Also, I just looked at my other dt3 spare carb. Didn’t think it had the washer in it but it does I guess and it is the same as the one installed on the bike. I did however add another thin washer to it and put the bowl back on and could feel it seat for sure. So maybe just a new O-ring.

I get the 007 O-rings at O'Reilly's Auto. Hardware store ones may not be fuel rated.

Yep saw your added two pics thanks, & My Bad, can see the washer now.
Have to be careful adding another washer as threads damage & break off on the Oring holder--jet setter. Just try a new Oring & if i was a betting man i'd bet it'll be good.
Probably need a rev out after all the rich running.

Just FYI, if I remember correctly a standard  007 o-ring is not metric.  These carbs were built under license from Amal originally and some of the carb jet threads, o-rings, and things like slide cutaway measurements are still "standard" (many sources say the slide cutaway for the VM is in "mm" but that is not true.  It's actually the height in the "number of 1/16th inch increments".  For example, a 2.0 slide has a cutaway height of about 1/8" which is closer to 3mm than 2mm)

 

 

Things i didn't know but should have. Thanks MarkT.
Once when i couldn't find an Oring at home to fit the grove that was small enough but also fat enough to be compressable--i just fitted on over the jet setter up to the hex & it worked a treat as hex squashed it ok into the taper lead in, in the bowl. Just tricks i'm not recommending--but!!.

Good to know, thank you MarkT. Thinnking about it now maybe I just got the wrong size or material of O-ring last time that I changed it out. I'll try and make sure I get the right one this time. Interesting that there is some "standard" measurements.

The sets that contain "metric" 007 o-rings might just be converting the nominal fractional inch measurements to metric? 

"Industry" size designations for bearings (i.e. 6202) are pretty much universally metric. 

"Industry" size designations for o-rings (i.e. -007, or "dash 007") are pretty much nominal standard fractional inch measurements.

I dug out my old DT3 carb...  311E2 stamp. 


I know there are different jet setters...  the one in mine is ~11.72mm from bottom of nut to end.  (12mm wrench for reference)


The jet setter with the correct washer does sit down inside the casting slightly like yours does...  which is what gave the impression washer might be wrong.  On many Mikuni's the washer is flush with end of casting.

I also tried a 007 o-ring out of the Harbor Freight Metric O-ring assortment...  fit fine.  Very snug fit in float bowl.  Remember to put a drop of oil or silicone grease on o-ring before installing float bowl.

Thank you for your diligence MarkT! I am picking up the o-rings after work and I will measure my jet setter as well because as I said my original one broke and I took the current one out of my spare dt3 carb.

Now I’m confused. Got new o rings (from harbor freight) I got the standard set and put in the 007. Put the bowl on. Almost no resistance. I took out my rt3 carb even with a really worn o-ring (rebuilding the bike right now) there was noticeable resistance on that carb. To make me more confused the hole that the jet setter sits in looks noticeably smaller on the rt3 than the dt3. I did check and my setter is the same dimensions as MarkT’s. Also it so happens my spare carb does not have a bowl. I might have to find a dt3 bowl online and order a new jet setter and might as well get the Yamaha o-ring as well.