Two comments -

The 250 MX bikes use a flat slide FCR 37mm carb - a 40 is too big. It will have probems maintaining intake velocity to draw fuel compared to the smaller 36. You also have to realize that Bill Blue does a 34mm FCR on his 351s that should tell you a lot.

Second comment, you could pull the tensioner, take off the allen nut and jam nut, then slot the thread end of the bolt with a hacksaw. This would allow adjustment and holding the adjuster bolt with a screwdriver, giving clearance on the carb.

Now a question to you TM users, you say there is no float bowl drain?

That isn't too good. I find I had to drain my float bowl to get my 250 to fire up after about two weeks of not riding it (well below freezing and lots of snow and ice). Cranked and cranked... drain float bowl (previous owners stripped the allen head, so it took vise-grips)... bike fired up nearly instantly. Same deal as my 650. Seems the gas in the carb will have evaporation of gasses that are vital to have it fire properly.

I need a float drain, because I may not run the bike for a week or two at a time, especially in the winter months.

turn off the petcock and run the bike dry before you park it ? that's what I do.

My TM36 sat for almost 4 months. Forgot to turn off the petcock and run it dry. After about 3 minutes of cranking, it still started. Couldn't go for more than a couple weeks without draining the stock carb. I did add some stabilizer before it was parked last fall.
Interested to know if that smaller TM40 drain plug works. A bit more room would be nice.

Time for a Custom Kreiger (sp?) CCT, one with a shorter adjuster bolt.

I've ordered some miniature screws with a oring on the back side with the intent of drilling and tapping the long float bowl drain plug so I can get it all out easily

The lack of a float bowl drain is an inconvenience to me because it doesn't allow one to do the clear tube method of fuel level adjustment...but hardly the end of the world. On the need to drain the fuel like some do on the CV carb, this is unnecessary with the Mikuni. It has a pump, so you can just about always get the engine to fire with this carb unless you have moss growing inside the float bowl.:D

As far as wanting to drain the Mikuni float bowl, if you feel the need, why not just use a 17mm wrench to remove that large plug and let it drain on a shop rag?

Mine is a tight fit next to the starter, and I previously I was of course accustomed to draining it into a cup and putting it back into the tank. Odd that the TM40 comes with a shorter drain plug. I wonder if the main jet sits lower on the TM36 requiring the long drain plug?

What a pain. I never do that on any of my bikes, especially since the float needle seized up in a dry carb on my 650. If the float needle works the petcock doesn't have to be closed. If it doesn't, you end up with a puddle on the floor either way. When I turned the gas on with the stuck needle it filled up the carb and the intake. Fortunately the valves sealed perfectly. I sopped up the gas in the intake after cleaning the needle edges and carb body.

But that's beside the point. Just wondered - If I do the carb I'll have to figure out a drain. Gas apparently isn't too good here in Ohio.

Where do you think the pump gets its gas?

After about 2 weeks, the gas I drain out of the float bowl will not light with a match! I tried.

When the gas is drained the bike fires instantly. That tells you it isn't a rich or lean condition - it is junk gas that won't fire.

Maybe a perusal of the Mikuni catalog will reveal a solution.

I've left my bike parked with cv carb with petcock on all through December! It took about 3 minuets of cranking with the choke till she fired, but she fired!