I have to think others have come across this issue, and am wondering how they dealt with it. On disassembly of a carb for an early DT1 I discovered that the intake mount flange is rather bowed. Pics attached. I would estimate that the gap in the center is maybe 1/32". Not sure that a new O-ring will take up that much space. 

As the carb is pot metal using any kind of force seems out of the question. What to do? Fine wet-or-dry sandpaper on a piece of glass. Valve grinding paste? A few hours of slow figure-eight movements to true up the surface? Take it to a machine shop and have them mill it flat?

What have others done? Suggestions? 

 

Use a perfectly flat surface and some sandpaper. Start with an aggressive grit like 220 or 280 and as you close in on having the flange perfectly flat use a lighter grit, like 400.
I've flattened out worse.

I've always just filed them flat. Be sure to use an Oring that's not so fat that it forces them to bow again when Oring runs out of room to squash up. These old crap alloy carbs were bad for throttle sticking as tightening the flange nuts tends to pull the body out of shape--just be careful & flick the throttle or pull the cable at the carb to feel it bottoming & sliding fully up & down.

On the 1968 - 1971 DT1 there is no carburetor front o ring on the yamaha microfiche. Several people including myself have tried to obtain them from different sources. Have you mounted the carburetor and run the bike to verify if indeed “you do have a air leak there” with or without a o ring? A double gasket might save you some time?

I was looking at an expensive carburetor just the other day, the rear air intake is/was dented from a overtightened clamp. I thought about heating it up and decided just to let it be. If it broke - it would then be a cheap 248M2.

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The carb is for a '68 DT1, and probably from one. It was an eBay score for a '68 DT1 project still on-going ( ). 

The bike came to me with a later version with the idle screw at the side of the body. The one I show is for the "correct" version with the idle "screw" pulling the slide up or down from the top. My goal is to have the bike look and run as close to original as I can get (it really was a real mess   ). So when I spotted this carb I snagged it. As others have observed and commented on the one recently seen/sold on BAT, having the "correct" carb is a faux-pas easily seen and pointed out. That said, if it doesn't work well then the one I have already gone through (without the mounting flange issue) will stay on it.

I did start a thread back when I was taking it apart to document the many issues and took many photos but have gotten way behind in updating the progress. 

Just for your information . The Early 70's T 250 Suzuki also used Mikuni 26 mm flange mount carbs ,
with the idle adjuster up on the top cap . they are possibly more plentiful that original 68 DT1 carbs .
Obviously internals would need to be swapped over , but it's another option to get the " correct " look .

Had the exact same thing on my RT1.
I filed mine flat them finished it with fine wet and dry on a sheet of glass.

I just finished my 68 and did the same thing but mine wasn't that bad. Like Pete-RT1 says just file it close to being flat then get a sheet or 2 of wet/dry paper on a flat service. I go in different directions with the carb body until i get the surface finish i want. I also did the spacer block on both sides between the carb and cylinder. Then install new gaskets. No o-rings. I did all 3 of my DT1's when i bought them when i had the carburetors off for cleaning.

And that is exactly what I did. I had an old window that had a loose pane of glass ready to pop out. Used that (after cleaning). Filed it to take it down, then finished with paper on the glass.

The carb model used must have had another use somewhere, considering the O-ring groove? I don't recall ever seeing one and none referenced in the parts book. Gasket is sufficient I guess.

Thanks guys!