Clark Propane forklift runs rough at start
I have a Clark GPS25 MC lift with a Mitsibishi propane engine that runs rough at a cold start. Once it has started and run for a short period it smooths out and runs fine until the next cold start. It starts a little hard then sounds like it is running on one two or 3 cylinders until suddenly it clears up. Occasionally it will start right up and run fine from cold. There's no discernable connection to air temp or weather conditions, or how long it's been sitting, except very cold weather makes it needs more cranking to get the first cough of ignition.
We've replaced plugs, wires, cap and rotor, vaporizer, regulator and carb diaphragm over the past year, with no real change. We've run out of people to ask locally for advice, and I'm reluctant to turn it over to a dealer with a blank check to sort it out. Leaving the key on for an hour before trying to start it makes no difference. The ignition coil is now wired directly to the key switch to bypass any harness issues, 12V at the switch goes to a ballast resistor then to the coil. I have not replaced the ignition module as of yet.
When I first bought it it started fine, this appeared after more than a year of 1 to 2 hours a week of use.
Thoughts and suggestions? Help!