Engine Missfiring and Popping under Load

[JamesKT25]

Ok so. Its a 1.9 DG with LPG conversion.

Im having problems on long runs. It will potter along a treat at 55, but if i push it up a long hill or try to get to 65-70(overtaking) It will violently missfire and pop. It is running on LPG permanently as the petrol system is in the process of being completely overhauled, and i would rather it was all renewed before bunging petrol in it,(the filler neckrubber at the top is totally dead)

This problem ONLY happens after a decent run of around 50-60 miles, once the engine is fully up to temp. Annoying my temperature gauge, just reaches the bottom of the white box at full temprature

My first though was the engine bay temperatures causing a possible faulty coil to misbehave. But I'm honestly not sure...

Any ideas would be much appreciated... Im not sure if it could be the lpg system...

[kevtherev]

firstly I would be checking my thermostat to see if it is ..
1, there..
2, working correctly.

Then I would be looking at mixture (CO 1.5)
then the timing advance mechanism.

[JamesKT25]

Why would the stat cause it? I don't think its getting too hot as its done a 100 mile trip at 55mph no problem. If i stop for ten minutes and set off again it improves. I assumed it was ignition based due the the popping after a missfire,(fuel + hot exhaust) i will admit the log system may need a retune as i don't know when it was last done. Thanks foe your help and suggestions. J