Backfiring on overrun

[Jitterbug23]

Hello I'm new....

Just bought a t25 it's a 2.1 DJ running petrol and LPG. My problem is the same regardless of fuel type, maybe slightly worse on LPG.

It runs fine right through the rev range and idles fine too, but on overrun in any gear it backfires. I've replaced the plugs, leads, dizzy cap, roter arm and coil, but no change. I've checked the vac advance to the dizzy and that's ok. I've had a general check for air leaks and can't find any. I have ruled out an injector because they are bypassed on gas. I have checked the timing and that's ok.

Just done a compression test on all cylinders and they are all good.

I haven't touched the fuel pump or filter yet. My thinking was that if it's backfiring on the overrun then it's too much fuel not lack of? Next on the checklist though.

Any advice would be great. Thanks in advance.

[big red bus]

exhaust blowing

[CovKid]

Weak mixture can do it too but agree, more likely to be exhaust pipe not tight against head (blowing gasket or stud/nut loose or missing) or hole elsewhere in exhaust system. You should be able to find leak by feeling around for it on an engine thats not too hot.

[Jitterbug23]

Thanks for the tip. I'll have a good feel round it tomorrow and report back. My other thought was possibly a sticky valve or dirty valve as this wouldn't necesserily show on a compression test.

[kevtherev]

Backfire on the overrun is a classic fuel weakness.
Check and test everything that uses vacuum for leaks.
Gaskets can and do split with time

[CovKid]

As said, it can be either Kev but given that holes and leaks are not uncommon on these exhaust systems over time it would be wise to check that first before messing with or checking intake side. I only need a small leak in my exhaust system to get backfiring when I take my foot off the gas - particularly downhill. He does say he's checked for leaks up top.