Phil, as others have said this can be a feature of the bigger bore bikes.
Plenty of tweaking and you'll find a setup that suits your main style of riding.
I dropped the needle on my RT1 and it's solved the surging when off throttle, which to me was the worst bit.
It's not as smooth mid throttle but is fine when you give it some beans, so I just ride it like that on the rare occasion I don't pick a 175 for a ride out

Thanks for all your responses.

There’s a lot to think about and to experiment with.

I’ll start with stripping the carb and double-checking the sizes of everything and squirting another can of carb cleaner around it (from experience, don’t get that stuff in your eyes….).
Are all the sizes written on the carb components? Do I remember not seeing a size on everything that was listed in the service manual?  Do you have to physically measure some holes etc on some components?
Sorry, that wasn’t very specific. I’m just trying to think back to when i stripped the carb last time.

I can’t remember removing the main nozzle when I last stripped the carb. How does that come out? Twist/pull?

Regarding the UFO, is that a mod to the slide only? or is another part of the carb also modified? Do I send off my slide to be modified (I’m in the uk) or do I buy a new, modified slide? I’m not at that stage yet, I’m just curious.

I’ll report back with my findings…

Thanks again.

Original Mikuni jets and main nozzle will have the size/number stamped on them, alongside the Mikuni logo - this is a small square in the corner of a larger square.

Main nozzle can be gently tapped out from the underside (mine came out whilst cleaning with a pipe cleaner) and make sure that you refit it the correct way round - there should be a locating lug.

Assuming that your carb is like that of the earlier bikes, then make sure that the 'O' ring that sits over the setter jet is in good condition - you should feel some resistance when you attach the float bowl. New 'O' rings are still available from Yamaha.

That little o-ring in my carb was completely shot and falling apart, so I ordered a couple from an eBay seller, $7 shipped for 2. While I was waiting for them I stopped into a local mom-and-pop auto parts store and asked if they had any small gas resistant O-rings. They pulled out a selection and I grabbed one that looked like the right size,  put it on the nozzle, and it fit perfectly. The O-rings I ordered from eBay arrived yesterday, and they are identical to the one I bought at the local auto parts store for $0.50. So it's not a specialized Yamaha part. It's a standard O-ring, you just have to make sure it's gas safe. Usually the ones you find in a hardware store will not stand up to gasoline.


  

Thanks. I’m not too sure my carb has that o-ring. I’ll double check when I strip it down against the diagram in the service manual…

You very well may not have that O-ring on your carburetor, mine's a 1971. But it is good to check.

I experienced similar on my ct3, I improved it somewhat by up-jetting but the real problem was an air leak from the crank seal. I would recommend a leak down test just so you can eliminate it as a possibility.

Yes, the UFO is a mod to the slide, I think Rich also puts in a leaner pilot jet too, the UFO only will make it richer, like slobbering rich.
here you can see the UFO mod at the bottom of the page.
richstayloredporting.com/carb.html

I am planning on getting cylinder and crank case porting done and a UFO mod done to my 1975 DT400B.