I have a mikuni vm26 carburetor on my 1970 Yamaha DT1c that has a small cut out involving several threads at the top of the
mixing chamber [see photo].There is a gasket between the threads and the mixing chamber cap. What is the purpose of the cut out ? The gasket has been deformed probably by buckling into the cut out. The parts diagram doesn't show the gasket for 1970 but 1971 shows a gasket.
Do I need to replace or discard the gasket ? I am not sure it seals with the cut out. .

From what i can make of it looking at partzilla--is, the first model in "68 had the idle speed controlled by a rod with the idle speed adjustment on the top. Therefore the top had to be secured in the right position so the rod can pull or stop the slide ad the desired idle speed. Gasket won't be important from a sealing point of view but is slightly rubbery to give the screw ring a bit of a spring washer effect so it won't come undone unexpectedly. Maybe you have a mixture of parts if using an ordinary top. Or does yours look like this in the link with e pull rod 'just' visible drawn below the slide No19. Sorry for the long winded story.
www.partzilla.com/catalog/yamaha/motorcycle/1968/dt1/carburetor
Actually i'm now guessing you must have the right top or there'd be no way of setting the idle speed. Unless they used the same body on later years & just threaded the idle speed screw hole in the later body. I'll go look at my spare 250 carbs. I'm running a 32mm of a 360 on mine.

As RT 325 has said , that cutout was used for the top with idle adjustment with the threaded rod .
That changed by 69 , but carb bodies still had the cutout . As for the rubber gasket .
It was probably included as part of the carb top . My DT1 B was missing one too , so I robbed
one out an old carb kit I had lying around .

Yep, took a pic of my two parts carbs today--which i'll get around to posting--one of which is black so think DT3 went black or maybe later but yep still has the cutout. So guessing yours has the normal idle speed screw in the side if it's a "70.

Yes,thanks the carb has the normal idle speed screw on the side and round screw on cap.I will put a gasket in. The throttle cable is now out of the cap as is the throttle cable adjusting screw. I need to reinstall and adjust for correct cable free play. The manual I have says 0.040 inch[1mm].I am not clear on how to set the adjusting screw in or out to get the 1mm correct free play. Throttle closed at handlebar and throttle slide down against the idlespeed screw.Do I just start turning the cable adjusting screw until it just moves the throttle slide.Manual shows 1mm free play at cable and cable adjuster.

that looks like a 1968 DT1 carb. here is some pics of one of my 1968 DT1 carbs.


the 1968 carb throttle slide has no provision for the side mounted idle screw on the throttle slide.

I have the 1970 round cap [ attached] and see how it evolved from the 1968 that had the idle speed adjustment on top.Thanks, now I have to reinstall and adjust cable free play [see my previous post] not completely clear how to set free play properly at carb.

Here's a couple of mine still with the notch but with the threaded idle speed screw so later than "68 but still piston port for the unpainted one.