Hi David,
It sounds like you have about 21CCs that you are playing with. The 25cc engine that followed the 21Ccc engine is the FL-25 and you can recognize it by it being a lot easier to trim the flange off than the 21cc. The 21s are about a 1-1/4" bore and the 25 is 1-3/8" bore. I converted 2 - 21cc engines. By porting etc., like you are saying, and boring the carb spacer to 1/2" (it can't go much bigger) and then using a 9/16" bore Zama carb, I get 7000 RPM from the 21s and I got the 25cc engine up to 8000 RPM. All with a 16-8 MA Classic prop. The 16x8 seems a bit large for the 21cc engines because the 25cc can accelerate noticably quicker. I have another 25 coming and will try to duplicate the 8K. You are correct about this type of engine work being another hobby. I am working on a airplane design suitable for the 21cc engines. The intake port on these things is what requires the most work. Playing with the timing will mostly get you a more tempermental running engine. These engines were not designed to be high performance engines for our use, so I have learned when to leave well enough alone, at least for what I intend to do with them. I am real pleased with the 8k, and all of them are light and run pretty smooth. (All of mine do have the spring connected muffler.) Its been a while since I worked on these and my memory isn't getting any better with time. I just rememberd also adding two 1/2" O.D. outlet pipes to the mufflers. With the small original hole in the muffler the engine can't breath at all. My old memory just reminded me that I also machined .020" from the bottom of the jugs on the 21cc engines, I did this because they don't have much compression to begin with. This is one part of how I got 7000 RPM out of the 21cc engines. The 25cc engine is the same weight, easier to convert, has more power and machining the jugs on the 25cc is not necessary. The 25cc, FL 25 which is the last of the 4 bolt crankcases, I think, is the best engine for our use in my humble opinion. I will take and post a few photos of the FL 25 features for identification purposes. There is a lot of Poulan engines and that does cause a bit of confusion, but the FL 25's crankcase is unique and easy to spot, once you see one. Hope all this helps. One other point is that the 21cc engines only turned 6000 RPM to begin with, that is how they became a hobby. 6000 wasn't enough, 7000 should be usable, for the right airplane. Corrected bore diameters, after measuring latest 25cc engine. I have also seen the same basic jug with a 1-3/16" diameter piston and much smaller ports. Maybe around 18cc? It seems the same crankshaft, and flywheel are used often on different models.