rob10000

Hi, long time browser, 1st time poster.
I recently bought a new Ryobi weedwacker so I could upgrade my old single ring piston to the new, twin ring version. My intent was to swap the new piston and jug onto the Ryobi 31cc engine I bought from Jag years ago.
I bought this engine with the C&H ignition to power a clipped wing Stinger 1.20. I finally got around to finishing it after 3-4 years and while I had run the engine years ago to break it in, it hasn't been flown till recently. I thought the twin ring would be a nice little boost.
Here's what I discovered. My old jug does not have the vertical slits in the cylinder walls to bleed off compression. The new jug does. Obviously, I simply swapped pistons and not cylinders. This combination now turns a 18x6W APC "fun fly" prop at 7750 RPMs, and I haven't even fine tuned the timing yet! Recorded thrust with a cheap digital fish scale was around 14 lbs.
One of my flying buddies who also has a Ryobi would like to upgrade his engine as well to the twin ring piston, but also we would both like to know how to find the jugs that don't have the compression robbing slits in them. Does anyone know??

Volture

My Feedback: (1)

Vertical slits??? Are you talking about the transfer ports? How about a picture?

bn120

My Feedback: (1)

they are decompresion slits for easier starting good for consumers but not fo us.

Darin

bn120

My Feedback: (1)

I was also wondering if you would be able to swap cylinders and see if the rpm changes.

Darin

av8tor1977

My Feedback: (6)

I'm really dying to know what "slits". You must be talking about the transfer ports. Picture??

Anyone else?

Thanks,
AV8TOR

Volfy

My Feedback: (23)

Why not just swap the entire engine? They have the same bolt pattern and external dimensions.

I notice the new twin ring has a beefier conrod as well.

rob10000

I'll try and get a picture of the two different cylinders ASAP.

The vertical slits appear as narrow (1/8") grooves that extend deeper into the cylinder when viewed from the bottom of the jug, originating from the transfer ports. As noted above, their funcion is to bleed off compression as the piston rises in the bore. NOT good for R/C engine use, GOOD for weedwacker use!
You'll have to pull the cylinder off your Ryobi to see them.
I want to find more of the cylinders without the decompression slits.

CrazyHerb-RCU-delete

My Feedback: (2)

I have an old single-ring here too....maybe I should swap....
How many more rpm's are you getting v.s. the "slitted" jug?

CrazyHerb

bdtsr

My Feedback: (5)

The Tecumseh 98cc I just got has something like this. It's a bypass port on the side of the block. Good thing is that with 2 set screws I can block it off. The Tecumseh 138cc has a small hole above the exhaust port that does the same thing as well.

Flypaper 2

On this Ryobi, how far up the cyl. wall do these slits go?

flipflop

Here's a shot of the slits that they're talking about. My advice, go ahead and use the new cylinder. Depending on the prop, my twin rings were turning about 500-800 rpm faster than the old style cylinder with a Bowman ring. I was getting 7650rpm on a Pro Zinger 18x6 on the test stand when the motor was new. Although I haven't tached it lately, I've got about twenty flights on it in my Ultra Stik and it's still getting stronger every flight.

av8tor1977

My Feedback: (6)

I just find it hard to believe it's purpose is as a compression release, though I suppose it's possible. I'm thinking more of it being a type of boost port to help purge the cylinder of exhaust gases. Does it extend up higher than the exhaust port?? It might be an assist port to close the reed valve more gently too.

Just my thoughts...

AV8TOR

Volture

My Feedback: (1)

Hmmm... rather than being a compression release, maybe the slits are a method of exhaust gas recirculation to lower NOX emmisions in order to meet new California CARB or US EPA requirements??? The weedie engines have such low compression to start with (especially as compared to a chainsaw)that it seems improbable that an additional compression release would be of any benefit.

rob10000

Thanks for posting the pic of the "slits"

That should clear up what I am referring to!

As I mentioned in the first post, my original jug from Jag does not have these, and my compression is noticeably more than a friends with an identical engine, but with the slits.

It would be nice to find more of these cylinders without the slits, but noone seems to know what application they came in.

I was considering doing some test runs with the same 2 ring piston in both, but I am reluctant to put the piston assembly in a new cylinder temporarily and lose any of the fit between it and the unslitted cylinder

Volture, I don't think your guess is right because I've seen engines 3-4 years old that have the slits.

bdtsr

My Feedback: (5)

From what I've been told, these type of holes, slits, bypass ports are to keep emissions down as Volture has posted above by recirculating exhaust gases into the intake charge. Just like an EGR valve on a car.

av8tor1977

My Feedback: (6)

Hi,

I don't know, but that idea doesn't seem to work for me. Exhaust gas recirculation is used to lower the combustion temperature to eliminate the formation of oxides of nitrogen. These are generally problematic in high compression engines only. "Exhaust gas recirculation" is rather automatic in two-strokes by design, and a lot of engineering was done to try to eliminate the contamination of the incoming mix with exhaust gases. This is part of the concept of tuned pipes, and of Schneurle porting. The transfer ports are designed and aimed to try to purge the cylinder of exhaust gases as they introduce the fresh charge into the cylinder.

Interesting subject here....

Once again, just my thoughts.

Av8TOR

CrazyHerb-RCU-delete

My Feedback: (2)

Anyone thought about calling/e-mailing the company???
Get it straight from the horses mouth???

Just a thought...

CrazyHerb

CrazyHerb-RCU-delete

My Feedback: (2)

Is there any way to fill the slits?

CrazyHerb

johnmd

JB Weld?

Volture

My Feedback: (1)

Not for long at combustion temperatures!

CrazyHerb-RCU-delete

My Feedback: (2)

Just checked my single-ring ...and it has the slits...wierd...
I have one more to check...still in the one I use for the yard...
I may be able to swap...

Crazyherb

rob10000

ok, here's something to consider.

The Ryobi I got from JAG without the decompression slits was never intended for weedwacker duty.

I arrived at this conclusion by noticing today that the 4 round bosses where the front plate would normally bolt to are NOT tapped. A friend and I were comparing the two engines to try and find any clue as to where mine might have been originally intended. Jag isn't answering their phone, and I doubt they would spill the beans anyway.
Hmmm... what application would this engine be in that wasn't a weedwacker? Maybe a blower? Perhaps in that application the slits aren't needed for easy starting by John Q. Public...
I hope someone else can find a cylinder like mine without the slits and shed some more light on this topic...

av8tor1977

My Feedback: (6)

Look very carefully, and let me know if those slits extend above the level of the top of the exhaust port.

Thanks,

AV8TOR

rob10000

The slits narrow in depth as they extend higher in the cylinder. They extend well past the exhaust port.

ZM2000

My Feedback: (1)

My Ryobi also have the grooves and I dont know how to fill them up. Performance is 3500 min and 6200 max RPM with a Dynathrust 18X6 prop. I think that the grooves are definitely meant to reduce the power for weed wacker use. I am looking forward to hear if anyone has any novel idea to fill up the grooves.