I have a Yamaha VMX 540, and its ran perfectly fine until this winter. At first it wouldn't start. So i bought new plugs, and it still would not fire. So I took one plug out, to see if it was even getting spark. With one of the plugs out, the other cylinder fired right up. So i did the same for the other cylinder, and...nothing.
So i cleaned the carb on the side not firing, and that only helped a little. I got it to run idle for a few seconds, but when i gave it gas, it stalled. The carbs are sancronized, so i dont know whats going on. Any seggestions?

The carbs are adjusted at 1 1/2 turns. Maybe thats not right?

Have your pistons checked to see if both are running exactly the same, your problem seems to be with 1 piston not properly working if you show sparks on both new plugs.
Good luck, Sunshine

Yamaha ski-doo girl

take a compression test first

This might be far fetched but worth a try, A friend of mine had a simular problem, the fuel line going to one carb was blocked with ice in it. somehow it picked up water then froze inside of it, worth checking Good Luck!!