Teaching a Weed Wacker How to Fly

The next thing you'll probably do is to look at the weed wacker for a while. I don't know about you, but I felt some trepidation about taking it apart and throwing away most of it. Remember that at the end of this conversion you'll end up with a 28.5cc (1.74 cu. in.) model engine similar to a Quadra or Zenoah at a bargain price. Mine was $126.25 total.

This is where it all starts

Once you get over the original trauma, and make prayerful apologies to the assemblers, you'll begin to note that they use strange headed drive screws (PT type) in the assembly. The screwdrivers that fit these screws are available at Sears or any tool shop. You'll need three different sizes. If you can't get the screwdrivers, use Allen wrenches or a flat blade screwdriver, they will work too. Point of information: All threads are American Standard, so you can replace any of the screws.

Referring to the Model 700r Parts List that comes with the grass trimmer, you will save some parts, throw some away, modify some, and buy some. The list marked Parts you Save shows the list of parts you'll save.
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Note: If you try to buy the parts separately from your local distributor, they will cost you more than if you buy the whole weed wacker and disassemble it. ( Something about handling costs etc.)

This is all the good stuff

Note: You obviously do not have to take apart some items, e.g. piston and rod assembly in the cylinder assembly, etc. We will now modify some of the parts that we have saved. They are listed as Parts you Modify.

Flywheel Assembly

Modification of Item #5, consists of following the contour of the carburetor on the plate, retaining the lever stops, grind away the outside metal on the plate. Remove the bent shaped handle on the lever and make it flat. Put a 7/64" dia. hole in the end to accept a 2-56 threaded push rod that will end up outside of your plane. You can use a "Z" bend to hold the pushrod in place. The other end will be outside your plane, thread in and glue a knob on it.

You will now modify Item #6 by removing the return spring on the throttle lever. Also remove the low end stop screw and spring. In the plane the throttle will be under servo control, so these items are not necessary. Since the Ryobi was designed to run at a much lower top speed than we require on an airplane engine, we must open up the venturi to at least .400" dia. Remove the "E" ring from the end of the throttle shaft and remove the screw that holds the "butterfly" disk onto the shaft. Now remove the shaft. To open the hole, use your Dremel with a small diameter burring bit, and remove the aluminum only. Be careful around the small brass piece (fuel flow) that protrudes into the venturi. Do not attempt to

Remove slop from the Throttle Assembly

disassemble the brass piece unless you have a machine shop at your disposal, you'll wreck it. When you have removed enough material, scrape the inside as smooth as possible with a #11 X-Acto blade. Then reshape the funnel end of the venturi. Your final finish will be to polish the hole with Simmichrome (available in auto supply stores). Next remove the shiny indented plate that covers the diaphragm assembly, it's held in with two screws. Carefully remove the rubber diaphragm and you'll see a fine filter screen about 5/16" dia. Remove this filter and discard it, you'll be using an external filter in the gas line. Reassemble the carburetor.

Carburetor Modification

You will now remove and modify Item #7, the Throttle Adjustment Assembly by removing the "E" ring. Make a small .015" thick washer out of Mylar or Teflon to take up the slop in the throttle assembly. Open up the cross hole to .078" dia. and replace the (PT) screw with a 4-40 Allen set screw 1/8" long. Reassemble it to the carburetor.

Now modify Item #10. With a jig saw, follow the contour of the gasket mount. Cut out and save the center assembly. Clean up the cut with an X-Acto knife. You will also cut out and save the portion of the mount that holds the choke bulb assembly (Item #9). You will also remove the two gasket locating bosses, they will interfere with the engine mount. Now trim off the carburetor gasket to remove the two end holes.

Carburetor Choke Lever and Plate

You must now modify Item #26. With a nail set punch, drive out the studs that hold the ratchet lever and spring. You won't see the hole from the back because there is about three thousands of an inch (.003") of casting left over the hole. Punch right through the casting and discard the stud, lever and spring in two places. The next part of the modification is a little more complex. Have someone who owns a lathe face off all of the fins and stud bosses down to the first casting high point. Then have him skin off the center hole about .003". (Caution: The center hub sticks out further than the fins that you are removing.)

On Item #41, there is a muffler cover that is not needed. Discard this portion of the assembly. Remove the exhaust port from the muffler by twisting it off. Be patient with this part of the job, the original tube is very reluctant. Enlarge the hole to 5/8" diameter all the way to the inside end of the muffler. Put eighteen (18) 5/32" dia. holes in the end of a 5/8"dia. by 4-1/2" long steel tube. (use electrical conduit, that will go inside the muffler. This will vent any oil that will accumulate in the bottom of the muffler.
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Note: there are two more holes than the calculations call for, these extra holes are to prevent any back pressure.
The other end of the tube should have a 20 degree cut. Braze the tube into the muffler with the 20 degree cut facing the rear of the engine. At this time you should braze the rolled over seam on the muffler, as it will have a tendency to leak oil if you don't. Paint the entire muffler assembly with a high temperature paint (outdoor grill paint is good).
Next: throw away all of the rest of the parts that make it look like a grass trimmer.

Items to buy from B&B Specialties, 14234 Cleveland Rd., Granger, IN 46530; (219)277-0499 are:

*If you are using a spinner you may have to use whatever retaining assembly the spinner requires. Ask B&B for the screw with the tapped 8-32 hole in the head.

Carburetor Mount

The final Item to buy is the engine mount. This Item costs $25.00. You can buy a suitable mount from; Jag Engines, 16073 Muscatel, Hesperia, CA 92345, Tel: (760) 244-7270. Jag Engines will also include new socket head screws for the mount, longer screws can be purchased at Home Depot, the screws are a 10-24 thread. You will want to put in a hole in the mount, in the area of the throttle lever for clearance and to ease assembly.

You will have to construct a carburetor gasket for the engine. If you don't use a shim, the throttle travel will allow the arm to strike the engine mount. The best thing to do is to trace the shape of the gasket used on the original assembly onto a piece of 1/8" sheet aluminum and cut it out with hand tools. Then make another gasket with the same shape. When mounting these parts, make sure of the position of the small hole, next to the big hole, on the carburetor. You can use the old gasket as a guide.

Muffler Modification

That is about all there is to it, use a 16 x 6 or 16 x 6/10 prop with or without a spinner. Be careful, the torque is very powerful. When taking off, put in the throttle slowly and use whatever right rudder necessary to track straight. Let's go flying.
Now let's see, there is a blower out there that has a 20 cc engine and one with a 16cc.

Since the modification described above was made, Ryobi has changed the engine in the Model 700r ( now I know what they mean by "subject to change without notice"). It now sports a 31cc engine (1.89 cu. in.) displacement engine. All of the previously described instructions are the same except for some of the part numbers and the carburetor. It's modification is actually easier in that small brass bar can be easily knocked through with an aluminum punch. This leaves the carburetor body casting bare. All the steps involved in the opening of the venturi can be done on a drill press. When the venturi has been modified and polished, it can be reassembled. First, push the spraybar back into the casting, then reassemble the rest of the carburetor. You can now continue with the remaining modifications.

	<----- These are the parts you buy.
Your NEW ENGINE ----->

I must take this opportunity to thank my friend and fellow flyer, Leonard Rylander, who provided guidance and did some of the machining on my engine. He has modified at least three Ryobi engines that I know of and has flown all the weed wackers he could get his hands on.

The following is a notice that was published by the editor in subsequent issues of the RCM magazine:

Jag Engines. is in business and they not only sell complete converted engines, but supply all the necessary parts for the modelers to convert their own engines into great R/C power plants. They can be contacted at: 16073 Muscatel, Hesperia, CA 92345, (760)244-7270.

Any questions??

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