I have a 1979 Ford F150 strictly on propane. It's a 302cu". On cold winter day it started to backfire going up a hill and then every time I gave it gas. I found the partial problem to be a fuel filter. I changed the filter and the problem got better but it didn't go away. Since then I have replaced all hoses to eliminate a vacuum problem and have tested for vacuum and it is good. I've replaced my ignition coil with a high energy MSD coil, replaced the ignition wires, spark plugs and checked the timing - Good. I've rebuilt the fuel lock filter and the vaporizer (Impco model E) converter and have removed and cleaned the mixer (425). It still backfires under load, usually when I'm going up hill. Usually it will make it about half way up the hill then it loses power just before it backfires. When I let my foot off the gas and reapply light throttle it will still backfire. I pretty much have to stop and wait a few seconds at idle then I'm good to go for about another 1/4 mile before it will start to backfire under the load again. Right off the start I can pin it and it revs up and goes great....then stubbles and backfires after about 5 seconds of the hard acceleration. Sounds like fuel starvation and a lean mixture to me but I'm at a lose to figure out what else it might be.
Any suggestions on what I could check or what I can do to help fix the problem would be very greatly appreciated.

 

I am having this same issue! Did you ever find resolution Sauka? I've replaced my hoses thinking it might be a vacuum leak. Timing is ok. Already have high energy coil and good wires / plugs. Was fixing to rebuild the regulator and purchase a new propane fuel filter, when I stumbled across this post. Any assistance would be greatly appreciated. I'm driving a strictly propane 1974 Ford Bronco by the way. It is my daily driver, or was until it started conking out and backfiring when it lit back up.

 

Backfire thru intake/mixer is because of a lean mixture. Air leak, fuel starvation. mixer out of adjustment or wore out. The only other issue would be induction fire between adjacent cylinders in the firing order. Any cylinder that fires consecutively and are on the same side you need to separate the plus wires as much as possible.
Good luck