I have an RT3,and ive replaced the original carb with a Mikuni 32mm,as opposed to the standard Mikuni 32sh.{with flange}. I bought a carb inlet rubber to fit the new carb to the original alloy inlet with reeds.Thing is it doesnt fit,do anybody out there know where i could source one here in the UK

Is it far out?. On one of mine i slotted the holes to get them there & used allen screws as the heads are round & could get the allen key to operate where nuts would be difficult.

Mikuni might make an adapter if that's what you need?  They probably won't have it listed by model for a bike that's over 50 years old so you'll need dimensions for the bolt spacing and spigot size.

Here's a list with dimensions and Mikuni part numbers...  one thing is Yamaha often specified smaller body carbs than the "generic" carbs you buy today.  So the original 32 might have had the same body dimensions as a generic VM28 does today.  prelive.nichecycle.com/carb-flange-carbu...ubber-by-mikuni.html

(I know the original Yamaha-spec VM28 and even some of the original VM30's back then had the same body size as a VM24 today...  unless you find NOS, a new off the shelf VM28 will not replace the original because the body is too big compared to original)

Best bet is almost always to find an original carb.

The holes in the rubber boot dont alighn with the threaded studs,and when i fit the nuts there is not enough room for them,as if the circumfrance of the nuts is too big.This then distorts the boot,really need pictures to show this.I thought this was a common modification,to put a later model carb on ,as the originals are hard to find

It was very common.  First you need to get the correct adapter.  Second you need to remove the studs and use Allen head bolts.

On the RT 360 models the carburetor studs are fixed to an allioy intake manifold,this is then attached to the barrel with four studs,betwween them is the reeds..I cannot see away of removing the studs from the intake manifold,or should i discard this altogether

This is not intended to sound harsh but it might...  If you don't have the experience/tools/knowledge or whatever to be able to figure out how to replace the studs with Allen head bolts of the correct length, my guess is a "carb swap" to a different type might end badly or not at all?  There can be the need to fabricate solutions to mounting...  beyond the boot you're talking about there's need for an air filter connection and throttle cable that might need to be fabricated...   Then the new carb will need to be jetted which can sometimes be very frustrating.

Most of the time, staying with the original carb is the best and cheapest option. 

That said, here is a link to an ad for a stock manifold which was never supplied with studs...  the studs are separate parts.  The manifolds are usually not that hard to find NOS, so buying new would be an option vs. removing the studs from your manifold.  Ebay manifold link.

Grab one stud with alloy soft jaws in the vice & turn the manifold as far as you can to free the stud thread.
Then it'll probably turn out by hand or two nuts locked together.
Do that on each one.
There's various things you could do if you had the gear.
If there's enough meat to drill an 8mm short piece of thread & tap a 6mm thread down the middle then screw it into the manifold then you can use 6mm allen head bolts on the manifold with a trimmed washer to get in close.
Or maybe forget that!!. I do desperate things haha but usualy they work.

Or might be easiest to get a rubber complete manifold. I've go two on different 360 motors but probably not yamaha originals as i just make stuff work. I'll get a pic or two later today & see if have numbers that might give a model code.

This'll be confusing but i put all these pics in an album & linked the album so just go with whats useful. Adapter bolted to an old 360 carb bolts right up with 8mm bolts & nuts. Nuts are 12mm spanner & them plus nuts have a slight flange but bolts up npretty good. The 32 carb i sat in has a 40mm od. Manifold is way bigger like a 36mm hole. Can find 34 cast around the edge of the rubber but its bigger--or i've been at it lol. I used to run a 36 on mine once apon a time.
Ignore the clutch that's from an old rm250 from "81 i think & believe it or not it was still working ok haha. That flat slide carb on my 360 is a 32mm EI Carb--never heard of them?. The rubber manifold on that is kawasaki old 125 & has a triple reed block with original fiber reeds by memory. Bolt holes line up with the 360 cylinder reed housing but manifold & reed block are marginal at the edge with minimal overlap hence the rough as guts gaskets i must trim up. Its hell in my shed [& head] i can tell ya haha. Think the pic with the boost bottle hole bunged is on my RT2MX cylinder but i had a 36 on that so that manifold's no good but housing holes line up even though it takes a wider reed block than the RT2.
No useful numbers on the adpter shown but they must be out there with your stud spacing i'd think.
DT2MX would be a good guess as think they had a 32 but even a 34 probably had the same od on the carb into the manifold.
Link below of the manifold on cmsl is crazy expensive so ignore that.
If you go looking for a used one be careful because if you go MX250 they're a wider housing by memory, wider triple reed & some sellers might call their item for the earlier model but is a year out & when everything changed.
Are you still awake, ok Sleep well.

photos.app.goo.gl/srDssNwh4TZWgArT9
www.cmsnl.com/yamaha-dt2mx-1972-usa_model8401/joint_3101355500/