Who knows propane engines

[blown motor]

What would cause a propane engine to backfire? This is a used engine on a generator in a new situation.

[JonH]

same as a gas engine. carb backfire, could be lean condition or a timing issue. propane timing is different than gas. propane has less btu's per gallon than gas. only other difference is you are introducing vapor into the engine vs liquid. What size engine? was it already running on propane or did someone convert it?

[blown motor]

My son bought it from a dealer that brings these used standby generator units into Ontario from the US. Apparently they have to get replaced every five years in places like hospitals, etc. Dealer told him it ran smooth but he's having backfire issues. Right now they're working with gas line and regulator size but they haven't got it running right yet. I don't know much more than that.

[Headjam]

Check the ignition system. Make sure all of the cylinders are firing properly. I did a lot of work on multi-cylinder natural gas (and propane) engines and could make them backfire by disconnecting selective ignition coils on cylinders on each side of the vee. Most backfires are because unburnt air/fuel is getting into the exhaust manifold/system.

If the ignition system is okay, check the valves. Might be a burnt or mis-adjusted valve. Propane/natural gas engines burn very hot and are very hard on valves and valve guides. Focus on the components and/or settings (timing) that would cause unburnt fuel getting into the exhaust. Good luck!