Building a gas weed whacker

engine bicycle.

Disclaimer

Please Read!

Why a motororized bicycle?

Imagine zipping around town with a lightweight gas powered bicycle that attains at least 100 MPG! With

the capability to hit speeds of up to 30 MPH with a nice cruising speed of 15-20 MPH. All without

worrying about running out of gas as its a bicycle! I have achieved over 200 MPG simply cruising around

town! Up and down hills!

Why THIS motorized bicycle?

I have perfected this design over the last year using six of my personal bicycles. Two of the bicycles have

achieved over 1200 miles on them with no major engine problems! The other bicycles are still “new” and

probably only have 400-500 miles on them.

This design allows maximum engine torque to be applied to the rear tire and allows a very very quick way

to disengage the drive system from the wheel so normal pedal operation of the bicycle can be resumed if

wanted. Since the torque is NOT applied at the center of the hub or wheel as with other systems, there is

much less failure of spokes. There are usually only 6 spokes at any given time transferring energy from

the center hub to the rim. My system applies ZERO torque at the hub.

Ok, what do I need for this?

List if tools needed for this project:

List of general supplies you will need:

Lets get started!

What do I need for the engine!

leaking around the hoses. I generally stay with string-trimmer engines as they are the easiest to mount and fabricate the system for. Plus they really start nicely from the rotation of the bike wheel.

What do I need for a bicycle!

Generally you will need either an old 3-speed bicycle (I have been using womens 3-speeds as they are free) or any other 26 inch or larger bike with smooth tires. Make sure that its STEEL and not Aluminum or Magnesium. I weld to the frame with these plans. Older 10-speed bicycles are perfect for this too. You will need any lightweight bicycle that has 26 – 27 inch tires that are TOURING tires. That is, they are smooth tread. NOT Mountain bike or Trail Bike tread. The object here is to make a nice affordable on-road touring bicycle and NOT an off-roading monster. Besides, the thin smooth tires have much less rolling resistance so more energy is spent on top-speed instead of just rolling resistance.

Once you find your nice older 3,5 or 10 speed bicycle. Please take the time to fix ALL problems. I suggest new tubes and tires for starters. Along with properly adjusted brakes and nice cable action. Bent rims can be fixed with a spoke-tool and one of the many guides on the internet. A comfy seat is very nice. Don't worry about how the bike looks. Everyone will stop you and ask about the engine and how much you charge to build them one. Trust me, it attracts a lot of attention. Even with a 40 year old guy riding a very old womens 3-speed that is bright yellow. Heh.

The Workspace

I have found that the BEST way to work on this project is to strap the bicycle to an old table with rachet straps. This allows you to work on the project at normal height. Please don't use the kitchen table as you will be grinding and welding and it will destroy the surface.

I also suggest a fresh ABC fire extinguisher handy. You will be working around flammable liquids and other nasty stuff so please be careful and THINK.

Enough background. Lets get to building! I hope you read EVERYTHING up to this point. If not then GO BACK AND READ EVERYTHING!

Lets build a weed-whacker bike!

Now, grab your 10 foot ¾ conduit stock and cut off a 14 inch length with either the hacksaw (sucks) or using a pipe cutter (Nice)

Next, measure 6 inches from the left end. This will be our first index mark for the motor to mount on.

Next, you will need to carefully grind out the flattened area of the pipe to allow room for the flywheel to spin. I usea metal cut-off wheel in my handheld side-grinder. Be careful to not remove too much metal. Stop and hold theengine up to the back side of the pipe to check for proper cutting depth. You are trying to get the top 2 holes on themotor to end up halfway on the conduit for the bolts to hold it on.

Bolt the engine onto the conduit using stainless steel bolts with lockwashers, or use the original allen screws.

Be sure to flip the pipe so that the 6 inch long end is on your right now. Otherwise the engine will be bolted on thewrong side when you attach it to the bike frame.

Measure and cut a ¾ inch conduit piece for the lower mount. Slightly flatten the end and drill a hole to bolt to themotor. Leave a bout a 1 inch gap to allow for the pivot nut to be welded on.

Next, you will need a long bolt and a square nut. I use a half inch thick or larger bolt that is 7 inches long. The square nut should be roughly 1 inch on each side. Larger is better here so use whatever.

Large bolt that will be the pivot point. Use a 1/2 inch thick one. Thin ones will bend over time.

Grind the galvanizing off of the conduit with the 1 inch gap. You will need to tack weld the nut into this gap asshown. BE SURE TO DRAIN ALL GAS FROM MOTOR AND COVER WITH SEVERAL LAYERS OF WET TOWEL. Be sure to leave the huge bolt in the nut to prevent spatter from hosing the threads in the nut. Do NOT thread the bolt all the way though. Just flush on one side.

Move the motor to a safe place and put the frame in your vise and finish welding the nut from all sides. Be careful to not burn through the conduit. Concentrate all the heat on the nut first then pull into the conduit.

Let this cool and remove the large bolt that was keeping the spatter out. It might be sticky to apply some oil if needed to help unscrewing the large bolt.

Small scraps of metal, or wet leather stuffed into the holes should also work to keep any welding spatter out of the threads.

Back of mount

I added the extra cross member just because. Its a cut piece of conduit that was hammered flat on the ends and welded onto the frame. Really stiffens it up.

Next we will prepare the bike for mounting the engine:

Secure the bike to the table surface using straps. Make sure that the bike is pretty close to true vertical as you will need a nice straight bike for welding. Otherwise things just won't line up right.

Eyeball the rear of the bike just below the seat where the two frame rails split around the back tire. There should be the rear brake calipers mounted there somewhere. We are going to attach the motor-mount above the brake calipers on the frame later on.

This next part is tricky so please follow along closely:

Assemble the motor to the mounting bracket with the screws and washers as shown way above. Insert the large long bolt that fits the nut that was welded to the bracket. Screw the bolt in so that about ¾ inch of threads is showing through the bracket nut with the head of the bolt on the flywheel side.

Hold up the motor bracket assembly to the cooled off bike frame and eyeball things to make sure that there is plenty of clearance for the brakes and such. You may need to extend your bike frame nuts by welding on another set on the existing ones to allow for brakes as I did on this bike.

We are now going to weld the engine pivot bolt to the bike frame stand-off nuts. Please pay close attention to the spacing between the flywheel and the tire sidewall. Please note the position of the ignition coil also.

Welding the engine to the bike frame pivot point.

You want the flywheel as close to the tire as possible without rubbing. Please take note that the pull starter tangs will fly outward as speed increases so please leave room for them also. (This engine had the removed). Your engine will probably look like this with the long flywheel nut. Use this to make sure the motor is aligned with the tire. Should be as true as possible.

Have an assistant hold the engine to the bike frame nuts and align the motor to the wheel as stated above. Tack the large bolt to the bike frame nuts at this point. Just enough to hold the motor on to the frame at this point as adjustments will probably be needed before final welding.

Weld away!!

When done, you will have something that resembles this

Mine have always been 14MM. Once the long nut is removed, measure the distance from the flywheel to the outer edge of the rear tire. This will be the length of the drive pin.

*Note: The flywheel fins break off very easily. Jam a screwdriver at the magnet casting as it's thicker there.

To make the drive pin, cut a piece of conduit to the length that you measured from the steps above.

Maybe add another ¼ inch just to be on the safe side.

Next, find a nut that will slide over the crankshaft threads of the motor (the flywheel nut you removed earlier). The nut must also fit very snugly into the conduit piece also. Once you have located the proper nut, you will need to carefully shave it down to half of its original thickness. This will allow the long flywheel nut to hold the drive-pin/tube onto the flywheel.

Once you get the nut ground down to roughly HALF of it's original thickness, you will need to press it into the conduit tube carefully. I use a hammer and tap the nut into the tube then follow up with a trip through the vise to make sure its nice and square.

You will need to grind the welding smooth on the pin assembly. Check for true by placing the drive-pin on a level surface. This thing will spin at over 15,000 RPM so it has to be balanced and true.

When you get everything nice and straight. Slide the new drive-pin onto the engine and install the long crankshaft nut with a 14MM deep socket to hold everything together to check out the alignment with the tire..

Now we need to add some traction to the drive pin. Mark where the tire rubs on the drive pin by setting the motor down and eyeballing the tire placement. Remove the drive-pin and lightly clamp it into a bench vise. Do NOT distort it.

Carefully grind off the galvanization and put lots of little welding dots all over the area that rubs the tire. I like to feed some wire then tap it onto the drive pin. It takes time to get a nice even covering of little dots. You will probably need to do it about 3 times after the initial run of the bike as the weaker ones tend to wear off quickly. This drive pin is only on the second time of welding.

  Make sure to lightly grind off any HUGE clumps. We are shooting for small sharp points that will grip the tire and not slip.

Install the completed pin with the 14MM deep socket and tighten it down securely.

Now we need a way to pull the motor assembly down on the tire with a nice even pressure.

I use a hoseclamp and a nice thick spring to maintain a nice even pressure on the tire. Simply slip the hoseclamp through one of the spring eyes and slip it onto the end of the motor frame.

Next, you will need to take a length of ¼ inch round steel stock and bend a small hook on one end. Using a MAPP gas torch and vise/hammer works great for this. Let it cool down and hook it into the spring assembly on the motor bracket.

Look for a slot or hole down by the rear tire axle. There is usually one for a fender mount, or sometimes there is enough behind the axle shaft to hook into.

Pull down on the bar and mark a good location for the bottom hook to start bending in relation to your bike frame. Remove the bar and cut/bend to the proper length. Each bike is different.

You can also use a bungee for this, but I prefer the spring as it gives a nice even pressure and it doesn't stretch over time.

Here is the pull bar installed on the spring and hooked into the bike frame behind the axle shaft.

To adjust tension on the tire, simply remove the pull bar, loosen the hoseclamp and rotate the spring higher or lower.

A note about the tire wearing out:

As long as you get the proper tension on your drive pin, your tire will not really care that its being driven by a welded bumpy tube. Imagine how harsh the road is at times! Just keep an ear on the tension to adjust it properly.

Some slippage when accellerating is fine. Don't make the tension TOO tight or you run the risk of crankshaft damage. Try to avoid wet grass and any liquids on the road. Just drive slowly and allow the tire to dry off before giving any gas. I have seen the kids eat a tire on one of my bikes when they were riding in the rain. Took about 1 hour, but they have no sense of “Take it easy”. Full throttle on a wet tire will wear through eventually.

I have about 1000 miles on the same tire with my Yellow bike. Just maintain the proper tire pressure and buy a quality road tire.

To disengage the motor from the bike, simply pull down on the pull bar and remove it from the bike frame then slip it out of the spring eye. Use a bungee to hold up the engine as it will pivot on the big bolt.

Throttle Cable:

If you are lucky, you can re-use the original throttle cable from the weed-whacker as they usually have a special swivel or connection on the carburetor for the cable. But, you will need to extend the length to reach the handlebars.

To adjust the throttle cable movement, simply loosen up one of the above hose clamps at the cable union and slide them together or apart depending on the slack in the line. A tighter cable will allow full throttle operation, but too tight and your idle speed will be high.

Test the throttle cable movement. Full lever movement should make the throttle on the engine open all the way. Released should allow the idle screw on the carb to hit the throttle stop.

Adding a Kill Switch:

If you look on the ignition coil, you will notice a plug. You probably disconnected a wire from it during disassembly. This is the kill switch wire. You will need to attach a female spade plug to this with a length of wire up to the handlebars. Simply attach a switch between this wire and the bike frame. Once the coil is grounded, it stops making spark shutting off the engine. I use a wooden spring-type clothespin that has one side zip tied to the handlebars. The open end has the wire wrapped around it so that when it's pressed shut, it hits a contact on the other side attached to the handlebars. Be careful as you will get zapped from this if your finger touches the end where the wires are. :) Maybe I will install a real switch sometime.

Getting it to run!

Once you get the cables connected and moving smoothly, its time to fire it up! If your engine has been sitting around for a long time, it will need to have the carburetor taken apart and cleaned. You should also replace the little gas line and pressure line (if equipped). Clean out the tank “clunker” while replacing the hoses.

Get a can of carburetor cleaner and proceed to remove the carb from the engine. Disassemble the carb and spray the ports and screens out. Look for any debris that may be blocking passages. Be careful of diaphrams and gaskets as they might need to be replaced if torn. A local small engine shop should have the Walbro carb kits for you if you need one.

Check the sparkplug to make sure its gapped correctly and clean. You can lift up the rear of the bike and spin the tire by hand with the plug out and you should get spark.

Mix gas with a quality 2-stroke oil. I have been running my engines at 32:1 mixture. Or 2 ounces of 2-stroke oil to 1 gallon of non-ethynol gas.

Carefully fill the gas tank, full choke, make sure your in first gear and start pedaling. The engine should start up.

Pull the choke off as needed and apply throttle. You will probably need to adjust the needle jets to attain the best running. If you have 1 needle then its a snap, get the engine up to temp by running it and adjust the needle so that the engine revs the highest at full throttle. It takes time and it's a lot of getting off and on the bike.

Two needle carbs are the same idea, but there is a low speed jet that will need to be adjusted for the partial throttle end. It's a bunch of tinkering until you get it just right.

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PROBLEM SOLUTION

Hard Starting       Check choke position, spark plug.

Will not rev up when riding        Check high speed needle settings, clean carb out

Will not run at slow speed         Check low speed needle, clean carb, set idle screw.

Dies Check gas level.                 Turn needles out for more gas.

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Great! It's running! You want more power now.

2-stroke engines generate more power than an identical 4-cycle engine since they are firing twice as often. To achieve more power from a stock 2-stroke engine, you need to get more fuel into the cylinder and the exhaust gases out as quickly as possible.

Lets start by removing the muffler on the engine. Most are held on with some nasty tight springs. Use a big pliers to remove the springs. Once you get the muffler off, it should fall apart in two sections. Remove the baffle inside if it has one. They are usually a tube with a bunch of holes drilled in it. Should just pop out. Re-assemble your muffler and re-spring onto the engine. You will probably need to adjust the carburetor as you are allowing better air flow into the engine. It will be a tad louder too. But, the power increase should be noticeable.

For more power, you can increase the hole size in the muffler to allow more exhaust to flow out. Punch out or drill out the existing holes. These engines can get LOUD so be careful to not piss off your neighbors.

I also try to avoid gas with Ethanol added. These little motors will run on 10% Ethyl gasoline, but your power and mileage will suffer. I use plain old 87 Octane unleaded gas with no alcohol added. Just 32:1 oil ratio.

Driving the bicycle with motor assist:

Simply set the choke if needed, make sure the bike is in first gear and start pedaling. Once engine starts, reduce choke. Apply throttle as needed up to top speed. To stop, release throttle and apply bicycle brakes. Without throttle being used, these engines are really weak on the slow end. The bicycle brakes are more than enough to stop.

Expect about 20 miles of moderate to hard driving on one 20oz tank of mixed gas/oil. More throttle will use more gas. Some of the neighborhood kids have cruised all afternoon on one tank and merely put-putting around.

If the drive-pin is slipping on the tire, (you will hear the engine revving up but no speed gain), you will need to adjust the spring tension as described previously. Don't go too tight or performance will be affected and you will greatly shorten the life of the engine.

As far as the legality of these? I have been stopped several times and have been told that eye protection is needed since they are motorized, no license plate is required as they are under 49cc. You will probably need a small headlight and such depending on your local laws.

Please check before riding on public roads. It is YOUR responsibility to check your local laws before riding. If in doubt, disengage the engine and use the bike as a regular bicycle. :)

Have fun!