I’ve got this problem and I’m struggling to find these two parts here in the U.K. I’ve looked in the archives and it seems people are suggesting it’s possible to fit the main jet directly into the main nozzle. Can anyone let me know if it actually works, thanks.

The O ring is still available from Yamaha.

If you haven't done so already, try Motocarb or Allens for the needle jet setter.

Niche Cycle in FL is really well-stocked
Also have had luck on ebay searching by "mikuni XXXX) where X is the stamped number/letters on the jet, or using the Yamaha with the yamaha part no off of Partzilla

On some models and needle jets the main jet thread is the same as the jet setter.

It's hard to see the numbers in your photo but it looks like the needle jet is a 188 series? So you probably have a 250 or 360. You CANNOT screw the original hex main jet into a 188 needle jet, threads are different. You can replace the 188 series needle jet with a 159 series which does have the same threads as the original hex main jet. If going with a 159 series, get the same size... looks like P-0?

Probably easier/cheaper just to get a jet setter... any good Mikuni shop should have one. 159 series needle jets are also very common. 188 series needle jets are harder to find if you need one.

Oh, and in case it wasn't obvious, there are TWO jet setters with different threads. You need to make sure you get the one for your model with the 188 series needle jet.

I'm pretty sure that means the earlier model jet setter.. pretty sure the later models may have had the jet setter with the same threads as the main jet. (i.e. 159 series needle jet)

Further to Marks post . If your needle jet was the 188 series , you could fit a Round Head main jet
directly into it , as it has the same thread pitch . The size of the jet required to match the hex head Main Jet
would have to be determined , as the hex jet is number rated for flow , and the round head is rated for apperture
diameter . This is just another option , but I'd recommend taking Marks advice and buy the correct Jet Setter
and O ring .

Thanks for all the excellent info, it is a 360, I’ve sent a few emails to Uk shops for the parts. Allens performance shop has this great page listing O rings by size, very useful just in case these parts ever completely dry up. 

Yes, the round jets have a completely different numbering system.

A bit of trivia is that the 4/042 hex main jet so commonly used actually has flow measured with fluid entering in the threaded end a exiting the flat end... the way the fuel would flow if installed in the float bowl jet holder. This comes from the original Amal design (and in fact, some threads in Mikuni are standard, not metric).

The round main jet was supposed to be for fuel flowing in "reverse" from the flat end to the threaded end... like if the jet was installed directly into the needle jet where the jet setter goes...

This is likely the reason for the different threads?

Anyway, as time went on and carbs got spigot mounts that allowed for main jet changes by rotating carb and going through a simple cap in the bottom of the float bowl, the common hex jet continued to be used... even though when a hex main is installed directly into the needle jet the flow was "backwards" from the design and the precise flow calibration stamped into the jet was no longer accurate... the flow rate changes slightly based on direction of flow.

(Mikuni main jets have different "lead" and an "exit" tapers... precise machining... not just a hole drilled into a brass bolt)

It turn out the difference in flow is either so small or can easily be compensated for so in the late 70's the common hex jet was used even though it was "wrong"... and the threads in needle jets were changed to allow using the common hex main jet.

So that's my story behind the screwy "different threads" (pun intended) in the needle jet... it prevented using the hex jet "backwards" and bypassing the jet setter... but then eventually it became common to use the main jet "backwards". Go figure.

I might try it out while I’m waiting for replacement parts. The bike was running okay with that raggedy old oring. Typical situation when you’ve got a deadline to meet and things start to mess up. I’ve entered the bike into the Red Marley hill climb this Easter Monday, not long now ! 
 redmarleyhillclimb.com/

MarkT said:

Mikuni main jets have different "lead" and an "exit" tapers... precise machining... not just a hole drilled into a brass bolt

That's why it's best to start with new parts in unopened packages from reputable sellers. I've found jets that were drilled out and needles that were reshaped.