Try gapping the plugs to less than 0.032"
The spec in the manuals is only applicable to the Gasoline version.
As this is a wasted spark system, the spark has to jump two plugs in series. !!
The previous backfires, may have created a vacuum leaks...check the vacuum hoses, etc.
The air leaks this problem has created, has contributed to many a dusted engine.

I have same problem on a 1994 hyster with 2.2 gm. The engine make one back fire in intake when we start hot. Just after the back fire, we re-start and engine run good. You can work with it all day. Just make one back fire when start. If you wait about half an hour, it don't make is back fire. We put new cylindrer head gasket, verify head, new wire and spark plug, new vaporisator and lock valve. We try another coil pack, new crank position sensor. I need new idea to resolve problem too :(

I would suggest using a Vacume Guage on your GM 2.2 or 2.4 if it goes erratic you have a Valve hanging up.
This happened to us recently only when Hot under load.

it is the spark plug wires i have one that we service and we replace the wires every season. the heat breaks down the wires use good wires.

check the start delay fuel solenoid. (blue solenoid between carb and regulator) I have seen many of them get over loaded with propylene from the L.P. fuel and sludge up when cool. Once the engine runs a little bit the propylene warms up and the diaphram opens and closes properly. If the diaphram sticks open it will dump fuel right into the carb and possibly cause your problem. That solenoid was originally put in place to slow the timing of the fuel into the carb. The early G.M. engines used to fire off too quickly and break off the nose cone on the starter.

if its an old pre emission model
try taking the carb apart and clean it out
if it has an o2 sensor do not take it apart

also on pre-em model
it should get about 1.5 - 2 lbs of fuel pressure at teh 1/4" pipe plug on the regulator (impco cobra or jb2)
more or less and its not gonna run right
usually on of the diaphragms get a tear and flood the system and you get hard starts and other issues
to see if its flooding take the vapor hose off the carb and let a lil air in while you crank it
if it starts right up and or you can get it to smooth out that way
the regulator is bad
also if it has a vacuum lockoff
once or twice ive seen them go bad and pull fuel through the vacuum line into the carb again flooding the engine
so check that too

itd help to get the serial number too

also new models with emissions
the fuel pressure dosnt feed the carb
the updraft from the carb turns the regulator on
so make sure your vacuum is good

On one unit we have replaced the complete ignition system. Module, coils and wires pick up. It seems as though the engine can't pull enough fuel from the regulator while cranking. I suspect worn carbs but they are not cheap. I was hoping someone else had run into this same probem be fore I order a carb.

Are the spark plug wires in good shape?

check the valvetrain timing?
i forget if these are chair or belt
i think it has a chain
check valve lash and i forget how the ign times itself but check all the sensors (on the pre emission hysters with coil pack GMs)

does this have the emission system?
if it does make sure you have the trigger wheel (for the rpm sensor) bolted correctly to the crank pulley it only has
3 bolts so you have a 1 in 3 shot of getting it right
at 120 deg out either way youll never get it to work right
i had a hyster GM in my shop the guy took the engine out for a reseal then put this wheel back in the wrong position
and it had very similar results like youre describing
but if no1 took the engine apart recently this may not be the problem