Just getting back to this model airplane thing, after about a 50 year absense. (Played with the control-line stuff as a kid) Like to tinker with engines, so thought that I'd try a "conversion" for my first attempt at RC. Got a 31cc Ryobi, & spent several hours on the engine stand, trying out different props, carbs, reeds, modified manifold, etc.. No matter what I tried (even some 10% nitro glow fuel) performance was terrible. Finally, put a degree wheel on the engine, & checked the exhaust. Couldn't believe the reading. (118deg)!!!!
Bought another Ryobi, & compared the exhaust port timing on the new jug. Was about 138 deg. Still very mild, but a long way from 118 deg. Put the new jug on the engine that I had been running, & gained nearly 700 rpm. Pic included to show comparison. Low port is the lo-perf. unit. Time to break out the die grinder! Geo.K

Check out www.shark-racing.com for mucho info on timing

Put a bigger carb on it. I replaced the stock carb with one from a Mac chainsaw. I think it was a Walbro WT 348. It was the exact size venturi as the opening in the backplate.

Bend the reed stop on the backplate. Bend it out untill it just barely clears the crank pin. This will let more fuel in.

Clean up the transfer ports on the cylinder with a Dremel. Anything you can do to smooth out the fuel transfer. Just don't remove too much material unnecessarily. That will INCREASE the swept volume in the crankcase and actually slow down the fuel/air mixture going through the case and into the cylinder===BAD. If anything- you want the fuel running through the case at the same speed as stock or faster. DECREASE crankcase size--STUFF IT-- to make the charge go faster through the case. Never actually done it myself, but it's supposed to work VERY WELL.

Get an electronic ignition for that sucker. It's pricey, but it will make your engine lighter and it wil "spool up" faster when it's not turning up that extra 2lbs. of magneto weight. I used a synchrospark ign. on mine. TOTALLY not necessary on such a small engine- but it idled at 1300-1500RPM and throttle up was really nice. Hand starting was a breeze. 2 flips on choke and then 2 more to start it.

Generally, if you use a BIGGER carb-- then you'll be spinning a SMALLER prop at HIGHER RPM. And if you use a SMALL carb- then you can turn a BIG prop at lower RPM.

Sounds arse backwards, but if you think about it-- it's true. In order to get a bigger carb to work properly- then it needs to have a lot of air moving through it to pull the fuel in at the right mixtures. That means using a small prop and letting the engine rev up more--7500--8000RPM or so.

Using a smaller carb will actually have the engine turning slower with a big prop because the engine will suck enough air through the small carb at lower RPM to make that carb work fine. So put a big prop on the small carb and swing it at 6500--7000RPM.

Big carb and big prop--- waste of time. Engine won't suck the gas in right and it will just generally be a P.I.T.A.

Your engine can only suck so much air through it. You need to match your props and carbs to achieve the desired effect-- i.e. RPM and thrust desired for a given prop size.

I was using a stock carb to start out with, but too small prop. I had a 16-8 on it and it ran like crap. It was turning 7800RPM- but running out of fuel. I couldn't open the needles enough to feed it the gas it wanted. So I switched to an 18-8 and it slowed down to 6400RPM and ran like a top.

After I switched to the bigger carb, I went down in prop size again to let the engine rev up and suck plenty of air through that big venturi. It was ripping along at 8000RPM with a 16-10. Engine was happy as a clam on 40:1 mix and needles were adjusted to normal positions, not all the way out as it was when I initially started.

Increasing the RPM of the engine gets all the air moving around fatser and increases the vacuum at the intake. You need the increased vacuum to get the big carb to work right.

Lowering the RPM slows it all down- but still enough vacuum and suction to make the stock carb work right at those RPM.

Gassers are torquers. They love to run under high loads and grunt along with no problems.

Just a few ideas to mill around with. These are just my opinions and personal experiences with this particular engine. Others might call me crazy- but thats how I got the most out of my Ryobi.

RCING1, thanks for the tip. Great site, with loads of info. Too bad there is nothing on the ryobi. The port timing figures in the "hot rod" section look a little too stout for a slow reving Ryobi. Think that I'll try to "sneak up" on the 160 deg exh. figure, a little at a time, & check the results about every 5 degrees. Thanks again. ....... Geo.K.

I'm using 164 degrees on my "hot rod" Kioritz 23.6cc, but the intended rpm range is near 9000. I'm thinking 150ish might work well for a lower revving Ryobi.

Good luck,
AV8TOR

Once I got the engine running decently, with the new cylinder, proved out what Rcpilet said about the carb/prop thing. The larger carb did little for the large prop, but made quite a difference for the higher reving smaller prop. Had already made a new crankcase back to improve the "stuffing" show. That's the easy part. Have to trim the bottom of the cylinder, & the rear of the piston, to clear the new cover. Also have to design/fab a new reed/manifold assembly, to work with the new cover. Two or three protypes made out of wood, then try to make the real thing.
Agree with AV8TOR, that 150 deg on the exhaust, would probably be a goog place for the low reving Ryobi. However, while going up in 5 deg increments from the original 118, found myself at 155deg, with no improvement. Only them did I think about the possibility of the intake ports also being misplaced on the "bad" cylinder. Checked them, & sure enough only had 85 deg! I probably gave the cylinder the final kiss of death by not being more attentive, but will proceed to increase the intake duration, & check things out, before tossing it into the trash. .............Geo.K.

Hi
Measure the width of exhaust port. I am read that a width equivalent to 60 % of piston diameter is a good number to have good torque.
I have widened a moped engine exhaust port at 60 % and runing good.And have a good life also.
This ratio is to have a safe port width to not allow the ring to go into exhaust port when going up.
If I remember well , the ryobi have a exhaust port with an aluminium piece in the half, do not remove it, it make the life easy for ring , and you have two halves, each at 30 % width of piston diameter.
Of course, you must round very gentle all the ports enlarged in the cilinder surface to avoid scratch the ring.
Others readers in the forum can help with it ?
I have a weedeater engine converted in a test stand and it have a 60 % of piston diameter in the exhaust port in stock form. Aprox 34 mm in piston diameter and 20 mm aprox in the exhaust port width.
Good luck
Have fun
Jose

Hi
The transfer ports , modify it to point the gas mixture to the center of head ( or next to it) from my experience with moped engine, I make it to obtain a straight way to the flow of mixture. Less turbulence.
Remember the geometry of ports in a high performance glow engine , point up.
Have fun
Jose

Thanks for the tips. The exhaust port width is very nearly at the .6 bore diameter that you advised. Seeing that it doesn't have a divider in it, will probably leave the width as is. Transfer ports look rather challenging, without a mill. May try to jig the cylinder to the milling vise of my lathe. Gonna have to think about that for awhile. ......... Geo.K.

Hi
In the moped transfer porting I have bought small grinding stones (general use not any special, they are chinese and soft but they are for only one use)and put on an electric drill, better of high speed, this must be in the hardware shop and are cheap , and with patience I modified, the material was iron but soft, I believe the aluminium is easy to grind.
Put a mark with permanent ink marker , it is the limit of grind work.
Be careful do not touch the cylinder surface.
If you have a friend with a Dremel better, but with care you can make with electric drill.
Have fun .
Jose