I will try to look more if I can finally get my shed door open tomorrow...  I'm pretty sure I found the right part #.  I found a VM34 and it has the 40mm diameter top....  cap is missing though.  Probably in the shed. 

cap gasket

If I get a chance tomorrow I'll look in my milk crate full of carbs. I may have one.

Thanks a bunch all you guys. While looking closely at the gasket I noticed a line across the face, it was not all the way through, but I remember when I "tested" it, I did hear a slight crack and at first glance I did not see any separation, but this time I seen why it did not separate, one side was still intact. Even if I did get it softened up, it has been severely compromised.

I can probably make you one, have to check my gasket punch out kit. you need in thin rubber? OD 40mm with 1/4 band.

Yup, that is the size, I guess in rubber, maybe neoprene?? The one I have doesn't even resemble rubber, it's so hard. Mark T included a link to EBay for one of these. Curious it never dawned on me to look there. I have already gotten so many things for my ride from there already.

I'll check first thing tomorrow. I'll let you know if I can. If so I'll make you one of each and will get your mailing address and just mail them to you.

Some carb kits supply the cap rubber gasket .

Yes I seen that on EBay for 48 bucks. I ordered a rebuild kit without one, for $12. The gasket is available stand alone for about 8 bucks with shipping. A few of the guys here offered to send one my way, It would be nice to have a Yamaha one, as long as they don't have any kind of investment into it. Then I would gladly reimburse.

Hey Hackman, I checked, the largest punch in my kit is 1", sorry.

Might be overthinking this, this gasket is not a precision piece nor a piece that should cost $10-12, trace out your old one onto a old inner tube as a previous poster already suggested, cut it out with a pair of scissors or x-acto knife and glue it onto the mixing chamber top, one and done and free and only took 5 minutes. Good Luck