I remember being taught that if you convert an existing engine to LPG, you will get [as a rule of thumb] aboiut 10% less power & about 10% poorer fuel economy, but since the fuel was about 1/3rd the price of gasoline, then it didn't matter.

Then I was told that if you reguild the engine as a dedicated LPG engine, then you can raise the compression up to about 12 to 1 & this would pretty much restore the missing 10% in both power & economy, which makes it even cheaper to run.

LPG hes a number of very real advantages, but one or two drawbacks, as you are now discovering.