Motorcycle Repair: XS400 Carburettor, air fuel mixture, yamaha xs400

Question
Hi, I have a Yamaha xs400 Special (81 model) The previous owner took the original airfilters off and replaced them with small ones fitting to the carbs directly, resulting in too much air flowing into the carbs. I drilled main jets bigger to compensate, but have a flat spot and sometimes misfire in lower revs. Once throug the flat spot, it goes like hell. What can I do to get rid of that flat spot?
Tx (Drilled main jets from 1.5mm to 2mm)

Answer
Hi Vos,

I suggest replacing the main jets with the stock sizes, maybe one size larger if any.

Tune the air-fuel mixture screws and pilot jets to rid the flat spot.

Read spark plugs to learn if mix is lean or rich. Adjust carbs accordingly.

Respectfully,
Mark Shively