Propane 460 backfiring

I have an F150 with a propane powered 460 in it It has been there for about 12+ years and recently it has started backfiring under load only above 50 mph. It has the standard Impco 425 mixer with full time propane. It was sitting for about 7 years but it never did this till recent. From a stop if you floor it the truck will kick down, go through first and second and when it hits 55-60 mph it will start back firing/poppinh. If you are just cruising at highway speed sometimes you might get 60-70 mph till it might start popping but only if there is a change in the load (like going up a slight hill). Would running out of fuel cause it to start popping? I am just wondering if there is some kind of filter in the regulator or what. I looked at the cap and there is a bit of carbon on the contacts but I am in Canada and have NO tools or timing light to check things out.

Could be freezing up, check your water lines going to the heat plenum, also could be lean as you said.

IDT-572 wrote:Could be freezing up, check your water lines going to the heat plenum, also could be lean as you said.

I never took the lines off but the coolant level was the first thing we checked and it is up.

Jason,

I had one do that about 17 years ago. I was running dual 425 Impco's, if you are in a rich/lean condition it can actually burn a small hole through the throttle plate. Check that out and then the mixer will have to be reset. I think propane acts opposite of gasoline as far as rich lean goes. I had a very rich setting once and after a 3 hour drive it burnt holes through 2 pistons TRW 2443's LOL.

Hope this helps some Mark

jason on the fork lifts at work I have drain the tanks on them about every 2 years the will get a oil build up in them and it will get in the diaphram and have take it apart and clean it.Propane has a oil in it something like the a/c oil you put in a a/c compressor in your car and it will build up in the tank and when run low it will clog the diaphram up and make it go to popping and run crazy.Don't know if that is your problem but might be something you might want to check.

Jim

My father inlaw actually drives this truck now so I went over and fixed a few other things today. I did find the hose from the converter to the mixer was loose on both ends so I cut a bit off the hose ends and tightened it up. From looking online there is supposed to be a filter in the lock off too but I didn't have a chance to check that or test drive it. I am going back to Louisiana in the morning so he will have to take it and have it looked at.

The oil thats in propane is what makes its smell called ethel mercaptan and if it gets in the system it will cause exactly what you are discribing. Check all filters especially the one in line to the vaporizer from tank. If propane tune isnt right it will eat a lot of things like pistons, valves, mixers, and heads. Dont ask how I know.