Hah! Thank you for the response! Two reasons why many of us are here! Fun project motorcycles and great reads! I guess the best way for us to find out stuff is to try and fail and document, so I'll try my goofball ideas and post the *ahem* amazing results, warts and all! Love your threads!Thanks for the support mate.
I stuck with the CV carb for a couple of reasons. One being they are very easy to tune and offer a good compromise for low down power and high rpm power. They also self adjust pretty well to different air conditions (altitude etc) and compensate for minor tuning errors. A slide operated carb may offer better throttle response but at the cost of bog if not controlled well (rider control) and if it does not have a pumper circuit. It may even flow better at high rpm, but no one knows because no one has done back to back testing (on the SR at least), including myself. Another reason is purely business - improving on what is already a good system is often better and more reliable that starting from scratch with an entirely new system and trying to make it work. There is also the cost factor - a new carb can be pretty expensive and is not always in the budget for an SR build. I have had the SR on a few different dynos for several days at a time and think that this would be required to get the VM to run well and be able to test it with different air filters and exhaust set ups. But thats just me, I like to do things thoroughly rather than just stabbing in the dark. I have also spent countless riding hours with a home built O2 sensor with and air-fuel ratio gauge set up doing countless jets swaps to test different set ups with the CV carb, so I am quite invested in it.
The only reason CV carbs can be finicky with air filters is if they cover the vacuum intake port (think cheap EMGO pod filter), otherwise any type of filter will work.
In the past I have somewhat successfully tuned a VM34 carb for a DT400 and thought they were pretty easy to work on. But beware, there are different needle jets (emulsion tubes) for the VMs for two-strokes and four-strokes - the two-strokes have somewhat of a shroud at the base of the throat of the carb - this is to do with the different type of vacuum two-strokes pull.
The XT250 came with a 28mm roundslide pumper carb. Already there you have two huge differences with the VM34 - the throat diameter and the pumper circuit. Without going into all the details there, these two factors affect a lot.
If anything, I would want to test a TM33 flatslide pumper carb. 33mm being a good compromise for low end and high end power and the pumper circuit giving good low end response and resistance to bog. The flatslide also flows much better than the roundslide. Alternatively, I would be very keen to test a LECTRON carb at some point! They look like the business! But again, would probably require a lot of testing.
I have had little time to work on my bikes recently and this long winded response shows that I am spending too much time working at the computer and too little time in the workshop! Time to get on the wrenches again Best of luck with your build and carb!