My Yamaha '77 XS 400, mitsumi carbs, starts very hard, regardless of choke setting. When it finally starts, it roars at 5,000 to 8,000 rpm. I've lubricated the throttle cable to make sure it isn't sticking. The throttle link arm seems to be almost resting on the remote throttle stop screw when that screw is all the way backed off. I've disassembled and cleaned the carburetors, but that didn't help. Also removed and cleaned the gas tap and gas filter. The vacuum tube from the tank is a little loose where it goes into the inlet stub, but it was like that when the bike was running okay. This problem started when I fired up the bike for the first time this Spring. Had been running okay last year. I'm all out of ideas to try to fix this problem. Any suggestions would be hugely appreciated! Thanks.