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Does anyone know which Mikuni Carb model best fits a 1965 Monza 250?

You might find an old flange-mount MIkuni or Keihin in the 24~26mm range that will match up to the standard Ducati studs. A search of salvage yards or, if you're rich, a call to Sudco or other carb supplier might yield results.

As far as I know, none of the Mikuni manifolds used for spigot-mount Mikuni will fit the standard Ducati intake studs. If you do find a special made one it will be very expensive.

If you want to use the later spigot-mount, round-slide Mikuni you'll have to fab your own mount, which is what I did.

Here's what you do: once you find whatever carb you want, get a piece of plastic PVC pipe. If you have access to a lathe, bore out the ID of the PVC pipe so that it slips snugly over the stock Ducati manifold, then turn down the outside diameter of the PVC pipe until it has the same outside diameter as the Mikuni spigot. Slide the PVC pipe over the stock Ducati manifold (use some gasket sealer to prevent air leaks), then use a short length of water hose and a couple of hose clamps to join the carb and PVC pipe. You should also open up the inside diameter of the stock Ducati manifold to match the ID of the Mikuni. This merely needs to be a gentle taper. Total cost should be less than $5. I've been using this setup on my 350 for years with zero problems.

Some folks just bolt on an Amal concentric carb from a Triumph or BSA, because these match the Ducati Studs (an older Amal Monobloc will also work). While both these carbs are probably better than the stock Duc carb, the Mikuni is far and away the best choice.

Awesome!!! This helps out a lot. Do you think this Mikuni VM26-606 will work? Below are the specs.

Carb No. Size - MM Main Jet Pilot Jet Needle Jet Jet Needle Throttle Valve Air Jet Needle Valve
VM26-606 26 190 22.5 O 0 5E75 1.5 .5 2.5

Does this flange look like the one you designed for your Ducati?

-Alfredo

That nice manifold will probably not fit your Ducati, the stud holes are too far apart.

If one of those trick Mikuni manifolds would fit, I would have used one! They don't, that's why I used plastic pipe.

As far as size goes, 26mm is best for a 250cc street bike.

For jetting, look around on the web. For a starting point, you could always using the jetting for any older 250cc single. Sudco might also have jetting specs. The jetting for a Mikuni on a twin-carb Triumph 500 should also be pretty close (it's two 250's).

Or, just bolt it on, start the bike and see how it runs. If it's rich, lean it out, if it's lean, richen it up!

One other thing, the throttle cable might have to make a very tight turn to clear the fuel tank. Mikuni makes a metal, 90° tube thingy that prevents the cable from kinking where it exits the top of the carb. I've never had any problems, but it's a mod I've always wanted to make.