maybe a bit late but here goes, sounds like your not getting the gas through at the right pressure, not enough of it

the Lipton lpg system

I am an oldtimer now, dont do forklifts anymore, but miss them, a bit about Lipton
L Lipton ltd was a UK forktruck national wide company in the 60's /70's known as Lipton the forklift people, agents for Komatsu, they also had a forge where they made the gas system to convert the lift trucks


look at the unit, to the left you have the smaller valve housing held on by screws, this is the high pressure side, to the right you have the low pressure side, this has a vacuum pipe attached
below you have the solenoid with 2 wires, pos and neg

the big nurled brass nut to the lower side with a couple of holes in it to aid removal, these can be tight, this is the filter unit, these can get choked up, there is or should be a small magnet to the end of the filter just sitting in there

usual faults with these units are the valves, high pressure to the left, low to the right, these are held in by two screws, the valves have tiny rubber o rings which can go hard and not seal, usualy found the high pressure side didnt give to much problem

the low pressure valve, under the right hand side larger valve housing cover can be adjusted, by moving/bending the arm slightly up or down, remove the valve cover, [remember Gas tank valve disconnected and excess let out of vapour unit] you will see a sliding clip holding the diaphram to the valve pin, sometimes this clip comes off, slide off and remove diaphram, you will now see the valve, holding down with finger pressure the sealing part of the valve, bend slightly the longer arm with other fingers, but to do this you need to have a pressure gauge fitted, usualy on the vapour unit or on the dash
The pressure output of the low pressure side should be 4PSI,
no more no less, also check the vacuum pipe,
you dont say which Lipton model unit it is, later ones had a microswitch to the low pressure valve housing, this has 3 wires going to it,
do you have any problems with the unit icing up??

you will have checked the distributor, contact points, etc etc, be careful, remember to disconnect the tank valve and eject whats in the vapour unit by pressing down on the pin inside the lpg tank connector on some item away from yourself when working on the unit