LPG will cause pinging when running rich.  Toss the rules of liquid fuel combustion out the window when working with vapor fuels.

Franz

In the school buses that I've dealt with ,the engine timing had more effect on engine ping thn the fuel richness. If fuel is too lean the engine won't have any takeoff power at all. Engine timing too high will burn holes in pistons.

Could be either.  Just another reason for careful reading and research!

If you have a critical mixture going into the cylinders -- e.g., just lean enough for stoichiometry -- and the spark lights it, you get detonation.  (That counts as ping!)  (Any leaner, and the engine won't run...)

If you run too 'rich' and then generate regions of criticality (e.g., through pressure effects) anytime during combustion, those regions will detonate (e.g., react at high speed rather than smoothly) and this will be perceived as ping.

Listen to what Franz is telling you about the characteristics of gas fuels in real-world engines...  

Thanks Overmod.

I learned a lot developing lean burn engines running L=1.5+.  More than one engine bit the dust when we were experimenting with different AF ratios.  We even configured one 6 cylinder engine with multiple intake manifolds capable of producing different AF ratios in each cylinder.

We learned that vapor fuels perform very differently than liquid fuels, and that many liquid fuel "Experts" have to re-learn the physics of combustion.  All in all, it was a very rewarding experience.

Franz