WEED WHACKER CONVERSION
While this was actually written 12-15 years ago for the old 80's and 90's Homelite weed whackers, the technique could be applied to other brands or even chain saws. I made a few changes in an attempt to update it.
THE BASIC REASON
Homelite weed trimmers were popular among model airplane and boat enthusiasts. One main attraction was low initial cost. Another was the relatively minor changes truly needed to get one flying. Other reasons get into low fuel cost, lack of attention needed by the engine after setup, etc. A good string trimmer candidate for conversion should have the crankshaft coming out one end of the crankcase and turn in the clockwise direction when standing behind the engine looking across where the propeller will mount. The propeller is usually mounted on the flywheel end of the engine. Some likely candidates are Homelite, McCulloch, Toro, Ryobi, Stihl, Echo, and Weed Eater. Some of these are no longer made, Many of the Weed Eater brand are too small or lack the porting necessary for good power. The Echo, Stihl, & Makita brands are rather pricey. The old style rear carb Ryobi 31cc and its variations used by other companies are often converted. The air box is removed and the engine can use the same hubs as the Homelite. But the carb and muffler are in the rear and this presents a mounting problem. Though some people have used stand offs, some companies make fancy box assemblies for mounting convenience. The old style Ryobi is narrow and easy to put a cowl around and comes with a Pitts style muffler, it is long and creates balance and vibration problems at times. The Homelite met most sport model airplane criteria with less fuss and a low price.
In the beginning conversion, lets keep it simple. Get a cheap candidate. You may find one at a garage sale with the cutter head or recoil starter damaged? Most of the Macs have a broken throttle cable. Some, the fuel lines have rotted off. There may be one in a trash can on the side of the road. Look on Craig's list or in the newspaper for sales at your favorite discount store. If you are looking at a used one, make sure it has good compression and feels good when you pull it through. You may could use some parts from one engine assembly on another engine. I once used parts from a bad Homelite that had foreign object damage on a new Homelite 30cc short block to make up a conversion engine. But before I go any farther, you need some experience with small engines. It is best that you have already ruined a few model airplane engines, lawn mowers, or had other hard experience lessons with engines in the past. If you feel chicken, there are people that convert engines for you?
Here are a couple companies that offer conversion pieces and or converted engines
And there are others . . .
THE BEGINNING CONVERSION PROCESS OVERVIEW
This consists of stripping the string trimmer down to where there is nothing left but the engine. This includes removing the muffler, The ignition system, the fuel system, and the back plate. The reason for this is on the Homelite, there is a large fan shroud that is cast onto the engine block that must be cut away. On a weed whacker the rear cover plate serves mounting purposes as well as sealing the crankcase so it too must be trimmed and modified.
These engines are typically assembled with self taping Torx head screws. These are generally in the sizes T-25 through T-27. Some of these Torx head screws also have flat blade screw driver slots. You should take note of the threads on your screws. Some have a triangular cross section that serves to cut or roll threads in the castings. Most of these have Loctite on the threads and you will need to warm them up with a propane torch to release the Loctite. The Torx heads screws are rather soft and I always replace them with Allen hex socket head screws.
FLYWHEEL AND PROP HUB
First you must remove the flywheel. As mentioned earlier this must be removed so you can cut away or unbolt the fan shroud from the engine crankcase. The flywheel sets on a tapered area of the crankshaft. It is kept from slipping on the crankshaft by a straight key or maybe a half moon (Woodruff) key between the crankshaft and flywheel. Do not loose this key. When you remove the flywheel nut assembly, the flywheel will still not come off the crankshaft. Some flywheels have the key cast into them and the key is not removable. You must use a gear puller or you can use two flat pry bars and a soft hammer which is usually a two man job. At this point you may decide to move along to other areas and come back here later, but I'm going to continue to talk about the flywheel.
At his point you need to decide what you are going to do with the flywheel? If you are going to use electronic ignition and make or purchase an new hub from someone, you may just want to set it aside and forget it. If you are going to use the flywheel, it could be used as it is, have the fins faced off on a lathe, break the fins off with pliers and grind the flywheel smooth with a die grinder, or like one fellow, mount the flywheel where it runs true in a drill press and sand off the fins.
In our philosophy of keeping it simple, There is really no reason why the flywheel could not be used as is with the fins. The fins cause some drag, weight penalty, and resulting RPM loss but it is not significant. The extra rotating weight will help the idle but will penalize your top RPM slightly. You made need the extra weight to balance a WWI model? However, there are two little spring loaded dogs or pawls that you may want to remove. They were for the recoil starter and are held in place by pins that are pressed into the flywheel. On the Walbro flywheels, the pins are in blind pockets and you cannot drive the pins out directly but can usually catch the edge of one from the side of the flywheel with a punch or chisel and drive them out. On the Phelon brand flywheels, the holes go all the way through.
You can turn the fins down on a lathe. Basic machinist technique. The radial run out of the flywheel in a three jaw chuck is not so important but wobble is. Try to get the flywheel seated squarely and firmly in the chuck so it does not wobble or twist. you can only turn the fins down until you touch the highest part of the counter weight or magnet. Do not cut into either area. While the flywheel is still chucked up, you should go in with a cutting tool and spot face the center area of the flywheel where your new prop hub will make contact. Places like Carr Precision or Brillelli can do this for you and balance the flywheel while they are at it.
I tried breaking the fins off with pliers and grinding the fins smooth with an air powered die grinder. A Dremel is not big enough. Balancing is mandatory. I used a Dubro prop balancer to check my efforts to balance the flywheel. The Dubro Arbor works fine on the flywheel. You turn one of the cones around with the flat side towards the flywheel to hold it square. The other cone is used to center the flywheel. It will take many trials and you will chase the balance around the flywheel 90 degrees before you are done. Although the Walbro flywheels are originally more in balance than the Phelons, they still show some imbalance even when untouched.
You could not bother with the flywheel and use a battery powered CD ignition (RCEXL)? You will need a magnet in the hub. It often works better to have the magnet in an adjustable ring that fits over the prop hub. The companies linked to early on in this post can possibly help you with this. The battery powered ignition offers less weight and bulk but you have to tend to the battery and the extra cost.
I consider the prop hub to be rather tricky to make yourself. The machining problems materialize when you need to turn the hub around to face the back side. Some people use a collet chuck to hold the hub so the backside face is still square. It is easier to just buy a hub from a vendor? The longer your hub, the more inaccurate it gets. There are considerations needed for the threads and their length on the engine and for your prop retainer screw. There are trade offs and compromises. Typical thread sizes are 5/16"-24, 3/8-24 and 8mm. A hub with a magnet is another consideration.
A SPECIFIC APPLICATION
The stock Homelite's horsepower peak is at 6700 rpm. Therefore you need to plan on using a 16-10 or 18-8 prop depending on the weight and size of your plane. The domestic props are noticeably out of balance so I recommend a Dubro balancer to balance your props. The magnetic centered balancer is not strong enough to hold a big prop. The cheapest Homelite had no clutch and a short shaft. The threads are 5/16"-24 and it sticks out beyond the flywheel about 7/8". Your prop hub will have 5/16"-24 threads all the way through it. To meet the AMA safety rules, you will need to use a 5/16" stud and a Higeley 1" diameter safety nut. It will almost take a 3" spinner to look right on a 18 X 8 prop? You will need to trim the prop openings in the spinner with a half round file to fit the prop.
The reason I mentioned a cheap Homelite above is that the more expensive Homelites have a clutch in front of the flywheel. The shaft on these engines is 3/8"-24 and sticks out some 3" beyond the flywheel with the end tapped 10-24. The hub in this case normally does not have threads and the prop and hub are retained with a single 3/8" nut. The shaft may need the very end sawn off and sometimes you have to run a 3/8"-24 die on the shaft to make the threads go halfway into the prop. If you are going to purchase a hub, you need to be aware of which engine you have so you can get the correct hub. By theory, you could build a shorter engine with a long shaft that you can with a short shaft if you think about how the hubs are made?
There are two brands of ignition coils on Homelites and they are the Walbro and the Phelon. The Walbro gives a nice spark but the Phelon, you better wait until after dark to look for a spark. Either will work on your engine but the Walbro is better. There are not too many people quick enough to hand start a magneto ignition engine so you best plan on an electric starter. The cheapest Hobbico will not start a whacker engine.
You may feel the need to mount a kill switch on your engine. On the coil next to the plug wire is a metal tab. If this tab is shorted to the engine block the ignition will not work. A good quality SPST slide switch is required. Keep the wires as short as possible as they can carry RF. Keep in mind the vibration and select your parts accordingly.
Clearance between the coil and the flywheel is important. I set the coil on the magnet with a business card between them while bolting down to set the clearance.
The latter Homelite mufflers were a large streamlined affair That works well but is heavy and may hit the fuselage on some planes. The old trimmer from a few years back and the latter commercial trimmers had a little square box muffler that was inexpensive to purchase new # A-04837. This muffler comes apart when removed from the engine and is easily modified. Gut it, block the original hole out the lower rear. Braze in a couple 1/2" pipes out the spark plug side of the muffler. If the muffler is plated, you will need to brush off the plating before the braze will stick. On all the factory mufflers there is a little punched hole near the exhaust port that is part of an exhaust gas recirculation scheme. Peen this hole flat and braze it shut. You can also get nice after market mufflers and headers for the Homelite as the Small G23/G26 Zenoah engines have a 34.29mm/1.350" spacing and it only takes a little rat tail file messaging to make these mufflers fit the Homelite which has a 1.5" bolt spacing.
CARBURETOR AND HEAT DAM
The stock carbs on the Homelite weed whackers has a 5/16" (20) venturi. They can be either Zama or Walbro. The engine will perform with the original carb if it has both a high and low speed needle. The mixture needles are mandatory on a model engine as the conditions are drastically different from weed whacker duty. I'm refering to the venturi size and not the size of the carb where it fits onto the engine. The engine perform much better with a 3/8" (24) carb but a 7/16" (28) carb is about the biggest you can use without high rpm mods to the engine itself. The small 33cc chain saws usually have the 7/16" carbs. You can buy them on Flea Bay cheaper than new ones. You may have to cross reference the numbers? I bought new Walbro WA-167A carbs with the primer bulb and no choke. This Walbro has a smaller throat where it fastens to the insulator or heat dam on the engine which makes it an ideal choice. The insulator or heat dam works better if enlarged for the bigger carb.
A note here about carburetors. A Zama runs as good size for size as a Walbro however, due to a lack of documentation on Zama Carburetors, and the high cost of repair parts for Zama Carburetors compared with Walbro's, don't go out and buy a Zama. If you already have a proper size Zama and it runs good by all means use it. If something goes bad about the Zama or you need another carburetor, get a Walbro.
Most Homelites mount the carb at quite an angle so you will need to run the throttle linkage through a bell crank or use the heat dam spacer from an electric start Homelite trimmer (Homelite # 06445). This straight pull mount will let you run the linkage directly to the throttle arm. This spacer also serves as a heat dam as well as the carb adapter. Do not be tempted to make one from aluminum for as the engine get warm, it will not restart until it cools off.
It may be necessary to make a few changes to your carb. On the throttle shaft there is a return spring. Some people leave it alone and others do not like the servo having to pull against the spring. So these people unhook the tang of the spring from the hole in the throttle arm. Do not take the spring off the throttle shaft. The spring serves as a spacer to position the throttle arm in the center of the throttle bore. To remove the spring, you have to remove the butterfly and the retaining screw is likely to vibrate out and go through the engine once it is tampered with. With the spring removed completely, the butterfly wants to hang up in the carb.
You want to use a Nyrod type push rod so RF is not carried back into the radio. Remove the idle stop screw or at least back it out of the way so you can close the throttle completely to shut the engine down like on the old glow engines. Set up the servo travel so when you run the trim all the way down or hit the idle cut off button, the servo will close the throttle and shut the engine off. Set the linkage up so with the transmitter throttle trim in the mid position, the engine idles perfectly.
I must caution here about the little hole in the part of the carburetor that bolts to the engine. The Walbro and Zama carbs are pumper carbs. They utilize the pulsating pressures of the engine crankcase to operate a diaphragm pump inside the carb. On the engine side of the carb there is a little hole adjacent to the bigger throttle bore. There is a corresponding little hole on the heat dam of carburetor mount adapter on the engine. On the the engine you can trace this little hole to the crankcase. The carb will run equally well in any position but that little hole must connect with the crankcase one way or the other even if it has to be run though an external line. If you turn the carb over, this little hole must still go from the carb to the crankcase.
The mixture adjustment screws were originally between the carb and the prop. If you turn the carb over, they will be on the bottom rear. It doesn't make much difference as you will have to shut the engine down to adjust the screws anyway for you will find it impossible to hold a screw driver in the slot while the engine is running due to vibration. These mixture screws will often have a limiter cap on them but the caps must be removed for your operation on a model airplane or boat is very different from weed whacker operation with all the air currents swirling around the engine. These mixture screws will need to be readjusted as you start the engine for the model application. While flying, the air pressures will again be different and you will likely need to readjust them again to have the engine running properly in flight.
In flight, the air flowing past the carburetor inlet will blow gas from the carb back onto your plane. There are a variety of so called velocity stack extensions that you can fasten onto the carb to prevent the fuel from blowing onto the plane. Some of these are straight pipes with an angle cut on the end and others are trumpet shaped and flared out larger on the end. Some extensions are aluminum and some are plastic. Get one that looks pretty and see if you can get it to work on your engine.
Some carbs do not have chokes directly on the carb. The choke may have been part of the weed whacker air filter? On a open engine installation, you do not necessarily need a choke as you can use your finger. Cowled installations are another mater. You may need a choke.
BACKPLATE AND ENGINE MOUNT
Cut and file the extra material from the backplate so there is noting left but just the part that covers the back of the crankcase. You may want to take a piece of coarse emory paper and sand the rear outside of the back plate level as you are going to cover it with the engine mount plate. You could also chuck it up in a lathe and face it off. If you use an electric sander, be careful that you do not melt the plastic. On the plastic back plates you need to make the flange evenly 1/8" all around. Measure carefully on all four corners as you take it down so the plate is still flat. On the metal back plates, you will need to take the lip off the edges of the flange. The aluminum engine mount assist the cover in sealing the rear of the crankcase.
The mount can be aluminum plate. It should be T6 temper but can be 2024, 6061, 7075 or whatever or you can even use 1/4 birch or maple plywood. The plate needs to be 3 1/2" wide and 3" high. The aluminum mount can have a 1 3/4" hole in the center to make it way less. The would mount can have a 1" hole. The top of the mount should be just under the tab that sticks out from the cylinder. You will need eight 3/16" holes in the plate. Four to fasten the plate to the engine and four to fasten the plate to the firewall of the plane. Use a gasket or the backplate to lay out the engine holes. These holes will be countersunk for flat head screws. The Homelites I own are 10-24 on the rear of the crankcase. The two bottom holes for fastening to the firewall should be about .300" below the and have a 2" center to center vertical distance and a 3" center to center horizontal distance. The top corners should be a 1/2" radius and the bottom corners a 1/4" radius. You can scallop the sides between the mounting holes. With an aluminum mounting plate, you can use 10-24 X 3/4" flat head Allen bolts and with the plywood plate you best use 10-24 X 1" flathead screws. You can buy these already made with the screws from the links in the beginning of the post.
NECESSARY MODIFICATIONS TO THE HOMELITE CYLINDER HEAD
Almost all the Homelite cylinders have an exhaust recirculation hole just above the exhaust port. This should be plugged. But be sure you do not do anything to penetrate into the cylinder lining. I used a 8-32 shortest set screw. The hole is just the right size for the 8-32 tap. Stick something lightly into the hole to see how deep it is. Mark your tap with tap slightly short of this depth. Tap into the hole but stop at your tape mark. Remove the tap and cut it off some and remark the depth on the tap. Retap the hole with the cut-off tap. Do this again to get some decent threads into the hole. Screw the shortest set screw into the hole so it is flush with the cylinder outside.
On some cylinders there is a compression release hole above the intake. If there is one on your engine it is easily spotted. This hole is 1/8" pipe and you can use a pipe plug. It does not have to be flush.
NECESSARY MODIFICATIONS TO THE CRANKCASE.
The only thing you need to do to the crankcase is to cut away the fan shroud. You want to leave the upper part in place to hold the coil if you are using the magneto. You will need to have the flywheel, muffler, and carburetor removed. The backplate must be in place and the exhaust and intake taped shut to keep metal shavings out. I took a short piece of 2 X 4 and in the center drilled a 5/16" hole all the way through. I counter bored this hole 3/8" diameter about 1/2" deep to clear the nose of the crankshaft. I then stuck the crank down into the hole and this held the engine so I could rough saw off the air box in a metal band saw. I finished it up with a coarse half round file. Do not cut into the crankcase itself and leave generous support for the coil. Dress up the crankcase with a file and carry the lines forward from the rear to the nose of the crankcase. If you have a long shaft engine, you will have to take a more manual approach.
It must be mentioned that removing the crankshaft from the crankcase can be serious business. If you have an arbor press and have done this work before then go for it but if not, try your technique on a junk engine first. No beating with a hammer.
I would bench run the engine before mounting it on a plane. This way you work out any problems without wear and tear on your airplane. With magneto ignition it is unlikely that you will be able to start the engine by hand flipping. A regular cheap starter will not turn a Homelite over. You will need the Tower/Hobbico 120 starter at least. A battery type ignition engine can be hand started after you get it set up.
You need to get fuel up to the carb. Once the carb is wet inside, you can rock the prop back and forth from compression to compression to work the pump in the carb. To choke the carb with your finger will draw fuel up into the carb. With an electric starter things are generally pretty easy. Due to the diameter of the prop, don't be reaching over it.
With a RCEXL type ignition, the standard procedure has been to have the choke on and flip until it sputters, take the choke of and three or four flips later it is running. Without a choke, things will be different?
I like to set the mixture just into the verge of four cycling or sputtering for the first tank to flush the junk out of the engine. After that I try to get the needles set.
The Homelite is generally used on a 11 to 14 lb plane
Edited to correct spelling . . . .