I have 2 Norvel 40's, one a plain bearing and the other ball bearing, and equipped with CL venturies, that have me stumped.  They are powerful and fairly light and I have been trying to get them to behave on a profile stunter.  They won't run rich even a fat two stroke.  They will take off in a fat 2 stroke then go really rich and quit within a lap.  Lean them down until they scream and they will run the tank out and let you do the pattern if you can keep up with it doing 4 second laps.  It also is extremely frugal on fuel - 15+ minutes on a 4 0z. tank.  I've tried Sig 25% castor 5% nitro, Sig Champion 5% & 10% fuel and have been running an APC 11.5-4 prop on a 550 square in profile stunter with a 4 oz. Hayes tank rigged for uni-flow.  Also using the stock Norvel muffler.  I've been flying UC for 60 years and have never seen this before.  Any engine gurus out there have any ideas?

   Hi Bob;
  There have been a few threads recently about engines that will only run well on low oil/synthetic fuels and the Norvel .40 may be one of them. When we talk about oil amounts, we only think of it in terms of percentages of oil content and forget the amount of alcohol left. It gets down to the viscosity of the fuel, as Bret Buck has mentioned in most of these threads. This made some sense to me, as I had experienced the same type of thing with vintage dirt bike fuel. In the old days, fuel mixture was typically 32:1, or one quart of two stroke oil to one gallon of gasoline. Some times more, and sometimes less, depending o the manufacturer of the oil. In today's market there are tons of oil available. I switched to a concentrate that came highly recommended and can be mixed to 100:1, but I don't go that far. The result was I had to go much smaller on main jets to get a proper run. This was because I had more gasoline in the fuel mix, making it richer. The same is true for when we have too much oil in our model fuel in an engine that is not designed for it. The more oil, or more viscosity, the less alcohol in it and it's actually leaner right off the bat. Somebody there at SIG must have some low oil fuel for a four stroke or something like that that you could test, or add some methanol to a quart of Champion to get near the levels you want to try. It just the materials that the engine is made from, all the the new metallurgy and ceramic stuff. They didn't have that back inyour combat days!!
  See you at the fun fly!
  Dan McEntee

Thanks for the suggestions - it gives me some ideas to try out.  These are powerful engines that are smooth runners.  I just haven't run into the quirk before as I spent most of my younger years flying combat but age is making me look at stunt more seriously.

Or one quart to eight gallons of gasoline.... I noticed the same thing, scratched my head, shrugged my shoulders, and moved forward....
I think that Dennis might be right about that engine not liking Castor.
There is a norvel forum, where you can ask the folks about that issue and they are very fast in responding.
The forum is on their website.

I have been mildly interested in the Norvel engines, since their little BigMig seems to be very good for 1cc.
Seems like they might be usable for stunt as well, if the proper venturi sized could be had.