cutarug

My Ryobi conversion has a 11.1 throat (28) , Bowman ring, cdi ignition, 18 x 8 prop and spins at 7600 static.
When it unloads in a dive the motor misses and I am not sure what is causing it. I have richened it and leaned it but this makes no difference.
The carby a Walbro WA39 has a new kit in it and the lever arm has been set with a gauge so I know that,s correct.
I ran a static vent inside the model from the carby vent on the regulator but no difference.
I am not sure if its a fuel problem or ignition, the cdi has a lipo and regulator 6v.
I was going to try lifting the arm a little to get more fuel, as I thought it could be starving for fuel but it seems to be four stroking when it unloads and the rpm increases.
Idle is good and reliable so no problems there, I have read somewhere in the forum that increasing the plug gap could help with this but why this would be a cure I do not understand.
Any one had similar problems and have a cure for this?

propellantchem

What is you plug gap now?
On the motorcycle forums, they shortened the plug gap down to 0.020" for a high end miss.

Hi gas pressure inside the cylinder makes it harder for the spark to jump the gap. Depending on your ignition system that may or may not be an issue, but it's a quick and easy thing to try.
If the insulator in your plug developed a crack it might be sparking up near the threads instead of the gap, hence closing the gap might help, if it does help you may want to replace the plug.

PC

earlwb

Besides what was mentioned, you may need to advance the static timing setting a little bit more. For example if you set it to 28 degrees BTDC you may need to go to 30 degrees BTDC or more even. But you don't want the engine to exceed its max RPM rating as it can throw a rod or break the crnakpin off the crankshaft. So maybe you really need a little more pitch on the propeller that you are using.

av8tor1977

My Feedback: (6)

I had a Ryobi that had a baffling miss to it somewhat like this, and it turned out to be the reed valve not seating. It would start and run normally, but had a high speed miss. I found the area of the reed seat actually cracked and had to replace the back cover/reed valve mount.

If yours is a fairly aerodynamically clean airplane, it is also easily conceivable that the engine is just unloading in a dive and four stroking; something a two stroke engine will do sometimes. Try going to a larger diameter prop and I'm betting that the problem goes away. If you don't have room for a larger diameter, a Zoar prop tends to load an engine down more than other props, so a Zoar 18 x 8 or 10 might do the trick.

If you have ever heard a chain saw operating, you might have seen this phenomenon. A properly set chain saw will four stroke slightly at full throttle but under no load. As soon as you add a load and start cutting with it though, it cleans out into a nice two stroke sound.

One last thought: How are you setting your high speed needle? The Walbro and Zama carbs have a fuel pump, and do not need to be set rich like a glow engine. Use a tachometer and set the engine for peak rpms on the high speed needle, then richen it only the width of a screwdriver blade. If you are setting it rich, like 200 rpms below peak or some such method, this could be your problem.

AV8TOR

cutarug

Thanks for your replies I have plenty to work with here and will try these in turn and let you know what happens.

cutarug

I have the Ryobi running perfect now, I set the plug gap to 20 thou timing at 30 btdc and still had a rich miss in a dive.
Next flight I leaned it out and got an extra 100 rpm out of it and that fixed it, it runs great, thanks AV8TOR , it was a little too rich.

av8tor1977

My Feedback: (6)

Glad to help and glad you got it sorted out. For me, my engines go through 3 states of tune. One, when I run them on the test stand and/or on the airplane but with the cowling off. That gets things in the ball park. Then, I do a tuning run with the cowl on, as sometimes that does mysterious things to the air flow and pressures and things often change a bit. Then, finally, with the cowl on we do some test/tune flights. Here the tuning is finely dialed in for best running under flight conditions, which while are normally pretty small adjustments, make for a perfect running engine while in it's natural element; flying! There are just so many factors; engine condition, engine particular idiosyncrasies, fuel, temp, barometric pressure, prop load, cowl characteristics, etc., etc., etc., that make it impossible for there really to be any such thing as "factory settings". Factory settings exist to help you get the engine running only, and are never optimal for your particular installation and circumstances. That's why we always have to tune an engine ourselves, and observing my method outlined above gets you there in a logical manner.

Sorry to take this opportunity to "get up on the soap box", but so many people believe in "factory settings" that it is an ongoing battle here in the forums to educate about/against the concept. People just don't seem to understand that a two stroke engine in particular has so many variables to consider in arriving at an optimum mixture setting, that each and every engine and installation must be individually tuned. (Actually, four stroke engines are susceptible to all the same variables, but they are more tolerant in their mixture settings and not so obvious when they are not tuned quite right.)

Congrats and have fun!

AV8TOR

cutarug

I agree with the tuning the cowl pressures do alter what the diaphram see's for sure. I ran the reg vent in brass tube flush with the carby inlet, so it sees the same pressure.
I also noticed a lot of fuel gets thrown out of the venturi so I fitted a velocity stack and that stops the fuel escaping and the motor is clean and dry now.
Good fun, I really like my Ryobi's, I have 3 aircraft now with them as power plants, Sparrowhawk, Volksplane and working on a Hawker Hurricane.
AV8TOR you should get on your soap box more often, lots of good advice there.
thanks
cutarug

av8tor1977

My Feedback: (6)

When you see a mist of fuel out of the carb while the engine is running, that is called "stand off" or "reversion." It is inherent in all two strokes, but much less so with reed valve two strokes. The reed valve snaps shut upon sensing any reverse pressure, thus preventing the phenomenon for the most part. (A little still slips past.) As you mentioned, a velocity stack helps minimize it, (or at least the mess it makes), and of course in our yard equipment, the air cleaner takes care of the task. Stand off can really wreak havoc on getting a good tune, as it varies with rpm. (More rpms equals less stand off). Think about it, the air goes into the engine and picks up a load of fuel as it passes through the carb. Then it gets blown back through the carb, where it picks up another load of fuel as it passes through the carb. Then, it gets sucked BACK INTO the engine once again, picking up yet ANOTHER load of fuel. How do you set your carburetor for that nightmare?? Once again, another variable of two stroke tuning. The stand off will depend on numerous things, including the intake port timing, the rpms encountered, the intake tract length and design, the air filter, velocity stack or lack there of, etc., etc., etc. Even exhaust port timing and any exhaust tuning going on gets into the picture.

Anyway, those are the vagaries of our wonderful little "3 moving part marvels"; the two stroke engine. It is rather true what they say; "You either love them or you hate them!"

AV8TOR

av8tor1977

My Feedback: (6)

Since you're a Ryobi fan, here's a couple of pics of one of mine. And no, one should not ever run a Ryobi that fast. They should be propped to no more than 7500 rpms static. But that is what it turned that 18 x 6 prop at: 8500 rpms!

AV8TOR

cutarug

Nice neat installation, is that a tiger moth less wings?
Your ignition does not look like its store bought with the rubber boot, no shielding and no flywheel, I see the sensor, so is that your own ignition.

av8tor1977

My Feedback: (6)

Yep, it is a Tiger Moth. Always loved the lines of that airplane. 76" wingspan on this one, and it and the Ryobi were a "match made in heaven". Nothing sticks outside the cowl, and it is airborne way before I reach 3/4 throttle. Way cool.

No, the ignition is not mine; it is a CH Ignitions unit, with their Synchro Spark unit as well. Starts super easy by a hand flip, and runs great!! Sorry, I just looked and I can't find the other pics I have of the plane....

AV8TOR

rcairboater

What is the wire on the spark plug wire? Is it a ground and how is it attached and what is the reasoning?

av8tor1977

My Feedback: (6)

The newer RCEXL ignitions and their clones have a shielded spark plug cable, and that shielding terminates into a metal cap that connects, covers, and shields the plug. That's what many people are used to seeing now. Before RCEXL ignitions were ever invented, we used CH Ignitions. They are great ignition systems and I believe CH even had a contract making them for the military as well. The owner retired and I believe the company has re-opened now by a fellow named Adrian. (Google CH Ignitions.)

Anyway, the CH system has a shielded plug wire, but a rubber connector that goes on the plug. The shielding itself is then connected to the engine somewhere convenient to electrically ground the system. Connecting that wire to a ground on the engine does the same thing the metal cap does on the RCEXL and other systems with a metal cap; ie, electrically grounds the system. A CH system must be used with a resistor plug. A resistor plug is recommended for the RCEXL systems, but at least theoretically they could run without one and not encounter RF interference because the whole plug is shielded by that metal cap.

The box is larger on the CH Ignition in the picture on the right because it is a twin cylinder unit I had here handy as I had been working with it; the RCEXL unit on the left is a single cylinder unit.

AV8TOR

rcairboater

Thanks for the pic. So how is the wire that is grounded on the engine connect in the rubber boot?

av8tor1977

My Feedback: (6)

The rubber boot just goes onto the spark plug like normal. (Think lawn mower, chain saw, weedeater.) The other wire is just twisted braiding from around the plug wire, and it gets bolted to any convenient spot on the engine itself.

That black wire on the CH unit is braided just like the RCEXL wire on the left, it is just covered with a rubber insulation and you can't see it. That is the wire with the yellow connector on it; just an extension of that braid that gets connected anywhere on the engine.

AV8TOR