Hi, Mark, it's me again, trying to make my VM26SH behave. I have been working on the carb and its issues for a while now. After becoming plugged up from old fuel, I took the carb apart, cleaned it repeatedly in my ultrasonic machine, and replaced several particular parts. These included the pilot jet (of course) and the valve and seat assembly. After installing the new components, it seemed to be leaking from the float bowl gasket, which did seem a little old and brittle. I replaced it with that part number that fits the Yamaha and a particular Suzuki. You recommended this gasket on another post. Now it seems to be leaking worse than ever. Yes, I have fiddled with the float height several times and it leaks no matter what I do. The float was tested and it floats. The seat assembly looks new and clean. I have observed a small amount of leaking from the overflow tube, but the massive amount clearly seems to be coming from the float bowl gasket. Can you offer any other suggestions? Do I need to add a second gasket to make it thicker? Thanks in advance....

Just my thoughts without thinking too deep but the fuel shouldn't get high enough to leak out the gasket if the bike it upright & level. Ok it'll splash around riding etc. so what's going on. If you invert the carb somehow held firmly on the bench or vice with a fuel suspended above, does the weight of the floats stop the fuel. Maybe not but how much weight can you add pushing on floats to stop it. Will it just not stop. Can you see if it's coming from elsewhere to get past the float valve. Can't think how that could be--but!!. Does it come out any high up breather or come out the bowl overflow breather. Sorry if I've not read your story fully. Will later but just throwing ideas out there. Be good if it had a drain screw to make a fitting with a clear tube held up the side to eyeball the fuel rising in the bowl to see where it climbs to etc.

Worth checking your float bowl is flat across the gasket mating surface
.

The fuel clearly seems to be pouring out of the gasket, not from any other orifice. I have taken the carb on and off countless times until I am quite frustrated with it. Right now the carb is disconnected from the throttle and the engine, with the fuel input tube connected. This way I can make it completely level or pick it up at any angle or turned in any direction while I open the petcock. The result continues the same, no matter what I do. Open the petcock and in a moment fuel begins flowing from the float bowl gasket joint. Any ideas?

Pardon my ignorance, but what do you mean by "flat across" the gasket mating surface? While I am at it, I should mention that the replacement valve seat assembly is an aftermarket part. (The Yamaha Mikuni part is three times the price.) Did I make a mistake not coughing up the big bucks?

Float bowl gasket surface can become bowed by overtightening the joint.

Remove bowl.  Pull out overflow tube.  Place a sheet of emery cloth on a flat surface.  Lap in gasket surface using criss cross and rotary motions until it is flat again. Refit overflow tube with a smidge of fuel resistant sealer at the bottom and refit bowl.

Rejoice in your leak free carb.

I found my carb leaked a bit originally . I found once I tightened up the float bowl screws with JIS drivers rather than phillips ,
I could get an extra bit of a turn on the screws , and no more leaks .

Okay, I found an error I made in my testing of my carburetor. The float is the problem. Now I find that this float # 156-14185-00-00 seems not to be available anywhere, even used from eBay. This part # apparently fits all the AT1 and CT1 Mikunis, but the AT2 models are different. Does anyone know of a cross reference for an alternate part that fits the 1969-71 models, or even a new copycat part number from China?

Post a pic to Imgur or someplace and link it here.  I have a big box of carb parts and I'd be surprised if I don't have one.  Or at least post the ID number on your carb flange.

Thanks for your quick reply, Mark. The mounting flange says 248 M2 E2. There is a 302 above the E2. The carburetor is a Mikuni VM26SH on a 1971 AT1C-MX.