WeedEater Outboard Motor Conversion
A delightful, quiet little 9 pound (4kg) motor for a small investment.

This is what there was to work with; a 25cc WeedEater FeatherLite 2 cycle with a broken drive shaft. It had been used very little over a  two season period. Maybe 2 hours of run time on it. As a plus, it is one that happens to be easy to start, and the drive shaft was broken at the end, making repair easier.

Horsepower estimates for this range from 3/4 to 1, but I will wager that opening the exhaust and running it into the water made it somewhat higher.
 
5 July 07

First I made some measurements to see how tight the plastic driveshaft guide can be bent. Looks like radius wrapped around a 1 gallon paint can will do.  (I know... Hardly scientific. I went by how hard it was to bend the guide by hand). A tighter bend will keep the prop from sticking way out behind the motor. The guide is very stiff to bend, so making it feed through a tube with that tight a radius is out of the question. The plan is the make a wood lower unit with the bearing bushing glued in place. (Well this is a prototype after all).

This what the original looks like. The deep groove in the middle of the bushing is for the crimp in the original steel tube.  The first surprise was when I tried to press it into an aluminum 3/4" tube. The diameter near the prop is about .050" greater, and the tube split. I decided to keep it simple and just press a tube part way on.
The plastic drive guide fits inside snugly to hold it in alignment (below).

Here it is with the aluminum tube and the nut that holds the threaded assembly together.


A paint can was used to trace the inner curve on 3/4" plywood.
The 3/4" aluminum tube and the bushing are glued into a scrap of hardwood.
Later the tube was trimmed so that  about 5/8" of the plastic guide goes into it.

Note that the bronze part of it protrudes on the prop end. This is to make sure the nut (which doubles as a thrust bearing) will clear the hardwood + about 1/16".



Outer curve is glued and clamped to dry overnight.

6 July 07

The hardwood brackets for the aluminum are drilled so they can be doweled to the the outer curve.


Hardwood blocks to align the aluminum tubes are glued, doweled, and clamped in place.

With the flexible shaft installed, the guide was inserted. (Boy, that sound simple. Actually it was a fight since it is so stiff)  Glue was applied to the upper curved 3/4" part and clamped in place. I wanted the guide to more or less assume a natural shape; more of a parabola than circle. Clothespins (clothes pegs) were sanded as needed and used as wedges to manipulate the guide at various places, while occasionally turning the driveshaft to make sure it was not binding or had a 'bumpy" feel. Glue was applied to each peg as it was fitted. It felt smooth, but it still had more resistance than I would have liked.

I cut 19 1/4" off of the plastic guide, measured to see how far the metal tube goes into the plastic motor housing (1 5/8") and the distance to the bottom of the square cable receptacle is (3"). That means the guide can be flush with the metal tube, and the flexible drive cable can extend 1 3/8" beyond that.

Now the drive shaft turns very freely. Apparently much of the drag I was feeling was due to the length of the shaft and the heavy factory grease. Before the motor is tested I will dribble a bit of 75W 140 weight synthetic gear lube into the top of the plastic guide.

A slot was cut into the bottom of the case to glue the fin to the bottom.



Next step is to affix the cut-off original steel shaft into the "gearbox" with J-B Weld. I wanted to use the aluminum tubing but the I.D. is a little too tight.

Then I want to start the engine and check to see if anything gets warm inside the "gearbox" before I glue a cover on it.


If you are following this, you may be curious why the my "gearbox" is a somewhat massive rectangle. Below is a Mercury outboard lower section that is typical of outboard and stern drive designs.

Note the plate above the prop laying in a horizontal plane. It does two things... Prevents some of the water from gushing upward as the prop spins thus wasting energy, and helps prevent air from being sucked downward by water turbulence, thus making cavitation less likely. I hope to incorporate this scheme into my conversion as I may need every advantage. It will be of plywood, as will the fin on the bottom. I believe the hydrodynamic bulge just above the shaft is intended to aid in directing water flow. It may be superfluous on a tiny homemade motor.

7 July 07
The flexible shaft was shortened and the end made square by careful hammering in the bench vise. I may clean it well and attempt to impregnate it with glue or J-B Weld later.

The motor was again attached to the shaft, a small quantity of gas-oil mix added and it started on the third pull after sitting two years.

The bronze bushing got very slightly warm to the touch, but about what I expected. I could swear it is much quieter and a bit slower at top speed than it was, but that may be because I am used to the newer model.

All seems well, so the cover was glued to the "gearbox" and was set outside in the sun to dry. Meanwhile I will contemplate if I want to lengthen the extremely short throttle cable travel (about 3/8").

 
 

Finished head weighs 27 ounces (.76 kg). It was sprayed with automotive undercoating.
Note that the shape belies the actual curve of the drive cable.
While the undercoating is drying, I will scout around the scrap pile for a suitable prop material. The Russian fellow's prop design will be used as a starting point. My "Main" engine - a lawn-mower.
I like the idea of being able to change the pitch in minutes for testing purposes.

8 July 07
I printed out the Russian fellow's prop drawing and glued it to a scrap of laminated marine plywood about .4 inches thick and allowed it to dry. The size of the template was a bit under the 4" he used, but the hub was the right size for the WeedEater nut. I used a disc sander to rough the outline, staying outside the printed shape to make up for it being smaller, thus making it the original 4" long. The hole is 5/16" for the shaft.

I checked the rotation of the motor, made heavy pencil lines on the edge of the prop blank to remind me which way it rotated and started to shape the 10°. I checked it again to proper rotation.  Made the blades with an airfoil shape mostly by eye, but all the while checking with a dial caliper to make each blade match as much as possible.

When shaped and sanded to my satisfaction, it was dipped in amber shellac (since I was out of varnish), and allowed to dry.

I filled a sturdy old plastic barrel 2/3 full of water, fired up the motor and lowered it into the barrel.  As it entered the water, it slowed quite a bit so I added throttle as it went.  When it was even with the splash plate it took about 1/2 throttle. It wanted more throttle as I continued to lower it. By levering the shaft against the side of the barrel I could detect far more thrust than I had hoped for at about 2/3 throttle!

Increasing  the gas beyond 2/3 throttle made it slow a bit. No idea why. Anything beyond 3/4 throttle is a waste of fuel. However, since this was in a barrel, a test on something that was able to move (boat) would be quite different.

I was holding it by hand in the barrel so the exhaust was near my ear. It was not nearly as objectionable as it was as a weed cutter.

11 July 07
The transom bracket for an old trolling motor has been repaired. (left over from one of my less fortunate eBay purchases).

Made a sleeve to hold the motor shaft in the transom bracket. That turned out to be very easy.  The bicycle gearshift for the throttle is a bit of a puzzle as to what is the best way to approach it. Cables are made one piece today as an assembly, so something will have to be cut on either the new cable or the old, and a new cable end fabricated.

Also got fittings to make an exhaust that goes into the water.

12 July 07

This is the original. A short section or 1/16" O.D. brass tubing such as found in hobby shops and some hardware stores was placed over the new (slightly larger diameter) cable that came with the bicycle gearshift lever and bent to this shape AFTER the cable sleeve, and fitting that enters the WeedEater housing was slipped on.


The tiller is the end of a discarded carbon fiber fishing rod I found at the dump. Height adjustment and swivel for the motor is via a piece of PVC pipe that matches the inside of the transom bracket. I found it necessary to glue the 1/2" hardwood strip to the PVC column because vibration made the motor slide slowly  downward when run.

 
 

Another note on the exhaust noise. In the shop with no load on the engine and full throttle it is objectionable.

As shown it weighs 9.0 pounds (4.08 kilo)  So far the investment was $7.50 for the gear shift lever.


13 July 07

Original muffler (left). Note the heavy springs holding it on, and the tiny half-circle exhaust exit hole. When the plastic rear housing is removed the crankcase innards are exposed!  Newer models have only 2 screws. Take care to prevent dirt from getting in while you work on a Weed Eater.


The copper elbow has a coupling inside it to fit inside the motor's exhaust port. The edges of the coupling were beveled with a file so that it will work its way in as it is used, to help seal it.  The gear clamp is wired to the rear spring. Oddly, the two factory springs are not in a line with the port.  The finished tube will hold it in alignment, so all that is required is one spring to hold the elbow against the port. Note the elbow is facing the wrong way in the photo.


A 45° coupling moves the tube to the rear; so that the the exhaust will clear the transom and transom bracket.
Holes were drilled, and a single stainless steel sheet metal screw holds each connection in place.


Above left
The lower bracket is aluminum angle, slotted on the crease and bent slightly to match the copper tube. A stainless steel gear clamp hold the tube to the bracket. I made sure the elbow in the port was aligned as I tightened the clamp. Later, I found it is for the most part self aligning.

Above right
Another 45°, but this time a male/female connector. The small end was annealed with a propane torch so it could be squashed to this shape to make the exhaust nozzle (upper center).

In the shop it is very noisy out of the water. The idle is much higher, likely because there is less exhaust restriction now.  It sounds more powerful, revs up faster, and starts easier. 


Barrel test 2
The motor is not nearly as loud outside, and a rather pleasant puttering sound in the water. I can barely pull the shaft away from the side of the barrel with three fingers, so I am expecting considerable thrust on the lake next week.
Micro lake

With the water level with the horizontal splash plate, the motor idles nicely (factory setting), because the exhaust is much less restricted now. Full throttle is not the screaming you are accustom to when used as a weed cutter. Those inside the house said they thought it was someone coming up the driveway from the sound it made.

25 July 07
 
Experimenting with the height at 1/3 throttle.

It was a beautiful morning on the lake with about a 10 mph breeze and 75 degrees. The motor started as easily as before and I had to raise it a little in the water to get the splash plate about an inch under the surface before it idled nicely.

As I expected, the noise was much less than in the barrel; just a gentle purr. About half throttle seemed the best for smoothness and quiet. Those standing on shore also said it was just a purring sound.

A wood chip took roughly 2.5 seconds to go from bow to stern, 2.2 miles an hour, (3.5 kilometers/hour, 1.9 knots) going into the wind.  At full throttle it churned quite a bit of water behind the boat and there was much better speed.  I did not time it at full throttle. Fuel consumption was too little to really measure. A longer test may tell me  more.

I am not certain of it, but it felt like a prop with slightly thinner blades would work better. I will try to find one with the 10 degree pitch that can be cut down, or make a second prop of my own design.

4 August 07  Use this instead of the wood prop. It only took one evening and works far better.
Ordered a piece of aluminum that will be appropriate for a home-made prop.  Flipped the PVC tube end-for-end that fits into the transom mount, then removed about 3/4" from the wood strip that the height screw bites into. That will allow me to raise the motor just a little more for the Flapdoodle. If I want to try the motor on a boat with a higher transom, all that is required is to flip the tube back the way it was.

 DIY Prop almost finished
Click image to follow link

I have acquired an ST-90, 30cc Weed Eater trimmer that needs carburetor work. It looks well used, but starts easily. This is the type with a solid shaft, but the motor rotates in the opposite direction from most small props. I happen to have a prop that should do that goes to a 20 pound thrust Sears trolling motor. If I decide to convert it, it will probably be the "mud motor" type outboard. The other option would be to shorten the shaft and make it more like my FL-25 motor. It has the same lower part as this commercially made small outboard for $450. Note that they are careful  not to show a side view that would reveal its weed cutter parentage.

8 August 07
I tested the new home made prop in the barrel. Very impressive. The vibration at idle is gone and it accelerates smoothly to any throttle setting.  The thrust is far greater. I have trouble pulling the lower part away from the side of the barrel. At full throttle it now bends the heavy rim of the barrel and pushes water high up the opposite side.  A very nice, quiet little 9 pound motor for very little investment.

12 August 07 Weed Eater fuel consumption
I acquired a syringe that is calibrated in 5th of a milliliter. The plan is to top off the tank, run it for a known period of time, refill it with the syringe, then calculate the hours per gallon fuel consumption at several different throttle positions.

Ambient temperature 92°F
Idle speed =  26.6 hours/US gallon of fuel
Full throttle=  10.8 hours/US gallon of fuel

However, the motor shut down after 4.5 minutes at full throttle. I believe the carburetor gets hot, because I can see bubbles in the fuel line. On a cool day there may not be a problem, but I really do not see much advantage to full throttle over cruising at about 1/3 throttle where the efficiency seems to be best and it is the smoothest running.

Afterthoughts
An easier way to make the cable end might be to use keystock (square steel) inside a brass or stainless tube all held with J-B Weld.  The hard part is getting the cable clean. Clean everything with acetone or lacquer thinner. Rough the cable with a grinding wheel. The plastic cable guide would have to be drilled to accept the larger brass end.

Do not attempt to remove the grease with a propane torch. It makes the end blossom like a flower.



The laminated plywood clamp for the tiller was remade in favor of a heavier design.

I have also learned I need some sort of bearing on top of the PVC tube so that the tube does not eat into the wood tiller clamp. Probably will use a polypropylene disc.

Carburetor Mixture
An e-mail to me by Sean Miller pointed this out:

"Just a note that when you open up a 2 cycles exhaust you really should richen up the carb. not only to take advantage of power gains but to keep it from seizing (too lean) especially newer emission engines that are already real lean as it is. to do it might be tricky on the carb though probably a pulse jet carb. might have a screw(most likely not) or may need shimming/bending parts."

I am not certain that I have "opened up" the exhaust, since it is now under water. 2 cycle engines need a little back pressure to work properly. Below is information that can indicate to you if you are running too rich, or too lean...

http://en.wikibooks.org/wiki/Reading_spark_plugs_for_racing

A better view of what colors can be encountered here:
http://www.nh-scooters.com/article/articleview/55/1/2/


Another precaution:  The bronze bearing was not designed for thrust; but for side loads when cutting brush. It would be wise to repack it with marine grease before each use to help keep water out and prevent the bearing surface from galling.  It probably would not hurt to dribble a few drops of heavy gear lube down the flexible shaft to help protect it also.



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Map started Jan 10, 2008