When my car is cold, the oil is also cold, and if I start on veg oil wih only what diesel is in the injector pump, it stalls, runs rough etc. It does sound like a cold injector pump, but as you say, see what Tony and Bruce say.
With my conversion (a "Tony" configuration and I would put up pictures as soon as I work out how) I have some heating before the filter, and a lot before the injector pump, so the pump is pre heated even before I put veg oil through it. When you car has been running a while, is it better?

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cheers
Chris.
1990 landcruiser 80, 1HD-T two tank, copper pipe HE+ 20 plate FPHE, toyota solenoids and filters. 1978 300D, elsbett one tank system. 1979 300D veg oil/ diesel blend for now.