Hustler Super Z with 25hp V-twin Kawasaki FH721V-AS12. Had one vented fuel cap and one unvented. Both tanks 3/4 full. Mistakenly tightened down unvented cap. While cutting level ground, vented tank self filled, unvented tank almost emptied. Yes, that seems backwards. Smelled gas "squirting" out of vented cap. Loosened unvented cap, tank levels equalized. Problem solved... I thought. Few minutes later lost power, as if on one cylinder. Some white smoke. Pulled plug wires, found cylinder near fuel pump was culprit. Both plugs carbon (not oil) fouled. Replaced plugs and that coil, adjusted all valves. No change. Swapped coil positions. No change. Still same cylinder. Coils properly gapped with no wobble on flywheel. Woodruff key undamaged and in place. Compression 120 & 170 (problem cylinder). Minimum spec 57 psi. Ran Sea Foam to remove carbon. Started backfiring through carb. Disassembled heads. Mild carbon build up. Sea Foam hadn't done anything to remove it. Don't waste your money. Cleaned heads and piston faces, lapped valves, reassembled engine. Valves, guides and seats look OK. All valves set 0.004" (0.003-0.005 spec). No change in performance. Disassembled and cleaned carb. No debris noted. Jets look fine. Did not pull caps. Blew compressed air through all passages. Changed fuel filter. Checked fuel pump. Fuel output OK, but hose to cylinder head is hard and should be replaced. Can't say that's the problem. Reset the governor arm. Verified no air leaks around carb or intake. Tried cutting grass. Weak at first and gradually lost power over 30 minutes. Almost didn't make it home. Again the same cylinder. The engine runs fairly well cold but has miss on bad cylinder. Can consistently locate bad cylinder by pulling plug wire. Spark plug tester light "looks" like plug is firing normally even when engine misfiring. It's hard to tell. The governor searches a little. Probably due to misfiring. Air filter is for externally vented carb and is correct. Both plugs carbon foul within minutes. Tried plugs with 2 step higher heat range. No change. Was going to replace carb jets if available, but read in forums of people with similar problems. Replacing whole carb didnt help them. So... Why is this thing fouling plugs so quickly? And why is it losing power on one cylinder consistently when hot? Its not the coil, plug, flywheel or top engine bearing, timing, fuel pump, fuel filter, valves, compression and probably not the carb. As a side mystery, why did the unvented fuel tank empty and the vented one overfill? That's just wierd.