Good fire at both plugs. Changed plugs and cleaned magneto anyway. Both cylinders have 115 pressure. If I disconect the right spark plug it smooths out. Disconecting the left plug runs very poorly. Have made sure all carb. screws and manifold bolts are tight it's getting plenty of gas. Anybody have any ideas.

Have you tried replacing the spark plugs??

Yes I have replaced both the plugs. I should've mentioned that. That was the first thing I tried.

if your disconnecting the right cylinder and it runs better, I would be thinking your ignition moduel might be bad

I will switch it out with another mower and see what happens. (I have a couple of 20hp B&S that I suspect will have the same module).