My Exmark Z with 60" deck and 25 horizontal shaft kohler starts hard. Seems to run OK once running until engaging mower then cuts out, misses and back fires. Thought I had it fixed when I located some electrical problems (fuel solenoid, 20 amp fuse socket dirty, not making good contact) finally gave up and took it to the local Exmark dealer, they called today scratching their heads. Seems it only runs on one cylinder but spark test shows good spark to both when it runs rough and backfires. These guys have been in business more than 25 years and I have confidence in them but can't explain this. The hour meter shows 1400+ but it may have been replaced as the unit is a 1998. The tech said compression looked ok. He said he checked all the normal stuff be he is baffled too. HELP!!!