I'm converting a Ryobi 26cc 4-cycle long shaft using parts from www.carrprecision.com. I'm planning on using it in a biplane of roughly quarter scale (12lbs, 72" span, 20x10 prop). I figure the low power-to-weight of a wacker engine would complement the short nose moments of the WWI era. The 4-stroke sound should add some scale realism to the fun, as well.

I've begun bench testing the engine to see if it is worth the effort of building the plane. The engine is stock (carb, muffler, magneto ignition). My main concern is the idle speed, it wouldn't reliably idle below about 2800 rpm, which translates to 25mph for the 20x10 prop. This is well above the stall speed for the intended bipe, so landings are going to be too hot.

BTW: I took off the muffler for one run just to see how loud it would be. There was almost no difference in noise at idle!!! However, the idle speed shot up to about 3500rpm. The idle adjust screw ran out of range and killed the engine at around 3000rpm.

Are these results typical for this engine? Will a CDI ignition improve idle? Or, is it better to leave the flywheel weight of the magneto ignition?

rwilder91, I've got the same engine almost ready to run, I'm waiting on the mount. The first thing is the stock carb must go. I'm going to be using a zama carb from an old homelite 25cc engine that I converted and put a bigger carb on. You need to have twin needle adjustments on these carbs for best performance. Also you may need to solder shut any hole in the carbs butterfly. The hole is there so that on the weedeater it would only idle so low but not kill the engine, some carbs have this hole and some do not. I havent ran my ryobi 4-stroke yet but past experience tells me the carb is the biggest problem you have right now. If you have any other carb lying around that has both high and low adjustment bolt it up and see what happens. Oh...and yes, CDI would improve things somewhat but once you get the setup right using the mag you might not see the need to spend over $100 on the CDI setup. Keep me informed of your results and we'll compare notes later. Piper
One more thing...check for air leaks on the carb side...it sounds like a too lean setting caused by sucking air.

Thanks for the info! I'll check out the air leak suggestion...

Actually, I rotated the carb 180 degrees for these tests, and maybe it doesn't seal in this position. The carb is a Walbro WT-339-1 which is a twin needle affair. I rotated it so I could access the needles while the engine is running 'cause the stock orientation has the needles facing the propeller.

I suppose your zama carb has a larger throat for more oomph at full throttle. Does this affect idle and transition? I've read that larger bore carbs may sacrifice the low end.

BTW: I'm collecting a variety of sizes of mousse cans to play with mufflers (the 3.5oz may be too small, the 5.0 looks about right). I also found a 12oz aluminum bottle of RAIZE energy drink that looks promising, although at 15x displacement it may be plenty large.

turned it 180? uh oh...the small hole that the pump in the carb uses to get its pulses would be covered up not allowing the carb to pump its own fuel when you do that. I just looked at my manifold to be sure and it only has the vacum hole that allows the carb to fuction one way.
the throat of my zama carb is the same size as the original but the venturi is slightly larger. Your thinking right about a larger carb causeing running problems but only in relation to rpm. Your running that 20" prop so your rpm would be much lower that the 18x6 im starting with. the larger prop causes a lower air velocity at the carb and would make carberation poor with too large of carb. I will work up to a 20"prop but I dont think the zama carb will have any ill effects with that prop only because I dont think I've overcarbed it, but time will tell. I have found that just making small changes is the best way to find rpm without blowing the whole setup. rotate your carb back to its original mounting and see if you have and improvement. Piper

Yep, I noticed the small hole you refer to, so I flipped the plastic manifold spacer around too. (Thanks, now I know what that hole is for!) What I'll check tonite is whether the plastic manifold seats properly to the head casting. Might need to make a paper gasket, or use a sealant.

How slow do you think these engines should idle reliably? Is 1800 a reasonable expectation?

Interesting thoughts about the smaller carb... I'm intending to use a 20" dia prop because it's close to quarter scale for a WWI bipe, and because I'll need 10..12 lbs thrust for a 12lb plane. ThrustHp shows that it might be more feasible to get it with 20" at 5000rpm vs a more normal 18" at 7500rpm. So, since I'm not looking for 7500rpm maybe I don't need a larger throat.

I've seen another post where somebody reports pulling a 20" (or was it even 22") prop on a Ryobi four stroke. I think they were pulling around 5500 rpm, but that might have been after some tuning.

BTW: Whose motor mount did you order?

use paper or gasket material for the auto shop, no selent.

Thanks, paper gasket material it is then.

Hey, here's a question for you and DougT...

The lore surrounding four stroke vs two stroke glow engines is that a 1.5:1 factor is needed to compare them. Thus a 0.70cid four stroke is deemed equivalent to a 0.46cid two stroke, and 0.90 vs 0.60, and 1.20 vs 0.90, etc.

Does that mean the Ryobi 26cc four stroke is only going to measure up to about an 18cc two stroke? Or are the rules different for gas vs glow?

Here's another line of questioning 2 vs 4 stroke...

I've been reading up on mousse can mufflers, which are used on glow and gassers. Some stuff says don't use any filler (e.g steel wool) to absorb noise, because there's too much oil in the exhaust. But on these four strokes with straight gas, this shouldn't be a problem, right?

Tell ya what I do. Take the original muffler, grind the crimping off, take the guts out leaving only the bolt guides to keep the muffler from being smushed when bolted back on. Use MAP gas and braze it back to gether in a vice, cut holes anywhere you want for copper tubing I get from the local hardware stores scrap can, braze the tubing in place and your done. No stuffing or costly aftermarket mufflers. This works every time and i love working with molten metal...lol