small engine propane conversion natural gas conversion

how to convert a small engine to propane the old fashioned way

this page is to document a very interesting setup my father in law
Donald fabricated to convert small gasoline engines to run on
natural gas or LP.

Sadly Donald passed away some years ago and his collection of small engines has been sold and scattered to the winds
but perhaps the simple technique he used to make them run without having to mess around with messy gasoline will help someone else. 

To convert 1-5 HP gas engines to natural gas or LP he modifies a poppet regulator (Maxitrol RV48 works well)
by removing the pressure regulating spring and cap. disassembling and replacing the diaphragm with a piece of a latex surgical glove to get the regulator
 to fully close but still open under vacuum .  That way if the regulator is oriented up side down so that gravity helps hold the diaphragm valve closed
 no gas will flow until vacuum created by the engine sucking air through the venturi draws the gas through the regulator.
 for use with LP gas he puts a gas grill type regulator between the tank and the modified poppet valve.

poppit regulator view for engine conversion
Next he constructs a venturi using copper water pipe brass shim stock and 3/16” O.D. thin wall brass tubing.

he selects the size of copper pipe that best fits over the air inlet of the carburetor on the engine and cut a piece 2” long.
 (¾” rigid copper water pipe seems to work well for most small engines.)
then he rolls 2 pieces of shim stock into cone shapes about 1” long with the large ends being the same size as the I.D. of the copper pipe
and the small ends being between 5/16” and 3/8” larger
for larger engines smaller for smaller engines but 5/16 to 3/8 seems to work well for most small engines.
He Solders the 2 cones inside the 2”piece of water pipe so that the small ends meet in the middle
then drills a 3/16” hole though the side of the water pipe and though the shim stock just to side of were the 2 cones meet
so that the side of the hole cuts the edge of one piece of shim stock .

Then he cuts the end of the 3/16 O.D. brass tubing at a 45 degree angle.
Then inserts it into hole in side of water pipe aligning it so
that the angle cut lines up with the angle of the shim stock as best he
can and solders it in place.

venturi top view for engine conversion  
venturi side view for engine conversion

next he fabricates a flow valve to go between the poppet regulator and
the venturi by drilling and taping a hole for a small machine screw
into one side of a piece of tubing so that when the machine screw is
screwed all the way in till it touches the other side of the tube it
pretty much blocks the entire tube. that way he can adjust the fuel air
mix by turning the screw in or out.

then he plumbs it all together with assorted tubing and hose in this
order, gas grill regulator (only needed for LP), modified poppet
regulator, flow valve, venturi. and then mounts the venturi on the air
intake of the carburetor of the engine being sure to get a relatively
air tight seal between the venturi and the carburetor.

diagram for venturi for engine conversion

this is posted for academic purposes only if you choose to use this information you do so at your own risk take proper precautions as necessary etc.
this was never intended for unatended use. if you intend to run an engine when your not right there with it i highly recomend that you buy a commercial vacuum flow valve. in this day and age were commercial kits are commonly avalable for most every engine this method is outdated and if you do not know what you are doing quite dangerous.

page by Kurt