I have the Kawasaki big bore exhaust on my KLX. I had to go down two sizes on the needle jet and down to a 350 main. Now these oval bore mikunis are pretty similar to the SP370 carb. Generically they are the same family. On my Suzukis I've had to jet down not up when derestricting the inlet and exhaust. Other's tried this [I think old fart] and got the same results. I'd say you're running way rich. Theory is that as the carb flows more air the suction effect on the main jet is so much greater that you have to jet down. It's counter intuitive but it works on these carbs. Generally I'd drop the needle too. A lot of the old articles say these carbs are rubbish, but I've found them to run really clean. I think a lot of the tuned back in the days stuff was blubbering due to running rich.
The SP370 and KLX use the same size, 32mm. The SP was known to have a very restrictive airbox and 32 is a big carb for a tiny valved 250, big for any 250 of that period, even the DR400 only went to 33, so I doubt the original flow was optimal. I've no idea why they used oval bores, I'd love someone to explain.
