Vintage Dirt Bike Q & A

Hi all,

I got a few Mister250s recently and I found this forum. Thought I'd introduce myself and ask a few questions.

Been riding dirt bikes since I was 5, but I took a break for a few years until last year. I also have some old Brit bikes as well.


For my first question, does anyone know what size/model Keihin came on the MR? I've been trying to figure out which one it is to get a rebuild kit or replacement and can't find any info. Also, who is a good source for air filters and other expendable parts?


cheers....tony.

It is a 34 mm Keihin carb, but can't find a specific model number. It is different than the 34mm Keihin on the CR250 as the MR carb has a pilot jet, smaller main jet and a third jet called a power jet. The CR carb has only the standard pilot and larger main jet. So any carb kit you find for a CR will not work on the MR. Likewise, needlejet, jet needle, idle screw and air screw are different on the MR.
I have never found a carb kit for this carb and I gave up trying to find parts. The used parts were all worn out when I got them, too. You can find float and float needle and seat from Honda, but not the needle or needle jet. Because of the age, if the needlejet and jet needle are too worn, all the jetting in the world will not fix it from running too rich and fouling plugs.

So, if you really need carb parts, I suggest getting a 36mm Mikuni from Sudco. On the Mikuni, a 35 pilot jet, 6f5 jet needle with the clip in the middle or down up from the middle, 159Q4 needlejet and a 270 main jet is just the ticket on my MR.

I prefer buying the Honda air filter over afterbrands. You can still get them and they are two layers and very well made. Uni also makes one, I think its the 4025ST (double check before buying, that is just from memory). I like NGK B8 ES plugs from any motorcycle or auto part store.
I recommend you get a manual. You can still find original Honda shop manuals or a Clymers manual for more detailed information.

Thank you very much, that is extremely helpful. I was hoping to avoid getting an entirely new carb, but looks like that is actually the easier way to go. I've been using the B8ES plugs and so far haven't fouled one. Of course I only got to ride it for about 3 or 4 hours before I low sided in a curve due to the less than adequate treads on the rear tire. That put me down for the rest of the day because my clutch lever broke and my masterlink snapped and like an idiot I didn't bring spares with me that day. Got all that taken care of and now it won't start. Figured the carb was all gunked up and filter needed changing....it was REALLY dusty that day. But that was just a shakedown ride since I had just picked it up with two others for almost nothing. One of the others is mostly ready to start and the other a parts bike. Tried to sell those other two at Mid Ohio this weekend but the track officials said we couldn't sell stuff in the campgrounds.

Oh by the way, do you use the slide and air jet it comes with on the Mikuni? And is the tuning kinda the same for most other applications? 1 1/2 turns out on the air screw as a good starting point? That's about where I have it on my Triumph and it runs great with the Mikunis.

Yes. But if you rode that long and didn't foul a plug, you probably don't need a new carb. Clean yours very well, clean out your gas tank and clean your petcock. Put in a new plug and I'll bet it starts second kick (choke on, no throttle). If not, check your spark. You may have loosened a connection somewhere. Good luck.

Ah, well I'll check that as soon as I put the carb back together. Problem I have now is I probably need a new gasket for the float bowl and it hasn't been easy finding one. Also, I looked around for a Honda air filter and was told they are on back order. I looked at UNI's site and they didn't have an MR specifically, but will the CR work? I would clean the one that's in it out, but it has started deteriorating and isn't really usable.

CR and MR same air filter (and whole bunch of other mechanical parts). Local Honda dealer isn't trying hard enough. It should take 3-4 days to get an air filter in. But a lot of people like Unis and White brothers. You can order them from sites like Vintage Elsinores. Be sure and look in "Resources" up above and visit some of those sites. Great stuff available for these bikes.
I'd darn sure go ahead and take your carb apart and clean the heck out of it. And having the new gasket is a bonus.

So are the '76 CR carbs be the same as the MR? I got the carb apart first and cleaned it out real good. The gasket was trash. Looked like it had never been taken apart and cleaned. Thanks again for the information. I feel like I'm starting all over again when I was building my Triumphs.

No, they aren't the same, as I explained above. They fit exactly the same and have exaclty same dimensions. But the carb is totally different. The air intake and airbox are the same, except for the top of the airbox. But the filter is exactly the same. MR, myself, and at least one other have posted what the differences between the CR and MR are, but God help me, I couldn't tell you what words to use for a proper search. This is a great site, but has a lot of painfully annoying tendancies. The search tool is not particularly good and the constant resetting to the home page if you do anything offbeat--like change the size of your page--can make you want to tear your hair out.