I finally completed the conversion. I used the LPG gas mixer I linked, and finally tapped into the exhaust manifold for vacuum (there is a blocked port in the manifold that seemed to be meant for it).
Overall it works just fine. I had to adjust the flow using a ball valve for full throttle, and use the idle screw to add fuel on lower throttle positions. Now I can throttle up and down, and seem to get full power (or close, given the propane energy density) at WOT.
One surprising thing, my neighbour brought over his CO tester, and since I can flip from gasoline to propane at the flick of a switch, we tested both.
Once it started (which is a smelly mess), the gas fed cycle is very clean, 2.5-3% CO.
It was better that the propane cycle, which was hovering around 4-5 until I used the main ball valve to choke the fuel supply to where it just became smooth. Even then I could only get 3-4%. I figure my crude fuel delivery system through the gas mixer is not allowing me to increase the air/fuel ratio to optimum propane values (15.2:1). Either that or my slightly compressed air filter is not flowing enough air.
Next steps include modifying the air filter and maybe adding a ram tube to see if i can improve air velocity and get a smooth output with a leaner mixture.
But even as it is now, the tractor is a joy to start on propane, with barely a wisp of smoke, while it s a terrible mess to start on gasoline (large dark smoke cloud). Still very happy with the conversion. Does not hurt that I can get propane for around 0.50 cents a litre either.
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