ORIGINAL: NM2K
My Sportster .28 would not swing a prop larger than a 9x4 MAS until well after a gallon of fuel had been burned. Keep in mind that I wasn't using a fuel with much castor oil, which I attribute to the extended break-in period. It was S&W 10% nitro with mostly synthetic oil and a tad of castor oil. It did get to where it would spin that 9x4 at a pretty good rpm and it would stay running during the expected duration (as long as it had fuel). I tried running a 10x4 Graupner nylon prop, but the little .28 would overheat and quit. It was behaving just as though it wasn't broken-in. I did see another fellow running a prop larger than a 9x4 on a Sportster .28, but he was burning fuel with 22% castor oil, no synthetic oil.
The few K&B .65 Sportsters that I flew or saw flying (none belonged to me) back then were spinning 11x7 wood props (Zingers). They ran well on the 11x7, but refused to pull larger props with heavier loads unless the fuel was made with all castor oil lubrication. Those folks running 14x6 props must have had a lot of running time on their .65's or were burning fuel with all castor oil.
Ed Cregger
I've been testing all sorts of props on this engine - and other than transitioning a little rough, it would spin that 13x5 (and all others) without incident. I tached every prop and between the 12x8 and 13x5 there is almost 1000rpm difference. 13x5 being faster than the 12x8 which I expect given the pitch. I cleaned the top-end of the engine and exhaust and reassembled it and I pulled the carb apart one more time and rinsed it out with Denatured alcohol and blew it out. There were some small burrs of brass on the main needle and in the spraybar orifice. I picked these out and cleaned up the edges. There might have been some debris or congealed castor oil in the main needle orifice/tube. It looked good so its back together. I havent had time to run it yet but I am hoping its got this little tantrum over with. I am just past 5 quarts of fuel in this engine so far. (1 whole quart was just for break-in)
I do also have a feeling this engine never will throttle well unless someone will tell me or I figure out what exactly causes them to run crappy between idle and WOT. The low speed mixture seems just a tad rich but in the full lean position gives me a smooth pinch test. No speed-up or quick stall. (this is before the last carb cleaning) If when I run it it does the same thing its been doing the last few times it ran I'm gonna make a short video and post it. I think I will have to bite the bullet and order some all-castor nitro fuel.. Nobody locally will order it for me so I'll probably order 2 gallons. Of course the all-castor fuel from SIG and Byrons doesnt come in quart bottles so hazmat fees apply. Do these engines lose a lot of performance if ran on FAI Castor fuel?