lpg problems. . . . knowledge needed.

hi all
after trawling through pages and pages on here i have resorted to asking for advice. i have a 96 disco v8 on lpg with two 35L underslung tanks. i am only returning around 100/120 miles to full capasity. i have had a full servic on the lpg system, new plugs, air filter and had it leak checked but non were found. i have a small blow from from one of the manifolds which i will attend to next but thats not going to make up what i'm loosing.:doh:
not sure what to try next so need some advice from you guys.

thanks

nick.

Your tanks will only fill to a maximum of 80%, so you are using 56l at most. This is giving you at worst 10mpg by my reckoning.

What sort of mpg do you get from petrol? You should expect to get 10-15% less on LPG. I think you need this info to narrow down the search area. Also don't forget that your ignition system (leads etc) needs to be spot on for LPG to be at its best. I assume that the timing has been advanced for the LPG as you have had it serviced.