I've been running home brew biodiesel in basically a straight B100 mix in a single cylinder diesel I have fitted to my Troy Bilt tiller going on 5 years now with no problems. I have another such engine on my rebuilt 1974 Ariens walk behind snow thrower that's been running home brew also going on 5 years now. No problems there either and that one is pull start only, living in an unheated garage. I do keep some biodiesel in the house, however, to have something to pour in to avoid a jelled fuel filter if I'm expecting a big snow dump. 'Never had a problem with either machine regarding fuel.
Our 2005 RTV 900 has gotten a few tanks here and there of the same home brew along with some commercial B100, but I've learned to quit feeding it biodiesel beyond October as the RTV has a fairly small fuel filter that objects pretty quickly to any cloudiness in the fuel. In fact, the 21 (?) hp RTV engine's filter is about the same size as the filter on the 10HP snow thrower engine, so there's little tolerance for cold biodiesel in the RTV as it is running a pretty small filter relative to its engine size.
The 2008 B7800 has had several tankfuls of B100 mixed into whatever dino diesel already in the tractor tank with no problems, even in the unheated winter storage shed last winter, but the 7800 has a fairly decent-sized fuel filter and the tank, being behind the engine, benefits from some heat fairly soon after start up.
I'm not sure but I think the 7800 has a fuel return line to the tank and because of that, some engine and fuel pump heat is returned to the tank possibly helping guard against fuel filter gel ups as well. I know the two single cylinder engines on the tiller and snow thrower have fuel return lines to their tanks and the fan blows air off of those cylinder heads and onto the tanks, adding more heat. The RTV, on the other hand, doesn't seem to have a return-to-tank fuel line, though I've never looked or had occasion to work on that machine's fuel system, so I could be wrong. In any case, the way the RTV is set up, its fuel stays pretty cold in the winter and, when combined with the small fuel filter as I said before, its the one machine that doesn't like biodiesel and that's only in the winter.
As for the quality of my home brew, it's never been tested, but I'm very meticulous when washing the biodiesel. I also dry and settle it very thoroughly, giving it a careful visual look over before dispensing it into the storage tank. Most of my feedstock is lightly used soybean and canola oil from the residential school I work at.
I don't think biodiesel is going to save the world, but I find that all of these engines have a much less irritating exhaust when running anything beyond B25 or so. I don't stand there inhaling the stuff, dreaming of french fries as some biodiesel fanatics seem to do, but I do breathe better around biodiesel powered machinery than when running dino diesel, so if I happen across some used veggie oil, I'll convert it to biodiesel. Ditto if I happen to be around one of the few Boston area stations carrying B100 too. (For Boston area folk, that would be Bursaw Oil in Acton.)
