Combatpilot.... Your work has inspired me to do some experimentation on my Ryobi motor. I have been flying it in a GP Giant Super Sportster for about a year. It flies ok but it could use some more power.

I yanked it out and put it on the test stand today and ran a couple of tests.

First off it had a very small carb on it with a venturi size of only 7 mm. I had an 8 mm venturi carb from another Ryobi WW so I put that on it. I have difficulty getting a stable reading of RPM but I think it gained about 50 RPM. I did not do a baseline today for the small carb but have used data from a run done in November installed in the airframe. The measured thrust went up about 1.1 pounds from 11.5 to 12.6. As I said one test was done in the airframe and one on a test stand so there is possibly a difference there. These data are corrected to sea level standard conditions as outlined previously.

I next put on the old style cylinder with the single runner transfer ports and the decompression slots cut above the transfer ports. The piston was a two ring and the crankcase the later configuration designed for the two runner cylinder. By your observation the cylinder head volume may be smaller too ie greater compression. The performance was abysmal!! It lost 630 rpm and required a lot richer mixture but boy did it idle nice! It got down to 1300 RPM before it started missing, but it still didn't quit! Needless to say idle is not the operation point we are most interested in is it! The obvious conclusion is that a cylinder with decompression slots makes a good paper weight!!! I'm not confident enough of a positive outcome to try filling the slots with anything.

The results of your testing point to a bigger carb as the very first thing I need to get improved the power. BTW when you all speak of carb size, eg. 11 mm, are you talking about venturi size? Also I noticed that even on the stock carb that came with the WW that the intake manifold dia is greater than the carb bore. You'd think that it would be of great benefit if the two were the same although with the aft facing step there is almost no loss in flow coefficient although the flow will decelerate.

Also, what do you suppose the benefit of cutting a waist in the reed is? More flow area when the reed is lifted? Also I notice that, at least for the backplate that's apart on my bench, the reed seat is not flat. The reed is suspended over a concave surface so that when closed there is space between the reed and the seat in the middle of the flat reed. In this configuration there would have to be some crankcase pressure to close the port to the carb. Is this the way they're supposed to be? I would think that if the reed were completely closed at static condition it would work better, but then again this is all a guess. I see in your most recent post that on your second reed test there was no difference in the modified reed so perhaps the question is academic.

One other thing. When I first saw the newer style two ring double runner transfer port cylinder design I couldn't believe there were blunt forward facing steps at the entrance to the transfer ports. That certainly would make a very lossy entrance flow coefficient. I just had to try and improve the situation. Unfortunately I did not do a back to back test to see what the performance difference was.

diceco

PS. There are always some sour grapes in the bunch. One needs to just throw those away and enjoy the rest of the grapes! I for one am glad you are publishing your findings in this forum.