wiring somehow. I haven't checked the points yet, been busy with work and haven't gotten back to it. Been looking on here with the search to figure out what I'm missing. I also have a new carb coming, although the old carb itself has been cleaned thoroughly. Just wanted to try a new one, 36mm Mikuni.

The lights have been removed so is there a simple diagram for chasing the wiring down to check it all out? I wanted to just have the essentials for starting it. For example, do I need the rectifier and condenser if I'm not using lights?

If your pipe, silencer, and air cleaner are all flowing well and your compression is good, your problems are very likely either carburetion or ignition.

I know it's a pain with the points/flywheel/etc., but I'd really suggest getting the ignition squared away before tackling the jetting. Your points are still available from Honda, and a flywheel puller can be found if you need one. Just for what it's worth - I replaced my MR175's points ignition with the CR125M CDI, and the bike instantly went from sluggish to snappy; I wouldn't have believed a simple ignition change could do that, but it did. Aftermarket CDI ignitions are available for your bike (they're not cheap).

Since your cylinder head takes the same 3/4" reach plugs as my MR175, I highly recommend NGK BPR7EIX spark plugs. These are projected-nose Iridium plugs, and so far they've been impossible to foul (the motor runs great too).

As far as jetting goes, it sounds to me like your original carb's pilot (slow) circuit is too rich, and maybe the needle circuit as well. There's an MR250 video on YouTube where the bike is smoking and sputtering from a similar over-rich condition, so I'd assume the stock jetting is too rich.

With the stock flywheel, your bike should idle fine if jetted/adjusted right. With my bike, I found I had to adjust the pilot air screw much leaner than I was used to, but the idle improved greatly (a leaner pilot jet will help a lot). I ended up adjusting my air screw pretty far out (2.5 - 3 turns) and then backing out the idle speed screw to brings the idle revs down. My MR175 has practically no flywheel now, and it idles OK if I run the idle a little high.

I had a similar carb on my CR125M for many years, the Keihin 36mm optional CR250M carb. Your MR250 carb is (I believe) the first power-jet Keihin. It should work great once it's dialed in, but the Mikuni should work well too.

Here's a wiring diagram for your bike. I bypassed my rectifier and the bike ran fine, but the condensor will still be needed for the points ignition.

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 haven't had a chance to make sure the pipe isn't blocked or check the compression. That's next when I get a day off...plus checking the points. I would love to put in a CDI and forget about the points, but I definitely don't want to spend a lot of money on this thing right now while I'm trying to get my roadracer ready for the season. You'd be amazed what a good electronic ignition will do for the performance. I don't know from this bike, but all my old brit bikes turned into new ones after putting a Boyer ignition in. And I got that plus an Accel coil in my race bike and it's a breeze to start and runs great.

I've been using NGK B8ES plugs but I'll try those others and see what happens. Another guy on here told me what jetting he uses for his Mikuni on the same bike, so I'll start there and see what happens. Not sure which slide though, I think the one I got has a 2.5 in it. I'll mess with that Keihin some more and put it on my other MR250 I'm fixing up to sell. I'm probably going to rebuild 2 engines all the way and put one in this bike I'm trying to get started, and one in the other to sell it. Thanks alot for the diagram! I figured it was pretty easy, but it's nice to have something to look at in case I get stumped.

I fought that oddball Keihin carb on my bike for three years. Exact same problem you have, no matter how I changed the jetting. I'm convinced the only way I could have fixed it was to replace the slide, needle and needle jet, which are difficult to find. The 36 mikuni cured all those problems. You know, I never did look to see what slide is on it. I think it's a 2.5. I'm running 35 pilot; 6f5 needle; 159q4 needle jet; 260 main over 80 degrees and 270 main in cold months.
Once weather warms up, I'll pull the slide out and get a magnifying glass and check the size.

Cool, thanks Mitchie I appreciate that. Seems like a 2.5 would be about right. Especially if maybe I'm running slightly rich now. I think the Keihins I have all have 3s in them. The Mikuni is on the way now, coming from Portland so hopefully it'll be here next week and I can start eliminating possibilities. Trying to get going by the 30th so I can join some buddies at Durhamtown down in GA for a weekend of riding.

No problem. I'll let you know soon as I pull it out and look at it. I do run my bike just slightly rich at 1/2 throttle and above, but it screams, pulls like a friggin tractor and I haven't fouled a plug since I made the conversion.
When I first made the conversion, I was fiddling with jet needles and needle jets and thought I had it right after testing around my house, where it is very flat and I can't ride for very long for fear of someone calling a cop. So I take the bike out to Stillwater to ride and I go on the blue trail, which is--of course--a one-way fairly difficult trail about 7 miles long. Well, about two miles into it, I start hearing some pinging, especially when I put a load on it, up to about 1/2 throttle. Well I couldn't ride backwards to the truck because trails are one way. So I had about another five miles of even tougher trails up ahead. So I pulled the bottom off the carb and bent the float up a bit, which maybe helped a little, but not enough to notice a real difference.
So I did the only other thing (besides pushing the bike--which I could not have done for a million dollars) I could think of: I rode it the rest of the way like a four stroke. I kept it a gear or two higher than I normally would have and just kept the throttle as near wide open as I could. With that huge flywheel, I just lugged the bike up those steep creek banks for another five miles without missing a beat. I probably rode that section faster than I ever have. With a lot of throttle, detonation wasn't a problem at all.
After that ride, I switched out carb parts to my current configuration and have ridden in all sorts of terrain for extended period of times without any problems whatsoever.
The lesson I learned was to test a carburetor set up under real conditions, not a flat residential neighborhood street.

It is a 2.5 slide

I had trouble a few years back, my MR suddenly ran awful! Perfect one day, ran like total crap the next.

Fouling plugs, oil splooge from exhaust, running just awful. I assumed something got into the carb, took it apart a few time, cleaned, rebuilt, cleaned again, etc etc.

I was baffled: came here for help, someone suggested the points. Bingo! They were fried! Put new points in (naturally, the night before a vintage race), bike was back to normal. I had to repack the Supertrap, as the earleir spooge had soaked the packing.

Note: order a flywheel puller (long reach) from Honda. Aftermarket ones are too short, even thought they are listed for the MR, in the end, they only work on the CR.

Thanks, I surely will. Right now I have put a freeze on my spending until my race bike is ready and I have the money for my entrance fees and what not. Hopefully I can get the points and puller soon and go for an uninterrupted ride with no problems.