G reg 2.0 petrol Fourtrak


I've recently bought a G reg 2.0 petrol Fourtrak. I've been trying to find out how many miles I'd get for £20 of petrol and think I may have let the fuel get too low. After filling up last week it back fired all the way home. I left it over night, back firing has stopped but the engine is very jumpy when ticking over and cuts out when I stop. Any advice on how to fix? Thanks.