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The timing is critical on any engine in order to make proper power, torque and idle. Propane has a different curve and takes less total timing than gasoline. Also the 425 mixers are preset for the proper power air/fuel ratio when manufactured so there could be a problem there. the idle mixture is the only adjustment that should be made after the timing is set correctly. Also platinum plugs are not good for alternate fueled engines, with the exception of liquid injection, because with the hotter combustion temperatures platinum absorbs heat and turns into a glow plug under load. A standard tip spark plug is sufficient and last as long as any other plug. It sounds like you need to start with getting the timing AND spark curve optimized first and replace those platinum tip plugs and see how it runs before going to the next step.
