Has anyone fitted a 34mm mikuni carburettor to a Yamaha Dt175mx? Any good?

Greetings from Tennessee 

Welcome from NZ. As much as i love ideas like that, i think its overkill on 'that' motor which as far as i know is only pretty much a DT175 so not ported enough to get the advantage. Not sure you could find a manifold big enough with 'that' bolt pattern to do the job. Plus would the bigger carb sit up off the deck--might sit on the case as physically taller--deeper.
I had 'of all things' a TY50 years ago & went 'way big' in the carb but the air flow was so low over the needle jet it wouldn't suck the fuel up the main jet good enough on full throttle no matter how big i went. In hind sight probably the slide needle was restricting it. Silly idea that sounded good in theory, 35 years ago.

Biggest carb that fits the original manifolds is a rare "small body" 30 that was used on a few mid 70's 125 MX bikes. 34 (and commonly found 30's) are a larger body size and will not fit.

And as was said, not worth it. The 24 is plenty big enough... I've tried a 28 from RD350 and if anything it hurt throttle response... no measurable performance increase. Even though I spent a lot of time jetting, top speed was down a couple MPH with a 28.