Several things that need looking at.
Mixture, could be a bit weak (like I'd put money on it TBH), possibly made worse by the system going out of tune.
Valve problems, LPG is very dry and can lead to valve wear. If you are unlucky like me it could also be caused by incorrect set-up for petrol, I could destroy an exhaust valve in about a minute with our first V8, 'till I figured out what the previousd owner had done (the donkey). Compression test first, though now I'm after a leak-down tester.
Timing, set it up for petrol, it will be a bit gutless and may sound a little hollow on LPG but should run ok. I run an ignition advance processor (had trouble with it mind you) because you can over advance the engine for LPG. Unfortunately that makes it harder to start and will damage the engine if left like that for petrol. LPG has an octane rating of about 112-115, unleaded is about 95 and 4 star 98 but LPG burns slower, so ideally a much higher compression ration and/or advanced timing, but like I say it isn't safe for petrol so set up as per the manual.
Carbs, you have stroms or SU, both should ideally have piston lifters for LPG as the variable venturi should be wide open. Otherwise it can cause some running problems and also premature needle wear, on account of there being no petrol on the needle to lubricate it.
Given that it's popping on over-run I'd look at mixture first (may need help with that) then your carbs. BTW have yuou noticed your MPG on petrol get worse since the LPG kit went on?
