My 1975-1976 Dt250 Carb Conversion
While trying to clean and re-jet my carburetor I couldn’t visibly see through my pilot jet and my welding/cutting tip torch files would not go through it, I could also see that the screwdriver slot was non-existent, I first tried a left hand drill bit with no success, I then drilled a 5/64” drill bit into the pilot jet & then tried using a EZ out.
I was having to put way more torque on it than I was comfortable with yet it would not budge, so I gave it one more twist and the whole pilot jet cavity cracked and that is when I noticed it had been gooped up with JB weld and partially clogged the little holes in the pilot jet. Best guess previous owner could not remove the pilot jet and drilled it out probably drilled the hole oversize and JB welded in a new or different pilot jet and then the next owner later in the bikes life tried to remove the JB welded pilot jet and damaged the screw slot on the jet and gave up, then came me with gorilla hands and strength breaking the pilot jet orifice, thus the no idle issue being the reason for him selling the bike to me, pure guess?

What to do? I placed a WTB ad on here looking for a original carb or parts & pieces with no luck, and couldn’t find any on eBay either,

I saw a previous post of another person in a similar predicament and Mako had recommended the Mikuni VM28/49 carburetor, but the thread went no further, so I decided to look into this option.
The Mikuni VM28/49 has all the same dimensions, but NO oil injection line nipple/orifice, I took my chances and ordered one from a reputable source, paid more for my peace of mind, too many Mikuni Chinese knock-offs out there. Don’t get me wrong on Chinese stuff, the Chinese will make good quality and poor quality, all depending on what the buyer is willing to pay.
Upon receiving it, I noticed the pilot screw is on RH vs. LH side of the o.e.m. carb no big deal, I adjusted it 1-1/2 turns out. The throttle slide and throttle cap are the short style, my original has the tall cap and tall throttle slide, I wasn’t sure if this would affect the installation of the throttle cable, I had seen a post by MarkT that you don’t want to mix a tall throttle cap with a short throttle slide, not sure why? I tried my original tall throttle slide and spring and throttle cap in the new carb they fit like they were made for it, gotta love it when 43-44 year old parts interchange with a modern day part, the needle jet is the same as well as the pilot and main jet style, I ordered a new #50 pilot and a new #150 main which are stock replacements for the 1976 DT250C, the VM28/49 comes stock with a #60 pilot and a #200 main.
On the carb body it seems to have a indentation boss on it about where a oil injection nozzle is on the stock carb. I ordered a oil injection nozzle and when it arrived I drilled and tapped the carb body and installed the oil injection nozzle and hooked up my oil injection line.
Everything else went as if it was a stock carb Installation, the inlet and outlet spigot are the same diameters and it bolted right up. The 43-44 year old throttle cap and the throttle slide and spring work like they came with this new carb originally, I installed a new spark plug in anticipation of reading it to see how the jetting was, the bike started on the second kick after having been dormant for a few months, Idles and revs smooth and crisp, test ride went great, after a 10 minute ride of varying acceleration levels I inspected the new spark plug, it was nice tan/caramel color.

To recap to make this brand new VM28/49 modern day carb work, you will need your old throttle slide & spring and throttle cap, your old or new jets, my particular sizes were #150 main and #50 pilot, I left the needle jet in the middle setting, drill and tap the carb body and installed a oil injection nozzle.
I might do some more fiddling to see if it is possible to use the VM28/49 original throttle cap, and throttle slide and spring, I’m thinking it can be made to work by swapping out the straight inlet fitting on the throttle cap where the throttle cable goes in on the VM28/49 with a 45* degree fitting as the o.e.m. one originally has, all of the conversion parts are readily available through numerous sources, minus the slide and cap, although I have seen numerous caps on eBay, this way your conversion will be 100% new.
I have roughly $100.00 plus using my old throttle slide and spring, throttle cap and my new main and pilot jet into my conversion.

More Pictures

Nice job! Thanks for posting, very helpful information!

P.S. The reason you don't want to use a short slide with the tall cap is the cap stops the upward travel of the slide... the tall cap/short slide combo can allow the slide to come up high enough so that the slide needle comes all the way out of the needle jet... and then when you try to close the throttle, the needle may not go back into the jet and you're stuck at wide open throttle (WOT). Not fun! And don't ask me how I know this can happen.

Thanks MarkT now that you explain the physical differences on the caps and slides it makes perfect sense. I did this conversion a few months back, I have since tried using the short slide and cap, I did have to buy a 45* fitting for the short cap to insert the throttle cable which the VM28/49 comes with a straight fitting originally, It works just like the o.e.m. carburetor so you can have a brand new setup, in hindsight the only reason to use the original tall cap and slide is if you didn’t want to have to buy the 45* throttle cable fitting for the short cap, another possibly is trying to make the original 45* throttle cable fitting work in the new cap, the difference is the original is secured with an “e” clip and the the new style 45* throttle cable fitting is threaded into the new stock cap, I have not looked into making that work but I’m sure it probably could be retro fitted, I also again checked the spark plug and it has remained the tan/caramel color that is optimal. One note is if you use the short cap you have to shorten the throttle cable a little, I shortened mine about 3/8 of a inch and soldered a new barrel end on the old shortened cable. I probably should have just left the tall slide and cap both for ease of throttle cable installation and o.e.m. appearance.

And of course after I did this 2-3 used o.e.m. carburetors became available on eBay and other sources, I left the new style mikuni on the bike I am now looking for some factory type painting methods to paint it black like the original.

So glad you got the carb to work out and great post!

I'm beyond tired of trying to fix a leak on my original DT250 carb... THANK YOU for this wonderful post! I'll be ordering this carb and saying goodbye to my float issues!

jscott14 check your PM

I replied with a video of the carb.

Lizeec, you inspired me to pull the trigger on this, and my new carb just came in! Unfortunately, we have a problem. It's not at all the same size! I ordered a genuine Mikuni VM28-49, and it DOES have a 28.8mm bore (just like my stock carb). However, the outside diameter of the cylinder side of the carb is much smaller (due to thinner wall material... my OEM carb is 5.5mm thick, where the new Mikuni is just 3.3mm thick).

Here's the O.D. of the new one:


And the O.D. of my old one:


In addition, the top cap and slide diameters are different.
Here's the I.D. of the top cap/slide tube on the new carb:


And the old cab:



Is there a chance that my carb is NOT OEM? The O.D. of the cylinder side is the same as my DT400, so perhaps I have an intake boot from a 400 with some frankenstein 28mm carb that has the same O.D.? But then why is my slide a different diameter?
I'm really scratching my head on this one, so perhaps the experts here can help me make heads or tails of what I'm up against. Thanks!

A couple of questions first up!

1) What year and model is your bike?
2) What series of numbers/letters are stamped on the outside of your original carb?

Thanks.

Alan