Is the main jet threaded into the main nozzle or into the jet holder in float bowl as standard?

Yamaha used different threads on main nozzle (aka needle jet)on many models and standard hex main jet will not thread into it easily. 

You can get a needle jet that accepts the hex main jet if you want to bypass the float bowl jet holder. 

What numbers are stamped in the current main nozzle?  Should be three numbers and a letter-dash-number.

P.S.  the two numbers you are listing are for 250 and 360...  you need the correct one for 360 which can be ID's by model code.

I have had some success in obtaining Mikuni carb parts from Steve at Motocarb.

Allens are also in the UK.

214 prefix is for the DT1 250 and 275 is for  the RT1 360.

Okay, that's not the main jet...  that's the jet setter. 

188 is the series and P-0 is the size...  a good Mikuni dealer might be able to supply both.  Make sure to save and use the thick washer that is on the jet setter in your photo.

If you can't find a 188 series needle jet, you can also use a 159 series BUT you'll need to use a jet setter (part with o-ring that seals into float bowl) with a different thread. 

If I remember correctly a member in the UK reported being able to get the parts from Motocarb...  not sure if Motocarb had the 188 or if converted to a 159 and new jet setter.

(159 series has the same threads as a Mikuni hex main jet, 188 does not)

The genuine Mikuni 188 P0 is no longer produced .
There is a seller on Ebay who has a few in stock still , listed under the part number .
They aren't cheap , but are virtually all but non existant .

If ya stuck & itching to go riding it & considering the female thread is toast you just 'might' be able to get by in the mean time by just screwing the main jet in direct, with the fat washer of course providing it doesn't hit the bottom of the bowl before the bowl is against the gasket. I shouldn't be teaching you bad habbits sorry haha. Waiting for thumbs down. Have you got a tap & die set.

Morning Simon.

If it's possible to do it with the barrel/cylinder in situ, have you tried removing the decompression valve and cleaning out any carbon build up with something like an old paper clip?

"He’s more than happy to push it back into the garage and give up."

At which point you offer to buy it off him (nice and cheap) and tell him that you're doing him a favour!