Whatever you do with slides--& i'm the expert at muckups & swapping things around--make sure the slide doesn't go above the venturi on full song. Or if it does then closely trebbble check how near it comes to getting off the guide pin in the side because bad things can happen when she won't shut off. I did see that on an RM400 40 years ago push starting it with very luckily 'not fatal' results as was in between two rows of parked cars & trailers. That case though was the results of someone [competitor or mechanic mate that knows everything] fitting the slide without engaging possibly the needle but definitely missed the guide pin & forcing the cap down on the spring. Still remember that as they pushed started it as no doubt the choke wouldn't suck like that & away she went minus the rider last seen hanging off the back like a flag & pusher lying flat on the ground. When it finally lay down doing about 8 grand some brave rider sat on the ground & sealed the end of the muffler with his boot which stifled it well while another person felt for the kill button. Team effort worked & lesson learnt.

1. It's a 1974 DT250A, and I have verified my VIN number against the list in the tech library to ensure that. Of course, now that I type that out, this could be the difference... the original post was for a 75-76. Perhaps those used different carbs.

2. There are no stamps on the carb that I can find that might help identify it. Of course, it has "Mikuni" and ISO stamped on it, but nothing else. I've even taken the bowl off to see if there were internal stampings. There aren't any. I've attached a few pics of the ORIGINAL carb.

I will be sure to avoid that! That's a funny story though, because I had the EXACT same thing happen on the EXACT same bike! And I was only about 13 years old. I found an old RM400 in a barn out in the country where I lived. I talked to the owner of the barn and it belonged to one of their kids, but had been abandoned long before, so they just let me have it. I knew enough about 2 strokes to get it running, but not enough to check out things like this very issue. I was a scrawny 13 year old, so I didn't even weigh enough to kick start the bike! I had to get my older brother to help push it to bump start it... but once it started, it was the meanest, most terrifying bike I had ever been on in my life. (At this point, the fastest thing I had ever been on was a KX80... a potent top-end for sure, but just a universe apart from a 400cc MX bike.) I rode that bike for probably a week or two before I ever got the guts to open it WOT... and then when I finally did, it stuck. LUCKILLY for me, I was riding in a pasture at the time, so after panic-shifting through gears 1-3 and realizing that shifting was only making my predicament worse, I layed it over on its side in the tall grass at maybe 35 mph and it finally revved itself dry of fuel. That was over 30 years ago, and I can still remember just about ever moment of it. It's probably worth mentioning that the bike didn't have a working kill switch. That particular fact also came back to bite me in this scenario.

Thanks.

If the original fitment, I would have expected to see 438 stamped somewhere on the carb. I'll have a look later at the carb on my DT360A and see where the number is stamped on that.

According to Partzilla, the carb for the 1974 DT250A is 30mm and for the 1975/6 DT250B/C is 28mm.

Maybe not what you wanted to hear, but does explain the differences. Anyway chin up, it means that you have two carbs to sell on ebay!

Edit - here we go, photo of 445 stamped on my DT360A. Your number should be in the same location.

It looks like the carb you took off is a larger "body size". Many of the 28's were the smaller body size... I think a couple 125/175 YZ/MX models actually went up to 30 mm or maybe even 32 with the smaller body but those are rare.

The take-off carb has a body that was often used on carbs larger than 28... like 30 to 34.

As said, prefix of number stamped in engine and stamping on original carb would be helpful... carb should have numbers on front of boss just below the starter (choke) lever. Also rubber manifold might have numbers on it near one of the bolt holes... I'd expect to see "498" or "500" on manifold... (carb should have same number plus maybe a couple more.)

I type too slow... I'm pretty sure the problem is you have a 1974.

Not 100% certain but the 74 360 and 250 may have used the same rubber intake manifold which explains using the larger body carb on the 250.

Look closer... because I can definitely see the number on one of your pics... can't read it though.






P.S. Not sure why you're replacing original carb.. looks great!




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I'm so glad we have eagle-eyed experts on here! Yep, there sure is a number that I didn't see (438/60):


Unfortunately, I don't know what that really means for me as far as next steps... is there a modern replacement for this carb that's as "bolt on" as this original post?

As for why I'm replacing it... yes, it looks near PERFECT, doesn't it? I have replaced every single brass and rubber part on the carb, I've glass bead blasted it. I've ultrasonically cleaned it. I've polished the rods that the floats slide up and down on. I've polished the INSIDES of the floats by using 1000-grit sandpaper on a toothpick in an electric drill. I have tried every single float height in .5mm increments that are considered spec by the manual. And yet, SOMETHING just isn't right with it.

The bike starts, idles, and runs perfectly throughout the entire RPM range... no flat spots, bogs, or anything. So I know the jetting is spot-on. The problem is that on any ride longer than, say, a mile or two, odds are VERY good that one of two things will happen:
1. It will either become fuel starved and go lean/die. (a few minutes waiting on the side of the road usually takes care of it... petcock/gas cap venting issues have been ruled out... fuel flow to the carb is EXCELLENT)
2. It will overflow the carb and spew gas from the overflow hose.

In short, the carb floats either find a way to get stuck low and overflow the carb, or somehow shut off fuel flow entirely. Back-to-back rides might give me each symptom. I'm a really patient guy, and very methodical. But I have struggled with this carb now for over a year. It's probably conservative to say the carb has been off the bike 50 times to clean/tweak in some way. I'm just sick of the headache. I want something that has a more modern float design so that the nightmare ends. I've already modernized the electronics with the 12v PowerDynamo system and an entirely new wiring harness. I think if I could modernize the carb, I could really love this bike.

If practical, it would be great if you could borrow a carb off someone that is known to be operating correctly, just to confirm that it is the carb that is playing up.

Out of interest, did you replace any jets with items out of a repair kit? Always best to use original Mikuni.

That number confirms it's the original carb.

As far as replacement, not sure... I doubt you'd find exact. I have a new 30 mm Mikuni that I could measure for you but I suspect you might have one of the OEM carbs where the manufacturer spec'd oddball dimensions... normally that size body is a 30 or bigger. And sometimes the inlet bell or the overall length spec is changed enough that a standard carb won't fit.

On to your problem... very odd... and the thing is it looks like yours has the "modern" float design which is the floats slide on pins in the float bowl. Mikuni still uses that design today and it's an upgrade from the brass floats or plastic floats attached directly to the float arms. More stable fuel level during rough riding conditions with the separate floats that slide on the float bowl pins.

First thought is the same as Alan's... new brass is not alike. Can you pull the float needle and seat out of your original carb and see if it has the size and the Mikuni "square within a square" emblem stamped in the end or on one of the wrench flats?

Second thought is the issue sounds like a fuel flow or vent issue... especially the starving and restarting after sitting... some of those issues can be maddeningly intermittent... once I saw a clear piece of plastic in a tank... virtually invisible in the fuel... that would randomly get stuck on the petcock blocking the flow killing the engine. Then it would move and fuel would flow great.

Anyway, what you might have is a venting issue on the carb itself.. odd problem for sure.

Hello jscott14 I apologize if I caused you more grief, my bike is a 1976 DT250C, I went to the garage and took some measurements of my old damaged carb and another new VM28/49 Mikuni I have waiting for another project I can barely make out the numbers as 498 62 on my old damaged carb body, also visible where my pilot jet cavity is cracked on the old one. The new Mikuni is an almost identical match to my 1976 carburetor, I suspect as other more knowledgeable than me the difference is in yours being a 1974 model.

I was kinda surprised that my 44 year old carb was so similar and adaptable to the new mikuni. I have included pictures of my old carb and new Mikuni waiting it’s turn on another 1976 DT250C