LPG and Exhaust Backfiring - Please Help!

Hi all, Just recently moved out to this great country and bought an 88 Pajero (278k KM's) to run around the countryside with the kids, however I'm having some bother with the LPG system that came fitted to the car. The engine runs as smooth as you like (when running on LPG) however the exhaust makes a 'bubbling' sound whenever I take my foot off the accelerator. When I switch over to petrol the car initially picks up nicely and runs for a about 3 minutes before there is a loud pop from the exhaust.

Within the engine bay it appears that someone has disabled the emissions components. For instance, there is currently no hose from the Air Injection Valve to the vacuum Outlet (this is the thin hose with the one-way reed valve). Also, the 4-Port Thermal Valve (located in the Thermostat Housing) only has one hose attached to it - the others are open! Is there a requirement to modify the emission control hoses when fitting LPG or has a previous owner 'fixed' a more serious underlying problem? I have taken the car to two mechanics however all they seem to do is adjust the timing and charge me a small fortune!

Should I rewire the vacuum hoses? Could my problem be due to the EGR (and other components) being disabled? Any help would be much appreciated as the weather is starting to pick up and we want to go camping!!!!!

Great forum!