I had a VM38 YZ465 carb in a couple of weeks ago for a rebuild.
The 465 is well known for poor hot starting and general carburettor misbehaviour.
I took a close look at the air adjusting screw which was not the normal short head type, it had an extended length enabling you to adjust it with your fingers.
I'm not sure if this is standard to the model or may have been an aftermarket part.
The problem was the part of the screw the spring fits over, was about 3mm too short. This meant that when you screwed the screw in to bottom it, to then screw it out to get the adjustment, the spring was becoming coilbound and stopping the screw hitting bottom by about 3 turns.
So when you then screw it out the 1 1/2 turns or whatever to get the adjustment, it was actually out 4 1/2 turns making the pilot circuit very weak.
We machined about 3mm off the bottom of the thread which ensured the screw bottomed correctly when screwed right in.
Unfortunately, I didn't take any photos at the time but it's worth checking the screw bottoms the same with and without the spring fitted to prove it's not the spring becoming coilbound.

Recently received a set of 4 x VM34 power jet carbs from a TZ750 for UFO fitment.
The customer told me they had been bored out to somewhere around 35mm.
At first inspection, it was obvious there were problems.
The boring job looked like it had been done concentric to the original factory bore.
Because of this, metal had been removed at the bottom of the bore which would otherwise provide the bottom edge seal on a closed throttle. You will see from the photo that with a closed throttle, the slide sits quite high. This would stop the carburettor from achieving a fully shut throttle and may leave it with an “idle”. 
Overboring if there is enough metal to do so, is normally done slightly high to retain the sealing edge at the bottom of the bore.
The point of us machining the body after fitting the UFO is to make sure the slide sits down properly on a closed throttle. It is not possible to do this if the metal has been machined away.
 
The second problem relating to the overbore is the amount of fresh air slipping down the sides of the slides as there isn’t a sealing surface there anymore. You can see this quite clearly by the light shining through at the sides of the slide.
The diameter of the slide in a VM34 is 35.5mm. The same slide is used in the VM30, VM32 and the VM34. You can see if you bore the 34mm version any bigger, it will cause problems which cannot be addressed.
Unfortunately, I couldn't fit these carbs with UFO’s for the reasons above. The carbs are not usable in the condition they’re in. The bodies are probably scrap.





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