I have found some Ryobi differences I didn't know about until now....

I have a Ryobi that had excessive vibration, so I pulled it apart. I also got out my supply of Ryobi miscellaneous parts. I found that there are two different style backplates, (carb mount/reed valve), and two different crankcases.

The engine I was working on had a backplate with the reed valve recessed into the plastic about 1/4". This would highly impede flow. I had another backplate on hand, and it is a much better design, with the reed valve unshrouded, and with a taper where the flow goes into the transfer ports instead of a sharp edge. Much to my surprise, when I went to fit this better reed/valve carb mount to my crankcase, it wouldn't fit.

Upon measuring my other crankcases, I found that one style case has a crankshaft area diameter of 2.220", while another has a diameter of 2.070". This is a difference of .150", and while the bolt holes are the same for the reed valve/carb mount, the better reed valve assembly will not fit into the smaller crankcase. It's o.d. is too big.

I do not know if this is where the difference comes into play, but the smaller diameter case happened to be a short shaft engine, while the larger ones were all from long crankshaft engines. Once again, I don't know if that means all long shaft engines have the larger case and better reed valve setup. I would like to know if any of you know for sure.

Since I was originally looking for a severe vibration problem, I then checked the balance. The short shaft engine was WAY off. It was nowhere near in balance, which makes me wonder if it originally had an off balance flywheel that helped to balance the engine. (I was not running the magneto, I was using electronic ignition.) The long shaft engines I have balance perfectly. 1/2 of the reciprocating weight perfectly balances with the counterweight on the crank with those. Remember that there have been questions about whether the Ryobi had a zero balanced flywheel or not?? I now think some models do and others don't!!

In the pics you can see the differences. Look at the space between the edge of the crankcase and the mounting holes for the backplate and you will see the crankcase i.d. difference.

My brother has a Ryobi with the better style reed valve, and it runs very well. With no mods other than a free breathing muffler and a slightly larger carb, it turns an 18 x 6 at 7700 plus rpms, and has very good torque. It will loop a 14 lb. airplane at half throttle without slowing down. This one of mine with the crappy reed valve was down 500 rpms in comparison, which was another reason why I had pulled it apart to inspect it.

What style reed valve do you guys have, and what's the scoop on these differences? Is there a certain model that has the better reed valve?

Thanks,
AV8TOR