SV830-0013 engine in Toro zero turn smells of fuel and very low on power

Hi all,
First time post. The Kohler SV830-0013 engine in my Toro 74815 zero turn is having problems. It's very low on power, smells of fuel, often sounds like it's faltering - i.e. at full throttle it's RPM isn't consistent.
Oddly, it has no problems starting, doesn't appear to be blowing smoke, the choke lever appears to be working, and it never stalls when driving. It only stalls if I'm cutting through more than light grass.
I just replaced both spark plugs, oil and oil filter and the air filter.
The mower show about 350 hours.
Any help would be most appreciated.
Michael
 

Thanks.
 
Both spark plugs were black and sooty and the gap looked fine. I replaced them anyway.
Air filter is brand new, and yes, I ran it briefly without the filter. No difference.
 
>> Have someone crank the engine over (plugs out) and LOOK for spark from both.
I presume they're both firing. If one cylinder wasn't firing, presumably I'd have problems starting the engine and it would sound horrendous? Engine starts fine.
 

Thanks all. I found today it's running on one cylinder. From my troubleshooting, it appears the left ignition coil is dead.