[Josh counts]

My suzuki vx 800 . .soon as the engine warms up . the rear cylinder cuts out. . could it be a bad coil? What are some other possibilities?.

 

[Easy Rider]

My suzuki vx 800 . .soon as the engine warms up . the rear cylinder cuts out. . could it be a bad coil? What are some other possibilities?.

Yes. Is it a carb'ed model or fuel injected ?

I would do plugs and some fuel system cleaner first (NOT Seaform).
A vacuum leak is also possible.

 

[MikeWay]

I'd do the same

 

[Aussie Steve]

If what Er suggested doesn't work I would suspect an electrical issue. To test the coil question swap them over and see if the problem moves too.

Good luck