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ryobi 31 modification

Old 04-18-2008, 11:11 PM
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luckytempo93
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Default ryobi 31 modification

hi everyone. i am building a rc airboat and i have a 31cc ryobi on it and i was wondering what engine work i could do to it to make it swing a 16X10 3-blade really well. i understand that 8000rpm is a no-no. so i want to keep it under that. what i am looking for is torque. if that is possible without too many rpm. i bought one engine and took it apart today to see what the insides looked like and took some pictures. but when it went together i didnt see the pins arount the piston groves for the rings. so i broke them. i just picked up another engine and took that apart too. so what im gonna do is make one good engine and buy rings for the other. so where should the new rings come from and any advice what i should do. here are some picts of what i have parts on the right are from the new engine.
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Old 04-18-2008, 11:16 PM
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luckytempo93
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Default RE: ryobi 31 modification

also i forgot some stuff. with the pistons and rods. which ones do you think are stronger? the one on the left looks more like an H-beam style and the one on the right has a "mountian" in the center. the cylinders are the same but the exhaust on the left has a beam down the center. can this be removed and than made bigger to be ported? and the third question. that canter picture... are those groves in the intake ports called compression reliefs? can the be filled in with jb weld and last?
Old 04-20-2008, 05:21 AM
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RhesusMinus
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Default RE: ryobi 31 modification

I use this engine in an airboat. I have done several modifications with to find some more power. Get yourself a Bowman pistonring, it will increase the compression, change the carb for one with 11+-mm venturi, widen the exhaust to at least 0.65 of bore and do NOT touch the bridge in the middle, set exhaust timing to approx 158+-, ditch the base gasket or get a thinner one to get a piston/head clerance of 0.5mm, then you will have a lot more power.

Personally I took it a little further, I widened the exhaust to 0.75% of the bore, used a exhaust timing of 162, if I remeber correctly, turned down the end of crankcase in a lathe to decrease crankcase volume, opened up the reedvalve and used 0.15mm shimmetal to make a new petal. I also opened up the transfers and matched it to the case and made a moussecan pipe. The pipe increased the rpm with 800+ or something(dont remember exactly) It turn an MAS 18x8 at 8900, and 18x10 at 7800. If you are conserned about the tourqe, start with a lower exhaust timing.

My engine was originally form a Stihl FS weedeater, but it is the same engine as the Ryobi, accept for the rearplate with the reed intake. I have the conrod on the right and the right cylinder from your pictures. Wich one is the best, I have no idea. Do the right cylinder also have the cuts in the transfers, if not I would use that one.
Old 04-20-2008, 01:23 PM
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luckytempo93
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Default RE: ryobi 31 modification

yes they do have the slots. what are they for? why should i leave the bridge alone? and does anyone know ehere i can print off a degree wheel?
Old 04-20-2008, 02:38 PM
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rangerfredbob
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Default RE: ryobi 31 modification

I've heard the slots make for easier starting, or lower emissions or something

as for a degree wheel, I did a google image search and came up with a few
Old 04-20-2008, 03:00 PM
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RhesusMinus
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Default RE: ryobi 31 modification

I believe the slots are for cleaner combustion with lower emissions like rangerfredbob says. I really dont understand why there should be compression reliefs for an engine with already low compression. You can remove the bridge completely if you want to, but dont make it much thinner. When the bridge is too thin it might overheat and score the piston. I think the piston ring is pinned on the intake side, but check it before you remove it. Usually the safe width of the exhaustport is 0.65 of bore when you use a brittle iron pistonring, otherwise 0.72 is ok, some even go to 0.75. If the exhaustport has a bridge, you can go even further. I widened the exhaust to 0.75 and left the bridge where it is, because it is better for the pistonring.
Old 04-22-2008, 07:08 PM
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clocknut
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Default RE: ryobi 31 modification

RheesusMinus
what do you mean ((opened up the reed valve)??
Did you in large the carb to the reed valve passage because of the larger carb or what? thanks
clocknut
Old 04-22-2008, 09:33 PM
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luckytempo93
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Default RE: ryobi 31 modification

no. the 11.11 walbro carb fit exactly to the back of the manifold. i just made a spacer bc the carb throtle would hit the backing plate. but the spacer was good cause of the added torque it added. but as far as the reed valve goes. if you take the backing plate off you will see the reed valve and a bent piece of metal on top of it. take pliers and bend it away from the valve just a little. i have heard 1/8 of an inch. i just bent it till i thought it looked good. lol. but DONT touch the reed valve. this allows the valve to open further and allow the fuel to rush in smoother.
Old 04-23-2008, 01:15 AM
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Default RE: ryobi 31 modification

It also allows the reed to flex more which will allow it to fatiuge faster and break.

Ask me how I know. [&o]
Old 04-23-2008, 07:27 AM
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luckytempo93
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Default RE: ryobi 31 modification

well what kind of material would you use in replace of that? or is there somewhere that i can get a better one
Old 04-23-2008, 11:02 AM
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clocknut
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Default RE: ryobi 31 modification

Thanks for all the info. I'm in the process of doing this engine for my 88" 1/4 scale tiger moth and I have received the ring form Frank Bowman I have in larged the exhaust port and I also took out the bridge in the exhaust port and I have the 11.1 mm carb for it. I have A good friend that thinks all weed eater engines (converted engines) are A wast of time and not Worth the out come so as you can see I would like this thing to really run good and good enough to possible change his mind.

Thanks clocknut
Old 04-23-2008, 01:32 PM
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luckytempo93
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Default RE: ryobi 31 modification

thats a good reason. i am building a rc airboat 4feet long and i too have a friend that thinks its a waste of time and thinks that i cant do it. boy is he in for a surprise. lol. how much did the rings cost you? is yours a 2 ring piston? thanks
Old 04-23-2008, 02:24 PM
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Default RE: ryobi 31 modification

Shim Stock works pretty well.
I've seen them made out of Carbon Fiber.

You can bend the stop. This does allow the reed to open wider.
Just remember to change them.

If you fly a lot, what ever that means, change them every year.
If not, change them less often.

It really is a crap shoot.
Unless you need that last 500rpm I'd leave it stock.

I tend to put powerful engines on my planes and fly at 1/2 throttle.
Instead of using a small engine and flying it at full throttle.

Here is another cage:
http://cgi.ebay.com/REED-VALVE-RETAI...ayphotohosting

He may have the reeds for it also.
Old 04-23-2008, 05:43 PM
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clocknut
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Default RE: ryobi 31 modification

The ring was only about $10-$12.00 form Frank.Mine is A single ring piston.I think A air boat would be a lot of fun until you had to go swimming for it
I have always wanted to drive A real air boat.Good luck with your engine let us know how it turns out and I will let you all know how mine does.I'm thinking of running mine on glow/gas.I think it will all depend on how good it idols down.
Thanks
clocknut
Old 04-24-2008, 10:40 AM
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Default RE: ryobi 31 modification

I am convinced that these engines are best left as built EXCEPT that conversion to electronic ignition to save weight and improve timing control (advanced at high RPM, retarded for better/slower idle) and swapping to a larger carb and minimum restriction muffler to let it "breathe" better at high RPMs are worthwhile. The other, more sophisticated, mods can provide some performance improvements but can also result in a destroyed engine. If you want a "real screamer", maybe a Ryobi should not be your first choice. It IS a good, easy to convert engine that will perform very well for what it is, for a very good price...
Old 04-24-2008, 11:23 AM
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luckytempo93
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Default RE: ryobi 31 modification

well in your opinion what would be a good engint that can be converted and mounted well. i loked rhe ryobi bc of the mounting positions that i could use in the front and rear. are homelite engines ok? looking for something cheep. lol like everyone else.
Old 04-24-2008, 12:45 PM
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clocknut
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Default RE: ryobi 31 modification

I need the small width of the ryobi to fit in my narrow cowling.So that doesn't leave many options.I just need A rear carb engine.I also should ask what engine you might suggest in the garden variety engine on this forum
Old 04-24-2008, 02:56 PM
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Default RE: ryobi 31 modification

The real question is what do you need an engine to do?
What plane are you going to put it in?
What is your flying style?
How much thrust do you need?
What size prop and how fast are you going to spin it?

I also believe these engine are best left pretty stock.
But which or what is stock?
There are high compression Ryobis out there.
I would like to see a Ryobi chainsaw.

A good squish band, free flowing exhaust and a proper size Carb. will wake up
most any engine with no bad side effects.

Start with these 3 things and if need be add to them.
7500rpm should be concidered the top rpm for this engine.

There are a lot of 31cc Ryobi engines being used.
Find out what you need and go from there.
Old 04-24-2008, 05:45 PM
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RhesusMinus
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Default RE: ryobi 31 modification


ORIGINAL: clocknut

RheesusMinus
what do you mean ((opened up the reed valve)??
Did you in large the carb to the reed valve passage because of the larger carb or what? thanks
clocknut
The carb was a perfect match to the intake, but I modified the intake to a trumpet shape, and fitted a new reedpetal made from 0.15mm shimmetal.
I also adjusted the reedstop to 3-3.5mm.

I dont know what rpm Ryobi recommend, but the Stihl FS36 manual says 10 000rpm max.
Old 04-24-2008, 06:52 PM
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clocknut
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Default RE: ryobi 31 modification

OK like I said before I'm putting it in A quarter scale 88"w.s. tiger moth so it doesn't really need A lot!
But it is like A American graffiti nite when you pull up to the stop lite and your are so cammed up that it about throws you out of the car and shakes YOUR TEETH OUT OF YOUR HEAD and every one looks and goes WOW!! and then the lite changes and you putt down the road.[8D]
It is great to have A little extra under the hood in case you need to just get out of some ones way.
clocknut
Old 04-25-2008, 01:13 AM
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Default RE: ryobi 31 modification

An APC 18x6 at 7000 rpm should be plenty.
I have a 77" Tiger Moth and a 25cc Toro and it is more than enough.

The 18x6 is a good prop for my plane
Old 04-25-2008, 02:09 PM
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clocknut
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Default RE: ryobi 31 modification

Ralph does the 25cc toro fit under the cowling OK? I would think that it does.I'll try the 18x6 prop I don't even have the engine back together yet but in the mean time i will pick up the prop.
thanks
clocknut
Old 04-25-2008, 02:43 PM
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Default RE: ryobi 31 modification

No it doesn't.
Actually it sticks out bottom and both sides.
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