Which equipment parts do i need to convert gas to dual usage with natural gas or propane.

The last generator (dual fuel), I worked on had a fairly large diaphram(maybe 6" across), with a line to the propane tank. 
 
There was also a lever/valve that you turned to go from gasoline to propane. You had to turn off the fuel 
valve as when you transfer over it would run super rich as it had two fuel sources.
 
Lastly, the carberator had an additional "port" (outlet) for a propane hose that went to that valve. As I re-call, that port was 5/8, maybe 3/4" .   That port, again as I re-call, was between the throttle plate and the engine.  
 
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I have a Generac  XT8000e and I purchased a tri fuel kit from Preston fuel, It was easy to install and it worked the second try. I have not run it on natural gas yet.