hello,
i have a 1995 gmc jimmy slt 4.3l vortec engine 4x4. the didn't ride on gasoline almost 4 years, it runs on lpg. for the past 2 month i'm having a backfire from the intake manifold and the chek engine start to flash (miss fire). i knows i'm having a fuel linq from the regulator in the spider injector. i have replace: air sensor, coil,ignition modul, spark plugs, wires, distirbutor, fuel pump. still at 2000 rpm i have a back fire.
today i put two bottles of injection cleaner "wirdth", and i could run on fuel but still with back fire. some one told me it could be a sticking valve or maybe a burned valves.
by the way, the egr sensor is ok.
help me please, don't know what to do?

 

Possibly a stretched timing chain.