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Briggs and Stratton WMB converting it to run on Propane


I have a Briggs WMB and am contemplating converting it to run on propane. I have found drawings to make a demand regulator using Tecumseh parts. I think I can use the existing suction carburetor, but of course I would have to re-jet it. Rebuild kits are available for this carb, so I could get a new jet and bore it out. I could also install a larger needle in the brass needle body. My question is; is there a rule of thumb regarding how much to enlarge the jet? Also, if there is a rule of thumb for enlargement that is expressed as a percentage, would that be percentage of jet orifice diameter? Or percentage of orifice area? Thanks.