Son of Weed Wacker

I received a lot of mail from readers after the 700r conversion publication, and answered all but one. Most of you had concerns about modifying the muffler. You don't have to do that anymore. Some had problems with the primer bulb. We found that you don't have to use it, the engine will prime itself just fine. The bulb is nice feature but for our purposes it is not necessary. If you elect not to use the bulb, be sure to seal the primer intake on the carburetor. Some asked about the choice of this engine. The choice was a matter of cost, weight and the thin profile of the engine after conversion. Speaking of cost, we have an outfit called" Harbor Freight Tools" which had latched on to a bunch of factory refurbished 720r models and they put them on sale for... are you ready for this?... $45.97. Now we can't beat that for starters. Some of us bought as many as the old pocketbook could bear. Heck, four of them for less than $200 is not too bad.

Now to the modification. I will have to be repetitive on some of this due to the similar nature of the modifications and to accommodate those who have not seen the original article. Refer to the parts list that comes with the Ryobi 720r. The following is a list of parts that you'll keep:

ITEMS YOU DON'T DISASSEMBLE
7, 8, 12, 14, 15, 16, 17, 18, 44, 49, 50, 51

ITEMS YOU'LL MODIFY
5, 6, 7, 10

The engine mount kit has a shim washer to level the mount to the plastic carburetor housing. One boss is lower than the other three. This kit also contains all the necessary Allen head screws for mounting.

The muffler kit comes with the two screws needed for mounting. It would be advisable to place an exhaust gasket between the muffler and the engine to prevent leakage and to avoid metal to metal contact. You can get the material at your local auto supply store. If you are real careful, you can salvage enough material from the old gasket that was on the engine.

The prop hub kit contains a rear hub with set screws, a prop washer and a prop nut. I suggest you get a package of 3/8-24 elastic stop nuts from your local hardware store. These nuts have a nylon insert built in that will not allow the nut to spin off even if it should loosen in flight. A good safety precaution for this engine and any 4 stroke engines that sometimes backfire and spin off the entire front end of the engine.

When you start to disassemble the weed whacker, all will be well until you get past the removal of the centrifugal clutch. It starts with a threaded nut that holds the clutch stuff on the shaft together. If it doesn't unscrew easily, wrap the engine flywheel with a towel and snug it into a vice. Then get a 14 inch monkey wrench on the nut and just lean on it. The next obstacle you might run into is the flywheel assembly. It might be stuck and hard to remove from its tapered shaft. If so, soak the shaft area with acetone. Put a short 8-32 screw all the way in using the tapped hole in the shaft. Now gently tap on the head of the screw. If it continues to be stuck, use a broad bladed screwdriver and very gently pressure it off.

After you have removed the flywheel, get out your prop balancer. We have found another use for it. Place the flywheel onto the balancer and note that the flywheel is slightly off balance. With a marker pen, mark the bottom of the swing. This mark will be used later as a reference. The flywheel is made off balance in order to compensate for the mass of the engine's other moving parts. You'll want to regain that smoothness later when you run the engine. After you remove the clutch ratchet arms, springs, and drive out the studs, have the flywheel fins turned off until the name on the casting goes away. After that, remove about .005" or so to face off the center hub. If you put the flywheel back onto the balancer, you'll find that it will be in pretty good balance. Now using the pen mark as a guide, grind off enough material from the opposite side to regain the original unbalanced condition.

You now should be ready to modify Items 6 & 7, the carburetor assembly and the throttle assembly. Do not remove the throttle lever return spring. Using a small diagonal cutter, cut off both legs of the spring, leaving the coils. The coils will act as a light spring washer to take up the play in the butterfly shaft. Now, remove and discard the throttle idle stop screw and spring. Its stop function will be controlled by the servo. Disassemble the carburetor, setting all the parts aside in the order you remove them and put them away so that they will stay clean. It's best to put your finger over any of the "E" clips when you remove them as they have a habit of flying to never-never land if you don't. You might even try to disassemble the "E" ring in a plastic bag, and let it fly-- where can it go? Sometimes I think the "E" stands for "escape".

The brass spray bar that can be seen when looking into the venturi should not be disturbed. It's locked in with a pin that you can't remove. The venturi must be opened up to at least .400 of an inch while avoiding grinding off the spray bar. You can use a small metal burring tool in a Dremel to do this. Then smooth off the venturi hole using a #11 X-Acto blade. There is no need to polish the hole any further. The smooth shaving of the blade is good enough.

When you disassemble the carburetor, remove and discard the fine mesh filter screen (about 5/16" dia.), you can use an in-line external filter instead. You'll also want to remove the slop from the throttle lever assembly (Item #7). That's the brass piece with an "E" clip. Make a small .015" thick washer out of Teflon or Mylar to take up the slop in the throttle assembly. A plastic milk bottle will also make hundreds of nice washers using a couple of hole punches. Open up the cross hole in the brass throttle piece to .078" dia. and replace the screw with a 4-40 X 1/8" long Allen set screw.

The next order of business is to cap off the brass primer fuel intake tube that is next to the "LOW" end adjustment screw. The easiest way to do this is to tap the brass fuel tube with a 2-56 tap as far as you can go. Then coat a 2-56 X 1/4" long screw with nail polish and screw it into the tube. When the screw is all the way down, coat the head and let it dry. Now, reassemble the carburetor and wrap it up to keep it clean.

The modification of the black plastic Carburetor Mount Assembly, Items #10 and 11, is accomplished by following the contour of the gasket mount with a scroll saw. Cut out and save the center portion of the plastic mount. The assembly now consists of the mount and the (Item #11) Reed Assembly. Clean off any flash from the cutting.

In order to modify the Choke Lever and Plate Assembly (Item #5), just mark off the footprint (shape) of the carburetor. Grind off all the excess material leaving the mounting holes and the detent area on the plate. Now remove the bent end on the Choke Lever so that the lever is now flat except for the detent. Round off the end and drill a 7/64" dia. hole in the end of the lever. You can fashion a 2-56 threaded wire with a "Z" bend at the end that engages the lever, and the threaded end of the choke lever will protrude through a hole in your cowl. Now make a hard wooden knob that has a 2-56 "T" nut imbedded in it and screw it onto the end of the rod. Shape and paint the thing as you please.

This concludes the modifications. Reassemble the engine, that now looks like it wants to fly. Set up the low and high ends as described in the manufacture's instructions. This is a good place to start. Mount the engine in a test stand, put a prop on it and run it. Now, you can make the finite adjustments to get the maximum RPM, and the smoothest low RPM.

Pushing the Envelope Above and Beyond In order to increase the compression on the Ryobi, we have removed .025" from the head assembly. We also installed a carburetor with a larger venturi (1/2"). The footprint for mounting was the same as the original carburetor. There was no appreciable difference between the larger venturi and the one that we modified. In fact, the adjustment on the low end is very critical with the 1/2" venturi. The 1/2" diameter carburetor came from our local lawn mower shop. We eventually went back to the modified carburetor.

George recommended that you run a new weed whacker for about 4 hours before you modify it. Then you do not have to break it in, in the plane. Sound advice. My spouse thinks I've gone nuts running the machine in non-existent grass. She asks, "Didn't the gardeners do it?" However, if you should buy a refurbished model, check and see if the engine has been run. Just remove the plug and look at the electrodes. Then of course, if you remove the piston, you'll see that the head has a layer of carbon on it.

The choke and lever assembly required modification to accommodate the 1/2" diameter carburetor. We had to enlarge the hole in the plate to 1/2 inches and add a brass circle to the lever about 3/4 of an inch. We silver soldered the two together. However, by using the original modified carburetor, you don't have to enlarge the choke system at all.

John Curran is a half owner of a tools rental yard. He has repaired more engines than I have ever seen in my lifetime. George and John have gone ahead and converted one of the engines to accept a C&H ignition system. They are using the C&H electronics and sensor, and they have designed their own Delrin ring assembly. The ring is mounted onto the four front tapped bosses on the engine. The C&H sensor is permanently placed at the 28° recommended position. They are using the modified flywheel and its magnet to excite the ignition. George's test engine (which was an old worn out dog) with a 18/10-6 prop, ran up to 6300 RPM. With a 16-8 prop, he ran up to 8000 RPM. The next step was to fly the ignition engine. The 17 pound plane performed flawlessly until somehow (and we're still trying to figure this out) the easy hinges sheared off of the elevator and crashed the plane. No one never even heard of such a thing happening. The test engine was wrecked. George took all the spare Ryobi parts he could muster and quickly made up a new engine. This one runs more reliably than the old one. He built a new plane, and has been flying it ever since.

Leonard has been experimenting with a pizo-electric ignition system using a crystal from a Bar-B-Que lighter-- no coil, no battery. We have no information on this as yet. We suspect, however, that the crystal may eventually break down at high RPM.

Leonard suggested using Drywall & Sanding Screen, medium grade 120 for non-slip washers between the prop and prop hub. Cut out your own, they'll last forever. The material is available at any hardware supply store, where the sandpaper is kept.

The above tests used a .400" venturi carburetor with the standard muffler as converted in the original article.

Second test was with a .400" venturi carburetor with the Bennett muffler.

No further tests were made after the 18X6/10 test. The next run of testing will be with high octane gas (92) with an oil mix of 50:1.

Only RPM readings will be taken and tests will include carburetor adjustments whenever deemed necessary to gain the highest RPM which relates directly to the static pull. As you know, the static pull tests give no indication as to how the prop dumps torque in the air. Only flying the model will confirm performance. We also saw no difference in performance with the modified original carburetor. The higher octane fuel does make for a smoother running engine. It seems that the higher octane gives the piston a more even push. The lower 87 octane, although perfectly usable, delivers more of a "bang" to the piston head. Most of us are using the higher octane mix. It seems to run much smoother. The extra couple of pennies for gasoline is still more economical than glow fuel. The latest input is a special prop that put the aircraft's performance right over the edge with a gain of 200 RPM. It's a MENZ, 18-10 ULTRA and it is available through Cactus Aircraft, 10380 E. Heritage, Tucson, AZ 85730; phone/fax (520)771-0087.

I would like to publicly thank my daughter, Veronica, for helping me set up a web page that contains the original article " Teaching a Weed Wacker How to Fly". We have set this up for your convenience, it's been a long time since its publication. You can download the text and the pictures from the web page.

My E-mail address is: joeg2@juno.com
Feel free to avail yourself of the information previously published and write to me at the E-mail address and I will get any other information for you, if I can.
For those of you that can't beg or cajole access to the web site, I would advise that you try to get a back copy of the August 1995 issue from RCM magazine, the information on the original conversion will be of value. Those of you who will be using snail mail can get in touch with me through RCM and they will forward your mail to me.
OK that's about it. Don't be afraid to do the conversion. If we can do it so can you, and you'll have a great engine at great price. Good luck and happy flying.
-Joe Guzzardi

But, before you go...

Some final hints for all

And here are photos to help you with the conversion.

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