I have been reading Jennings' book also and lurking on this thread as well. I am pretty much a newbie to conversions but I do have a Ryobi that I converted and am still getting used to it. Right now it is down due to a crapped out ignition.

But from what I have been able to understand from Jennings, port timing and compression are not as important as port scavenging. In other words, how quickly we can get the exhaust out and the fresh new charge in. This is mostly controlled by the area of the exhaust ports and transfer ports and also by the direction and smoothness of the transfer ports. I am sure everyone has heard the term Schnerle porting on our glow engines. This is a reference to a man who came up with one of the best ways to arrange the transfer ports of a 2 stroke engine to bring the charge into the cylinder in an upward direction that then pushes the exhaust out. That is why the transfer ports are set at 90 degrees to the exhaust port, to keep the fresh charge from going straight across the piston and back out the exhaust port.

And from what I have been able to understand and how this would be applied to the Ryobi is:

First, there is not much gain to be had by changing the intake any as a reed type induction is pretty well regulated by the engine anyway. However the larger carb is needed to supply as much as it can take.

Second, getting a good seal on the piston is important to seal the exhaust out of the crankcase as exhaust into the crankcase dilutes the fresh fuel charge. Not to raise the compression. So a Bowman ring is a good thing to have.

Third, in order to get the exhaust out and the fresh charge of fuel in (which is the definition of scavenging), the exhaust restriction caused by the stock muffler must be changed. He goes into a long description of resonances and tuned pipes on the exhaust and how they help to draw the exhaust out and then stop the fresh charge from coming out but for our purposes, just opening up the muffler or using a conversion muffler is about the best we can do here.

Fourth, the transfer ports are good as is but if more scavenging is needed for more power, the best machining mod that can be done on this engine is to increase the area of the exhaust port. Not the timing.

Lastly, for the last optimization, the spark can be dialed in more closely using an electronic ignition.

From what I have inferred from all my reading on this forum and from Jennings book, the best steps we can do to convert a Ryobi are.

1. New ring from Bowman that will create a better seal in the cylinder.

2. Get a 10 to 11mm carb to match what the engine can draw from the intake reed.

3. Gut the muffler and make at least a 1/2"outlet for it or switch to a custom made conversion muffler.

4. Switch to electronic ignition so that the spark can be dialed in optimally. Not to mention making it easier to start.

At this point the engine will be close to the best it is going to get. But however, if one has the knowledge and the skills to go further to hop up the engine, the exhaust port area would be the best place to start. Jennings book can be used to make calculations on what the best areas would be best for the rpm desired.

So, this is what I think I have learned and gleaned from all my reading. Do all the assumptions sound correct? Or is my understanding off?


Jim