Make sure your spark plug wires are good before you try this one. There's a reason I made note that WD-40 is flammable. I saw a guy try to locate a vacuum leak with WD-40, but instead the misfire was caused by spark jump. The WD-40 made a nice little fire. Propane could get a lot more fun.
I agree 100% with the spark plug wires causing a fire but I also want to note that propane dissipates quickly below its flammability concentration. WD40 or other flammable materials in liquid form coat whatever you are spraying and will cause a fire to burn longer and create more damage than propane if you are reasonable with how you use it. Of course propane is more explosive so go around one suspect area then let it dissipate before trying another suspect area.