av8tor1977

My Feedback: (6)

They don't seem to like to idle well much below 2000 rpms. It's normal, though I can't explain why.

AV8TOR

combatpilot

I think its cause the crankcase volume is so big that it dosent work well at that speed. one of my tests today I decreased the volume and it idles so slow you can darn near count the blades as they go by lol. didnt do anything for high end though.

Im taking mine when the engine has runn for a minute or two at full throttle and stabilizes. I think this is a fundamental difference in our numbers. that and the fact your running an apc prop which i think runs a little faster anyhow.

In my opinion the biggest payoffs so far are. #1 making sure you have a good cylinder to ring seal amking for the most compression you can get. # 2 making mods to up the compression ratio. # 3 a larger carb. my 12.7 mm venturi made huge gains. thse so far seem to be the best of the bunch and everything else plainly just dosent work or i havent had time to try it yet. Jack had great results with his exshaust port mod as where mine yeilded no results. important thing to note is we have different cylinders we are using. Any mods to reed valves had no effect at all so that makes me conclude belmouthing the intake opening cutting down the reed or a litghter opening force reed is not worth doing and has no effect at all. A stock back plate in my opinion is more than adequate and a stock reed is more than adequate and all my testing to modifying these yielded nothing. stuffing the crankcase and reducing the crankcase volume had no effect also expect it idled very well. again in my opinion a stock backplate un modified is more than adequate and any crankase stuffing which is complex only improves idle.

I think if you make sure your cylinder/ rings seal good weither its the two runner cyl or one runner cyl and take stepps to raise the compression as high as you can get it ie removing or reducing base gasket thickness and use a larger carb of 12.7 or 11.1 mm venturi WITH stock ignition and no other special mods should pull an 18-8 prop around 7500 rpm and i do mean around or about. anything else despite what you might read in my opinion and according to my testing is just plain bull. I do still need to make the electronic ignition to see what gains are their and i need to do the port timming again now that i have a good cylinder to see what gains are there again if any.

diceco

I did NOT remove the base gasket. It's stock and in place.

IMHO the amount of compression increase from removing the base gasket is minuscule and coupled with the resulting near interference of the piston and the deck, and the changes in port timing. ... It's not worth it!

diceco

diceco

Say combatpilot,

Have you gotten any thrust numbers with your latest motor configuration?

diceco

combatpilot

todays progress: in my testing today I borrowed a tach from a friend and compared it with mine. his would hold a very steady rpm reading close to what i was getting before i dropped mine. mine was all over the place and was never where his tach was. I put m ytach on the bech and fixed it. How did i fix it? i took a hammer an repaetedly smashed the piece of *%*^( till it was a plie of chips on the bench. Now i can stop wondering if my tach is accurate as now i know ITS NOT LOL.

Ok so i started on a project to to build a cylinder hone to try and refurb some of these cylinders. In a way it may have been a failure but it is important to post failures to as we can learn from this too. It may be usefull somewhere sometime and may even be good for these engines as a light polishing/ cleaning method if not done to hard or long.

Here is what i started with



I took the half inch dowl and slit it with the table saw



the idea is to use the foam to back up the polishing cloth sandpaper. i used the finest i could get at oriellys auto parts. it came in an assortment pack. i will use the rest so no biggie. the polishing cloth is placed one on each side so when wound up it has two surfaces to the outside with the grit out and the foam backing up the cloth. the scotch brite at the end is to polish the combustion chamber which worked marginallly. I put one screw in it to clamp it all together and hold the cloth in place.



the foam and the cloth on the one above was to long and worked better when i shortened it up like the one here. the end of the dowel extended through the spark plug hole and made a nice guide.





When you make one cut your foam long then wind it up into the cylinder. keep trimming the foam till it has the pressure you want.



I have a cylinder that is pretty badly scared as you can see by the piston. its hard to see from the cylinder pics and for some reason the camera wont take a clear pic of the cylinder. anyhow i thought it would be a good test subject.





here is what it looked like after honing. I could not hone the scars from the scores out and i concluded that the scores are already cut through the chrome lining so if you got one that is scored its probably better to trash can it and look for a good one. one season wonders are the best donors. thats where they bought it ran it one season then put it away and would not start next year so they bought a new one. nothing wrong with it just needs a carb cleaning lol.



In my opinion if this method is used it should be to a cylinder in good condition and done very lightly to just break the glaze and clean it up and polish.

ok so where from here. here is a pile of ryobi parts i got from the engine shop and i have three really good cylinders and one good two ring piston from the set. I decided since mine started as a cylinder that was questionable and had scores in it to go back to my cylinder leakdown test and see if i could improve this situation.



I started by examining my original set up again. the cylinder still was kind of rough from the scores and the piston now has the matching scores on it. the rings look fair now and it still had good compression. But how good. well to find out first i looked a tthe ring gap. the rings were removed from the piston then using the piston to keep it streaight in the bore it was shoved into the cylinder to a piont where it contacted the cyl wall all the way around. i then measured the ring gap with a feeler gauge and this is what I got. top ring .019" and bottom .018". I then installed it back on the engine test stand without backplate installed. I installed the leakdown pressure tester and as you can see it had a pressure of 94/100 psi. this is a leakage rate of 6% and is quite adible. If the cranks was rotated to move the rings to other positions it was much worse and the best spot was at tdc.



Ok now here is where we get some good info. I now turn the input valve off and count how long it takes for the cylinder to leak down to zero psi. it took 3 seconds for this setup to leak out.

When this setup was test run and remeber this cylinder has the 150 deg port timming change i took a reading of 7380 rpm at 13 lbs 6 oz static thrust. this is close to what i was getting last time i tried this same setup test.

I then picked out a really clean cylinder. when i say clean i mean no scores and good cylinder wall condition with very little wear and not physically clean. I then de carboned the combustion chamber and did no posishing of the head or cylinder. i did make sure it was as clean as i could get it with cleaner and compressed air. i then made sure the ring lands on the piston were free of carbon and had a good surface where the rings seal. i cleaned the rings and scuffed the ring faces with a houshold green schotch brite pad. I put the rings in the cylinder and measuered a ring gap of .012 on the top and bottom. not bad, better than the first cylinder but not as good as the bowman 0 gap rings. the rings were assembled back on the piston and installed into the cylinder and on the engine and test stand again with no backplate. when i installed the leakdown tester and turned on the air i was stunned at what I got.



Thats right 100/100 psi. thats 0% as best the resolution of my setup would allow to be seen. It was hard to hear any kind of a leak at all and it seald great. it was constant on the seal anywhere the piston was in the bore above the exshaust port of course.

Ok now when the valve was closed it took 13 seconds for the pressure to leak down to zero psi. thats a 10 second improvement.

When this engine was run with this cylinder set up and remeber it is stock without the port timming mod it ran 7530 rpm at 14 lbs 2 oz. finally broke the 7440 barrier lol.

From this testing I can conclude that zero gap rings may be nice but it is possible to get a good seal even with a stock ring and a good condition cylinder with no honing. Without a leakdown testor there is no possible way to know how good your seal is no matter what you have done to it. It is the ONLY way in my opinion to tell ring and cyl sealing condition. 0 percent leakdown with a 13 second count to 0 psi is pretty damn good and would very much like to see what others have. I would very much like to try the zero gap rings to see if it improves more. the resolution of my testor wont let me see any improvement in the gauges but i would see it on the count when the valve is closed and it is counted how long to go to zero psi.

Ok from here I have been dying to try this backplate modification. It is designed to reduce the crankcase volume. I will not go into construction details but here is the finished product.



This backplate mod reduses the crank case volume by a bunch. sorry i dont have a number for this.

when this was installed and run the rpm of 7530 and static thrust of 14 lbs 2 oz. no change what so ever on the high end so i concluded this mod is not worth doing. Even when tried again with the waiste cut reed valve there was no improvement. the only improvement noticed was the idle was improved drastically. it could be idled so low you could almost count the blades going by lol.

I am now wondering if I am about at an end with this engine. there dosent seem to be any mods left that might improve performance to a point that it would be worth doing the mod. most of what I have tried has had no effect good or bad. I dont know if im missing something that may yeild more power but i dont think so.

I think from here i got two things left to try. 1st iss to do the timming mod again and the 2nd is to do the electronic ignition. from there its gona go on the plane and im gona go flying lol.

combatpilot

Jack i still hacve a gasket in mine to but i reduced it to a .010 thick gasket from a .030. .010 is the thickness of a grizzley tools catalog cover lol. anyhow any lower it would hit and interfere. I think i did get a gain of about 130 rpm with this mod. i am to lazy to go back in the post and look it up. i will have to look at my notes when i go to the shop tommorow lol. the close tolerance does concern me. its not a problem till i start to get some carbon in there. even with this lowereining with the cylinder im using it only lowered the portt timming to 140 degrees.

thrust and rpm numbers on the post above.

Flathead

My Feedback: (11)

Back to your discussion about honing the cylinder, when I got my first ring from Frank Bowman, I ask him how to do this, and his suggestion was to give it only a light scratching with some very fine emery cloth and a finger (or thumb) if you have stuck your finger in a prop lately like me. My finger is healed, and I will be back to testing after a short period. Since I am an "OLD TIMER", I haven't managed how to put pictures on this forum, any suggeastions? Flathead

combatpilot

This method of honing can be done with a very fine emory cloth also. the foam can be adjusted to give as much or as little pressure as you desire. the pressure across the paper will be more even than it would be with your finger. your finger is gona give a definate pressure point. the cylinder that i honed to test i honed pretty hard and it didnt hurt the chrome at all. the chrome is pretty damn tough and if it can be hurt by light honning with this method then the rings are gona kill it altogether. again if you got scores the chrome finish is done ant the cylinder is junk. also i for got to add never hone dry. dry holes like a little lube. a few generous shots of wd 40 work great.

As far as posting pictures it really is a process and i described it in a poast somewhere. i will look at it and see if i can find it latter. remind me if i forget ok.

combatpilot

Here it is found it. hope this helps.

To post a picture i use a picture hosting site. The one I use is myspace. Ok so you put your pictures on myspace. you can choose there weither you want them public or private. There is also photo bucket, pretty much any site you can post pictures on will work. So when your on your photo hosting site you bring up the picture. Right click on the picture and it will bring up a menu. Select properties. then go to where it says Adress: URL and highlight it and copy it. this is what you will get ie here is one of my pics.

htp://c1.ac-images.myspacecdn.comimages01/26/_0facda2c09fb974da070ced452fb8de8.jpg

ok so now go back here to where your wrighting the post and click on the image button on the message tool bar and you will get this

[img][/img]

Put your cursor right in the middle of the ][ then paste your url adress like this

[img]htp:/c1.ac-images.myspacecdn.commages01/26/l_0facda2c09fb974da070ced452fb8de8.jpg[/img]

ok now notice there is a [img] before and after the url adress. the last one has the / after the [. this is important as it closes it out. this is what you should get when it posts

combatpilot

Here it is found it. hope this helps.

To post a picture i use a picture hosting site. The one I use is myspace. Ok so you put your pictures on myspace. you can choose there weither you want them public or private. There is also photo bucket, pretty much any site you can post pictures on will work. So when your on your photo hosting site you bring up the picture. Right click on the picture and it will bring up a menu. Select properties. then go to where it says Adress: URL and highlight it and copy it. this is what you will get ie here is one of my pics.

htp://c1.ac-images.myspacecdn.comimages01/26/_0facda2c09fb974da070ced452fb8de8.jpg

ok so now go back here to where your wrighting the post and click on the image button on the message tool bar and you will get this

[img][/img]

Put your cursor right in the middle of the ][ then paste your url adress like this

[img]htp:/c1.ac-images.myspacecdn.commages01/26/l_0facda2c09fb974da070ced452fb8de8.jpg[/img]

ok now notice there is a [img] before and after the url adress. the last one has the / after the [. this is important as it closes it out. you wont see the picture till it posts

diceco

Those are pretty good performance numbers! From what I can decipher from your description the motor is:

The older type crankcase and cylinder with the single transfer port per side
No port mods at all, ie. stock exhaust port with the bar separating the port
No reed valve or back plate modifications
A stock two ring piston
The base gasket reduced to a thickness of .010 inches
Stock magneto ignition with no modification to the flywheel, ie. no trimming of the fins etc.
An after market free flow muffler
A 12.7 mm Walbro carb
The prop was your DynaThrust 18x8.

Please correct any errors in my guess as to what the exact engine configuration is.

av8tor1977

My Feedback: (6)

If a person has a Ryobi with the low exhaust port timing of around 120 to 130 degrees, I absolutely guarantee that performance will increase noticeably when raising the port timing to about 150 degrees.

AV8TOR

desertdog

I have a 31cc MTD/RYOBI from a yardman weedeater with jag conversion parts and stock carb. It tachs as high as 14000 RPM. I think that is about 42 LBS of thrust give ir take.

desertdog

14000 RPM WITH A TOP FLITE 18X8 POWER POINT on my ryobi. I forgot to add prop size in first post.

diceco

There is something wrong with your tach. You sure it's not half that ie. 7000rpm?

diceco

desertdog

Its a new tach from tower hobbies. I spent a full day playing with tach and ryobi. this thing screems. I will check it again, but at first I did not belive my eyes so I called tower to make shure I was doing everything corect. I will do it again and let you know. But for now I have a x wife comming to visit. I must clean the house.

desertdog

It does tach 7000-1000 with no problem at all. at full throtal between 12000-14000.

Ralphbf

My Feedback: (27)

If you have the laminated rod it will eat it's self above 8500rpm.

My tach reads wacky with a low battery.

What does it read when pointer at a light bulb?

desertdog

It reads 3600 on the light bulb. This is the two ring steel rod ryobi. so I should not run it over 8500? I will try new batteries.

Ralphbf

My Feedback: (27)

3600 rpm is the right answer.

60 cycles per second times 60 seconds = 3600 cycles per minute or 3600 rpm.

There are no line running the length of the rod?
Then you have a better rod. 8500 is the top rpm on the El Cheapo rod.

You should consider a larger prop 12000rpm is a lot.
I'd aim for 8500 maybe 9000 on the ground that would put you between 9000 and 9500 rpm in the air.
That's enough in my opinion.

av8tor1977

My Feedback: (6)

Sorry guys, but a Ryobi wouldn't turn 14000 rpms with an 18 x 8 prop even on pure nitro....

AV8TOR

adrian222

My Feedback: (1)

Try putting the prop on the engine.

combatpilot

yea thats pretty much it. I do have the fins cut off the flywheel though. it is faced off even with the flange near the crank.

Jack did you get my last e-mail? im not sure it went through.

I dont want to sound like an a hole but im gona call bull***** on that one. Its just not possible. especially with a stock carb. Im sorry to say i think you got a bum tach. mine would read 3600 when held to the light also but would read 12000 plus on the engine and when i used my buddies it read 7500. How are you getting the 42 lbs of thrust? is that a number off a web site or is that an acctual test number from a test you actually performed? Are you darn sure thats a 18-8 prop or again like that one guy says is it on the engine lol.

Avi8tor that very well may bee. On my tests i just didnt get those results but mine started at 140 deg to begin with. this leads me to think i have a much different cylinder than what your working with. the engine i am using now i switched to the better comp cyl and am now getting 7530 rpm. I checked timming and it is at 138 degrees. I did some measuring andd some math and to get it to 150 i would have to raise the exshaust roof .065" I went ahead and did this. When i did this i did it in a mill and it came out perfect. the new timming measures to 152 deg. I was gonna test it yesterday when it started to rain and i havent got a chance to test again. i will today if i can, I want to get my electronic ignition on today to.

Also i Figured it out that mathematically ever one degree you want to raise it you need .0055" that 5 and a half thousands raise in the exshaust roof to get 1 degree. ok so if you want to raise it 10 degree you would raise the port roof .055" right?

diceco

Man, that's very encouraging.
So, to make a Ryobi develop a bunch more power:

Get bigger 11.1 or 12.7mm carb
Fix single ring pistons with a better piston ring (ala Frank Bowman?) THE TWO RING PISTON NEEDS NO MODIFICATION
Get after market free flow exhaust or modify stock muffler with bigger outlet pipe.

YOU ARE MOST OF THE WAY THERE!! Right??

Those are pretty inexpensive and painless mods.

diceco

desertdog

Thanks to all for the input. This is my first gasser, I want to put it on the giant super sportster. I just got a email from wacker engines and said most of the MTD engines turn a 18x8 at 7500 rpm. So you are probably right, 14000 just is not possible. I will put the prop on the engine this time and try again. I got the thrust reading from a website. Have a great day.