Yeah, I was very pleased, and somewhat surprised how this Ryobi turned out. It runs very well and has good power. My brother has a Ryobi that is basically stock except for electronic ignition, a slightly larger carb and a gutted muffler. His engine has the slits in the cylinder. It turns an 18 x 6 prop at about 7600 rpms. My modified Ryobi turns the same prop at 8500.

Answers:

1. I am using CH ignition. Electronic ignition frees up some power, as the mag uses some power to generate electricity, and also just to turn the flywheel with its fins. Being able to optimize the ignition timing also adds power. Mine is set at 28 degrees BTDC.

2. I miked the compressed gasket, and it measured .035", so leaving it out lowered the cylinder that much. I did not remove the ridges on the gasket surface. I used Yamabond sealer in place of the gasket.

3. Yes, lowering the cylinder increases the compression, but it also decreases the exhaust and transfer port timing. Some of the Ryobi's have a very low exhaust port timing anyway, (around 120). 150 degree exhaust timing is a good general number for an engine to run up to about 8000 rpms. You can go to 160 or 165 degrees for higher rpm use. Much more than that and you are into the range where a tuned pipe would be needed to effectively utilize the high port timing. You raise the port roof to raise the timing, but do so in small amounts and keep checking the timing, and remember to chamfer the port edge when you are done so as to not damage the piston ring or piston. Note that 7500 rpms is what is generally accepted as a good max static rpms for the Ryobis due to their relatively weak connecting rod. I will be putting a larger prop on mine to lower the rpms.

4. No modifications were done to the reed valve other than setting the stop at 1/8". No mods were done to the crankcase.

5. Yes, I used a carb with an 11mm venturi, which is a good size for 25 to 30cc engines run in these rpm ranges. The original carb on the Ryobi is very tiny and limits power. I get my carbs off e-bay, and/or from engines I buy from small engine repair shops. Do a search and you will find a long list of useable carb numbers with venturi sizes here in the forum. If you want to find the size of a carb with a given number, you can do so at www.walbro.com

6. When using electronic ignition, the flywheel is left off. With the first engine, (short shaft, small crankcase, restrictive reed valve), this made the engine quite unbalanced. With the new engine, it balanced perfectly without the flywheel, and runs quite smooth. Most people have no balance problems when removing the flywheel on a Ryobi, so I believe the first engine I was working with on this plane was an oddball and not commonly encountered.

7. Note that the Frank Bowman ring provides a nice power gain as well. It reduces the stock ring gap from around .030"+ to near zero.

8. WHEW!!

Here is the plane the engine is in....

AV8TOR