The actual "rate of combustion" for a single charge of the combustion chamber remains constant throughout the RPM range and is dependent on the fuel type as to exactly how fast the particular fuel burns.
LPG fuel combustion is slower than that of gasoline so LPG needs to be lit earlier in order to have the combustion at the proper stage when the piston crosses TDC from compression stroke to power stroke.
I will search for any manuals that apply to 2F models at work today and post tomorrow.
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