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#Post#: 70435--------------------------------------------------
Converting a Weed Wacker to be a Model Airplane Engine
By: 660magnum Date: March 20, 2016, 4:36 pm
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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
HTML http://www.carrprecision.com/
HTML http://www.carrprecision.com/
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.
THE HUB
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?
IGNITION COILS
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.
MUFFLER
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.
HTML http://images.rcuniverse.com/gallery/photos/23804/lg-14766.jpg
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.
HTML http://images.rcuniverse.com/gallery/photos/153113/lg-139287.jpg
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.
#Post#: 70436--------------------------------------------------
Re: Converting a Weed Wacker to be a Model Airplane Engine
By: 660magnum Date: March 20, 2016, 4:36 pm
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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.
RUNNING
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
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