Vintage Dirt Bike Q & A

I am trying to jet my 36 mm mikuni round slide carb for my mt250 with a cr top end on it. Its running rich and fouling the plug after about 30 seconds to a min. I have found some specs for the mr250 and used them I just thought it would be a lot closer than it was. I have an after market cr pipe on it from elsinores.com so it should flow pretty good I even took the airfilter off to see if that helped it didnt. I removed the airbox its got a big uni filter on it right now so its not the stock setup. I am just gonna keep going down on the jetting one step at a time till it stops fouling the plugs. I am at 260 main right now and 35 pilot the only thing that was different from the mr250 specs I found here was the needle jet as I have to mail order it. I have a well I forget the whole number but the last part is q-5 and the specs called for a q-4 I figured the q-5 would be close enough am I wrong? Lol I also have a q-8 that elsinores.com sold me with there jet kit that was well way different from what I have seen people use on here.

The MR250 uses a power-jet 34mm carb (I believe it's the first Keihin to use a power jet, but I could be wrong). The pilot and throttle valve specs may be helpful, but I don't think anything else will be close enough to be useful to you.

If your Mikuni was used when you bought it, I would assume absolutely nothing about it. I still get pissed off thinking about the months I spent chasing my tail with my 28mm Keihin, only to find some clown had drilled out the needle jet (the result was a ridiculously over-rich condition that no jetting change seemed to affect much - sound familiar?). Also, the throttle valve can have a HUGE impact on throttle response, although changing the needle diameter can have a similar effect.

Here's a posting redrocket190 made here in April 2006 regarding a '79 CR250; it seems pretty much on-point for your needs:

Mikuni VM36 Round slide
Air screw 1.5-2.0 turns out
Pilot 35
Air 200
Slide 2.5
Needle jet 159-P8
Needle 6FJ6
Clip #2-#3 from the top
Main 350

This is ball-park for Southern California. You would need fine-tune the air screw, needle clip and main to suit. http://www.sudco.com has full charts on alternative needles if you need them. Sam Alexander at http://www.jastco.com also has some ideas on radical set-up for these carbs. You will also need to grind down the bell of the carb to get it to fit between the rubber boots on the CR250R. The Mikuni is longer than the stock Keihin. I have a picture...

Just for what it's worth... a 36mm CR250M ('77?) Keihin carb worked very well in my CR125M. It should have been a disaster on that small of a piston-port motor, but once I got it dialed in - even though I knew jack about jetting back then - it worked cleanly for years. I can only imagine how well it would have worked on a 250. If all else fails with your Mikuni, these carbs come up on eBay from time to time, and the stock jetting should be very close.

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'm a bit baffled. It was probably my 36 Mikuni MR250 specs you found: 2.5 slide; 35 pilot; 6f5 needle; 159q4 needle jet; 260 main over 80 degrees and 270 main in cold months.
I also recently went back to a 6f4 needle (which is a tad thinner near the point) because of some slight detonation about 2/3 throttle when it was really warm and I was getting on it really really hard.

The needle jet might be your problem, but even with a richer needle jet, I wasn't fouling plugs like that. It was just gurgly rich at points in spectrum.
Be careful stepping down the main while you sort out this problem. If you are fouling plugs that quickly, it is likely you are never fully on your main jet.

My thoughts: I think the correct needle jet is very important, but might not be all of your problem. Do you have a strong spark?? Timing spot on?? Are you using new plugs?? I run a 8 plug generally. If I'm racing a hard long moto, I run a 9 plug. I haven't fouled a plug since I got the jetting sorted out.

Its a brand new carb from elsinores.com and yes mitchie I did find your mr250 specs. Do you think they should work I figured the engines were pretty similar after what I did to them.

I would think they'd be darned close. I know getting that carb right was one of the most frustrating tasks I've ever undertaken Try one part, close, but no cigar. Order another part, wait a week for delivery and start over.... A really viscious circle.
I guess it's kind of a crap shoot with each engine having a lot of variables.
Did you try the carb as it came out of the box??

Well I got tired of cleaning the plug so grabbed a spare it was a cheaper ngk its a 9 so a little hotter. I was using one of the 15 dollar iridium jobs funny thing is the cheap one works way better. I idled thru about an eighth a tank of gas and it ran out of gas. It seems the problem may have been the plugs I havnt tried a cheaper one like I am using now but it seems to work on the idle jet just fine now. I will take it for a spin and see what it does with a 260 main.

Be careful with a 260 main. I'm using an MR cylinder right now and using the 260/270 mains. With a CR cylinder and pipe, I'd expect that to be a 280 or 290.
You'll have to sacrifice a few brand new plugs, but I recommend a series of plug chops to determine what is really really correct at wide open throttle.

I know it sounds to low to me two but I have not been able to do a plug chp as its fouling plugs like mad. I did finally get it to run at idle all day long well I didnt let it run that long but it seemed it could. It took a 27.5 pilot way small but it works fine at idle now and good till I hit wot and then it falls on its face. I am pretty sure its still to rich as it wont start after it falls on its face cause its fouled. I have not tried to run it without the airbox its a unit that someone built to use a uni filter and a 36mm carb. The part the carb clamps to is a hair smaller than the carb but I dont think it would cause this much of a problem. Its like I am not getting propper airflow but I have no idea why. Where is the air screw supposed to be at?

Well I made two runs up and down the road before it fouled a plug with a 250 main in it. I did this with nothing but the carb on no air box or filter of any kind so thats not the problem. I have never owned a two stroke before this one so dont know a lot about them. I havnt sealed the pipe yet and it leaks exhaust out the front where it connects to the engine. Is this gonna cause it to foul plugs and seem to run rich? I am gettin kinda scared to jet it any lower its getting pretty low. The onlly other thing I could think of was this isnt a 36mm mikuni but I am pretty sure it is.

you have too many variables to easily sort this out. Seal the pipe, it might help a bit. Make sure this is a 36mm VM roundslide Mikuni. If it is, you are getting into a dangerously lean area of jetting. It could be falling off above 1/2 throttle from being too lean. I'd try a 280 jet and see what it does.
And the other thing that comes to mind is that you could have a leaking crank seal letting transmission oil in, causing the fouling. You can jet as lean as you want and plugs will still foul if this is the case.

I have pretty much givin up on being able to jet it with a 280 main the plugs black and a touch wet with a 240 main at wot. I can make a few runs till it dies with a 240 main in it. I have rebuilt the engine with new bearings and seals so probably not leaking but I did think of that to. Is there a good way to check for this as I dont really know. I did have a blaster that sucked in crank oil and this engine did it before the rebuild they both ran just smoked really bad they never did foul plugs like this is doing tho. It is a 36mm mikuni I had another for my xl500 and compared the two they are identicle so now I am thinking the pipe is the culprit. I put a different coil on it as well the 78 xl125 coil is just the same as the mt250 coil it made no difference tho. If the pipe was a pos could it possibly cause this problem? Its a loud pipe so it sounds like its flowing good I am just running out of ideas and am about to buy another pipe who makes the best pipes in your opinion? I am going to go even lower on my main against your advice as I just dont know what else to do the plug is still black it not tan or whit so I doubt its to lean. I guess if I burn up the piston I will just have to buy another one its not the end of the world.

Do you know what slide is in the carb?

Its a 2.5 I finally found the jet that came with the jet kit from elsinores.com thats where the pipe came from the one I had to get welded after I got it as it was full of holes. Ya this seems like a joke to me now he gave me a 190 main with his jet kit I just dont know anymore this seems way to low did he just throw some crap in a bag or somthing. I had this in there at one time and it reved real nice but I also had the oil mixed at 20:1 and so it wouldnt run very well. It was still to rich on the pilot anyways he gave me a 40 with his kit I got a 27.5 in it to get it to idle with out fouling anything. I am going to throw in a 230 main and see what it does elsinores.com obviously thinks I need a 190 main with this pipe to bad I cant ask him what the deal is.

Are you getting a lot of blow back out the carburetor? You haven't installed a reed kit yet have you?

Rick

Nope no reed valve or blow back.