Hey so I need some advice in which direction to look first

back ground info:
I have a hyster S120XLS serial# D004D08510X I just replaced the water pump along with the thermostat and belt due to a leaking water pump got it all back together yesterday ran it and burped the coolant system I let it idle and warm up then I reved it to a higher rpms multiple times no problems what so ever everything was perfect then I let it sit over night was going to return it this morning

Problem:
today I started it up and let it idle for 5-7 minutes to warm it up before I took it back to the shipping department well as I backed it up to leave my shop it backfired coughed and died.... so I checked the fuel tank (its everyone's favorite prank to turn the lp tank off) every thing was good and I had just over 3/4 a tank of lp got back on the lift and started it, it fired right up no issue idled fine reved it up and it gave a series of tiny backfires I figured it was still cold since we were down to freezing last night so I idled it around the parking lot trying to let it warm up anytime I hit the throttle to fast it would back fire and cough brought it back to the shop all vacuum lines are attatched and the LP filter is clean now I am lost as where to start looking for the issue


any help would be greatly appreciated thank you in advance