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Have you resolved this yet? If the problem is at very small throttle openings then the needle should have little effect. For most slide type carbs it mostly controls mixture from about quarter throttle up to nearly full throttle, and I gather your bike runs fine in that range. The mixture at smaller openings is mostly controlled by the cutaway and the idle system. Cutaway should be correct as BVF never offered an alternative size, but it could have been modified with a file! The 1982 edition of the parts list shows the idle jet as 35, or 40 “if required”, main jet 135 and needle jet as 70 (without cross-hole). Back in the day many riders found the larger pilot jet gave better running. I did, and also found that correct setting of the float height was important. Good luck!
Most of the changes between the 4-speed TS250 engine (MM250/3) and the 5-speed TS250/1 engine (MM250/4) were mechanical. Larger main bearings lubricated by petroil not gearbox oil, 5 speed box and revised gearchange mechanism plus new crankcases to hold these bits. The combustion chamber was still the same design in use since the ETS250, but the plug was moved to a central position.
The exhaust was unchanged, so was the cylinder barrel and therefore the port timing. Carburettor changes were minimal: main jet down from 140 to 135, and no cross-hole in the needle jet.
The TS250/1 annex to the workshop manual lists the changes, but says nothing about the change to the inlet pipe. It is mentioned in the parts list, but understandably this doesn’t explain why.
I was hoping that MZ would have said something about it in a press release or dealer bulletin, and that someone would remember seeing it. I guess you win some, you lose some.
