Flathead

My Feedback: (11)

Flathead is back, and with more questions. Weather and other problems has kept me from enjoying the hobby, but I have been looking at my RYOBI collection and now I have more questions. I aquired another off of a "TRIMSALL". From all appearances, this is a early engine with a different back-plate and reed set-up. I cleaned it up, including the original carb which matches the other small carbs and put a 18X6 prop on it, and got it running. With a little adjusting, I got an honest 7800 rpm. The exhaust port apperared to be stock and all that, but I looked further comparing this block to newer blocks. I finally discovered that on the piston end of the exhaust port, the width is a full 1/8 inch wider. Not the top or bottom, but the sides. I assume that all the porting that has been done to date, has anyone ported the sides rather than the top. Next question "When you port do you carry the porting to the outside of the block so that the port is the same size to the outside next to the muffler, or is it bigger out next to the muffler. Thanks for any help you can give me. If the wind ever stops (50 mph+ today), I plan on putting a 26 carb on the older engine and see what happens. Will keep you posted.
Flathead.

ckangaroo70

My Feedback: (51)

That sounds like real respectable numbers. I know I would be pretty happy with those numbers. My stock "Zama carb" Ryobi 31 spinning 18x6 APC runs in the 7200 neighborhood on 32:1 give or take a few hundred RPMs depending on the day. My concern regarding trying to gain RPM by modifications is that from everything I have read....the stock piston rod is only good to handle 8000rpm or less. Would sure be nice to see someone build a low cost solid billit aluminum rod and then I would really be open to modifications like better porting, Bowman rings, larger throated carb, less restrictive exaust, etc etc .

Ralphbf

My Feedback: (27)

I believe the trick here is running the Prop. that limits the RPM to less than 8000rpm.

Do the mods, run the bigger prop.

w8ye

My Feedback: (16)

It has long been said on here that a Ryobi type engine will not run for very long over 8000 because of rod/crankpin problems.

Porting and props should be limited to keep the rpms in the mid 7 area.

Flathead

My Feedback: (11)

Additional info on the TRIMSALL engine that I failed to mention. The RPM on this egine was achieved after the flywheel was trimmed and the larger carb was installed. This is one of those engines that appears out of balance without the flywheel, and it has a solid connecting rod. The conection rod looks much like the ones found in chainsaw engines that have mains on both sides. The bottom of the cylinder is not tapered at present, but will be after I tear it down again. It has one wide ring, and very good compression. I plan on installing a FRANK BOWMAN ring at some point. The prop I used is a 18X6 APC narrow blade. The engine has very little wear on it, and the reason it was discarded was the fact that the recoil assembly was totally destroyed and in pieces. "FLATHEAD"

ckangaroo70

My Feedback: (51)

I said that my engine was stock, but I forgot to mention that my flywheel has been turned down as well to remove the fins which I would guess helps the engine to spin up a bit quicker. With the fins left on I would imagine it does cause the engine to have to work a bit harder. Anyhow mine is not 100% unmodified, but not much else has been done in the way of any performance mods.

Elnino

My ryobi does well over 8000 RPM and i have not thrown a rod yet (touch wood) but my engine is the newer type with 'I' beam style rod and twin piston rings, not the older style.

ckangaroo70

My Feedback: (51)

What prop you running?

Elnino

hehe, Prather 280 - It's in a boat....

Flathead

My Feedback: (11)

I am putting togather another RYOBI engine, and have had it running. This block is one with two slots on each side, a double thin ring, and a somewhat oval shaped exhaust port (no center post). It has the heavy duty connecting rod as well. I have raised the exhaust port to 140 degrees, and squared out the corners. A 26 carb and custom muffler and EI set at 30 degrees with an auto advance. I was going to leave the gasket out, but there is not enough head room unless I bevel the piston. Presently it will turn a 18X10 wood "TOPFLIGHT" prop 8000 RPM. My question is "What is the maxium exhaust porting possible without ruining the set-up?" Has anyone had this experience? Thanks. Flathead 4

av8tor1977

My Feedback: (6)

You are probably "good" where you are at. I run 150 degrees on the exhaust, but higher exhaust timing biases the power curve higher.

AV8TOR

cutarug

I have been following this thread and have test run a Ryobi intended for a Sparrow hawk (50CC) or 2.00 FS size.
I used a bowman ring ,150deg exhaust timing, 12mm Walbro, replaced the cylinder gasket with a thin one , gutted the muffler and used a cylinder with slots, as I have never seen one here in Australia without slots.
I got 7400 from it even though it has not developed that snap you get when the compression is good and you flip it over.
My question is does the compression get better as you run it in and how long does it take.
I tested the ring in the bore and there is little to no gap but I expected more compression than I got.
I took the ring off the piston and reduced the thickness a bit on some fine wet and dry paper as it was just a bit thicker than the original and I will test it again.
I hoped to get 7700 from it which would fly the Sparrow hawk with about 16 lb of thrust.
Any suggestions for developing more compression.

av8tor1977

My Feedback: (6)

The only thing I can think of is to ask if you sanded the cylinder bore with 400 sandpaper as Frank Bowman suggests? This gives the ring something to "bite" into to break in and seal well to the cylinder wall. Other than that, just keep running it. It takes some time to break in.

In addition to gutting the muffler, you need to provide a larger outlet as well. This helps a lot.

AV8TOR

diceco

The compression and power will improve as you get more run time on it. In general I've heard that gassers will improve power output over the course of several gallons of fuel burned, although I've only qualitatively observed this.

The best thrust I've gotten out of a Ryobi 31 has been 15 lbs of thrust based on a sample of two, one early model single runner transfer port, single ring piston and one dual transfer port two ring piston. 16lbs of thrust is an admirable target and might be possible but it will be at the very max of it's capability from my experience. In order to get this kind of power a tuned pipe (aka expansion chamber) will probably be necessary and I've never made one up for mine.

diceco

CraigjP

anyone ever use ethonel as fuel?


CraigP

cutarug

I honed up the bore with some fine paper and ran it again and ran fine for a while then started missing and backfiring above 6000 rpm.
It has a electronic ignition but I am not sure if the problem is ignition or fuel as induction and exhaust timing can't change.

av8tor1977

My Feedback: (6)

Back firing is not likely caused by carburetion. Try changing the spark plug first, and check your timing.

AV8TOR

Rcpilot

My Feedback: (78)

I blew one up. It's been a few years and I haven't touched a conversion engine in a number of years. Seams the exhaust was arond 150* and we had dropped the cylinder down to .019 squish. Twin rings. Bowman rings.

It was turning 8800RPM on the ground with an 18-8 or maybe an 18-10 - again, it's been a LONG time and I'm getting a little older. It only lasted about 4 flights on a Stinger 120 after we modded it that much. Then the thing made a loud bang and the air conditioner on the front stopped spinning. Thats when the pilot started sweating.

It was fun to mod this and mod that. Then take it up to fly a few times. Back to the shop and mod this again. Grind that a little more. giggle giggle snicker snicker - let's go fly it again!! BOOM! [] Yup, that musta been too much.

I think it may have held together if we'd used a bigger prop and slowed it down. 8800rpm was just too much for that stamped rod.

Twin Star

One other thing. Many forget that static RPMs are much less. The actual RPMs can increase as much as 1000 in the air.

Elnino

Was that one with revised 'I beam' style con-rod or the original pressed type? Assuming it was the rod that let go as that seems to be the weak point. I am running the newer type and run in excess of 12k in my boat...

Rcpilot

My Feedback: (78)

It was an older stamped rod. Yes, thats what blew up. I think the bottom end let go and spread out. The top of the rod was still intact on the piston, but the bottom of the rod was blown out and the roller bearing could not be found. The rod got between the crank pin and the case. Blew a hole in the side of the case and busted the cylinder lug off that side. Wish I had pictures.

Elnino

Well so far i have not heard of any problems with the newer rod exploding. Perhaps it just needs more time but my engine is at least a testament that the newer ones are much stronger.

av8tor1977

My Feedback: (6)

Could you possibly post a picture of the newer style rod? What did the machine look like before conversion? Was it one of the newer Ryboi's that has the block and fan housing cast together so that you have to cut it apart to convert it? (Unlike the older Ryobi's that have a housing that unbolts.)

Thanks,
AV8TOR

Elnino

Mine was in a hybrid type. It still had a bolt on fan shroud but used twin piston rings with the slits in the cylinder. It was by MTD (who made these Ryobi engines). I would suspect than any newer ryobi type would have the newer type rod.

See this link for the newer piston/rod -

From: http://www.ereplacementparts.com/pis...-p-282359.html

av8tor1977

My Feedback: (6)

Thanks for the pic. I can't really tell how it is made from that picture however. It still looks like a stamped steel rod I believe... (But not quite like the older ones.)

AV8TOR