TB42 Idle issue

1990 GQ, TB42 on petrol and gas. I'll start it from cold, on gas, it will start fine and idle correctly. Once I start driving and let it coast, the revs will drop out and it will stall. Once it's warmed up I'll switch it over to petrol and it does the same thing, idles correctly until I drive off and I'm up to speed or going on a downhill, let it coast and the revs drop and it will stall. Not sure if anyone has had a similar issue or not. Temporary fix has been winding up the hand throttle and a lot of left foot braking but is obviously a pain in the neck,. Any help appreciated

You probably need to put a kit though the carby - fixed mine. The LPG idle is set on the mixer gas convertor usually.

Check all the vacume hoses - they might be cracked ot have came off.

It's struggling to idle on both fuels, does this indicate a carby issue or something else?

As Tank has written, check all of your vacuum hoses are fitted and not split. Failing that, while the engine is running, Lightly spray some WD40 and around the inlet manifold, carby and hoses.

If the revs change, (it may only be slight) it indicates that the WD has been drawn into the engine through a leak.

Running an engine on gas all the time can dry out some carby components.