Throttle cable repair, modifications, cleaning, and lubrication
by Had Robinson
You can go to this site for a complete explanation and diagram of the parts of a Bowden cable, the throttle cable used in paramotors.
Here are the steps to getting your throttle working working smoothly and easily. If it all looks daunting, take the whole thing to your local mountain bike shop and they can do it all but the kill switch wire and the shrink wrap, the latter being more decorative than functional.
Drilling an access hole (see below) in the throttle body makes cleaning, lubrication, and inner wire replacement a snap. Use ONLY pure graphite powder to lubricate the inner wire and the parts of the throttle body. Most hardware stores have pure graphite powder for lubricating locks.
The Bowden cable must never be bent sharply or it will be ruined. Ensure that the cable has smooth turns from end to end. Sharp turns increase friction and make the throttle harder to operate.
As your throttle cable ages, friction between the inner wire and the housing increases. This is caused by wear and/or dirt. Dirt is a particular problem if the wire was ever lubricated with anything other than pure graphite. Over time, the Teflon coating of the wire (if it has it) wears off and the little pieces can jam the inner wire. Some housings are coated with Teflon and it, too, can come loose, jam things.
Sometimes, the throttle will stick completely. Rather than buy an entire new throttle cable assembly (about $165), handy pilots can replace the inner wire, an item readily available from any bike repair shop for about $5. You can take the entire throttle cable to a bike shop and they can replace just the inner wire. The inner wire is what usually wears out or sticks. A local shop can do the whole job which involves cleaning the inside of the housing and replacing the inner wire. They can also replace the entire Bowden cable. Below I give the easy steps to get your throttle cable working smoothly, if you want to do it yourself.
The inner wire will last 50 - 100 hours.
If you buy a new inner wire, do NOT cut it BEFORE you re-insert it in the housing. If you have to insert a cut wire (without the weld at the end), clean the cut end with brake cleaner and then apply super-glue. Let it harden and THEN carefully insert the inner wire into the housing from the throttle end.
Use a rag to hold the hand throttle at about 1/2 throttle and twist the inner wire so it tightens the weave (usually clockwise) as you gently push on it. It is a tedious process but much better than breaking apart the throttle body and ruining the specially sized shrink wrap. If necessary, you may have to crack the throttle body open just enough to see where to insert the inner wire into the cable housing. If you are careful, it will not disturb the shrink wrap. Once it is fully into the housing, THEN cut it to the correct length with a Dremel type tool using a cut-off wheel. Wrap the area of the cut TIGHTLY with masking tape or the ends that are cut will go everywhere and mess up the cut.
Be very careful of the cut ends! The individual wires must NOT be bent or twisted.
1. Checking the outside of the cable for damage
With normal use, the cable will rub on various parts of the frame. If the cable has not been protected at these contact points by wrapping it with electrical tape or a piece of split hose, the kill switch wire can short to ground. This will cause an engine shutdown at unexpected times. In extreme cases, the housing can wear through to the inner wire and the throttle can jam or cease to function!
Carefully examine the entire length of the cable for damage. Apply tape or hose to those areas where the cable rubs any part of the frame or where the outer plastic weave is damaged.
2. Cleaning and lubricating the housing and the inner wire
Pilots will save themselves a lot of trouble and frustration by making an access hole in the back of the body of the throttle. All bicycle devices that work with Bowden cables have access holes so that the inner wire can be easily removed without having to disassemble a complex device, like a gear shifter. Miniplane should do the same because the housing of the Bowden cable rarely needs any attention. It is just the inner wire that moves, wears out, and jams.
Take a 4mm (5/32") drill bit and make a hole 42.5mm (1 11/16") from the opening of the lever, as shown. Tap the back of the throttle body on a hard surface so that chips from drilling the hole will fall out. There is no necessity to plug the hole but, if desired, a small piece of black electrical tape can be put over it.
With the access hole, the inner wire can easily be removed. Just undo the knarp that holds the wire at the carburetor and pull the inner wire free of the lower throttle lever. Straighten the end of the inner wire as best you can. The factory bends the inner wire through the throttle lever on the carburetor – something not recommended for prolonging the life of the inner wire near the end. To fix this, see step #3 below.
If the inner wire is frayed or damaged, it should be replaced. See the instructions later on this page on how to do that. If you start pulling a damaged inner wire out of the outer housing, you will not likely be able to push it back through the housing later if you need to. If the inner wire is damaged, the bad section will need to be cut off before pulling it out of the housing. To do this, wrap a piece of masking tape firmly around the area to be cut. Using a Dremel type tool with a cut-off wheel, cut through the masking tape and wire inside. If you do not use the tape, the small individual wires that make up the inner wire will go everywhere. You cannot use a saw or cutters. You must use a cut-off wheel or purchase the special tool used to cut these wires. Bike shops can cut it for you.
Put the throttle lever in the idle position. Carefully, push the inner wire out of the housing from the carburetor end. As you push the inner wire out of the housing at the carburetor end, look through the access hole for the round plug crimped on the inner wire at the other end. You may have to take a pin or paper clip to align it with the hole as you push it out of the throttle body. Grab the plug and pull the inner wire out of the housing about 30cm (12"). In order clean the housing, there must be some of the inner wire in the housing. It does not matter if you plan to replace the inner wire or not.
DO NOT PULL THE INNER WIRE COMPLETELY OUT OF THE HOUSING!
If you do pull it completely out, it is not too hard to thread it back through the throttle lever and the hole in the housing. Pull on the throttle lever and locate the hole in the lever with a light. Thread the inner wire through the hole. Push gently or you will bend things! Put the lever in the idle position. While holding the inner wire, spin the throttle body and the housing with the other. As you turn the throttle body, push very gently on the inner wire. It will usually find the hole in the Bowden housing and go right in. This will NOT happen unless the end of the inner wire has a weld. You can put a tiny amount of 5 minute epoxy or superglue on the tip and let it harden if the end has been cut. Alternately, you can remove the screws holding the throttle body together and crack open the back of the throttle body enough to see inside in order to thread the inner wire back through the Bowden outer housing. By cracking the back open just a little, you will not disturb the heat shrink around the Bowden cable where it connects with the throttle body.
To thoroughly clean and lubricate the inner wire with an access hole:
- Affix the throttle body in some way (in a vice) so that the access hole is facing up with the inner wire coming up out of the hole. Put the other end of the Bowden housing in a receptacle so that the cleaning solution will be caught.
- Put some mineral spirits (paint thinner) in a small glass and pour it on the exposed inner wire. Make sure the inner wire does not touch the hole. If you do it right, the mineral spirits will dribble down the inner wire and into the Bowden outer housing. Pour enough mineral spirits onto the inner wire so that it runs out the other end of the Bowden cable.
- Push and pull the inner wire in and out. This will help remove any grease, oil, or debris in the housing. If the end of the inner wire is damaged, you might not be able to push the inner wire back and forth in the housing.
- Repeat step 2 until the solution coming out of the other end of the cable is clear.
- Remove the inner wire.
- Using compressed air, thoroughly blow the mineral spirits out of the throttle and housing using the access hole in the throttle body. There must not be any solution left in the housing. A hair dryer or heat gun can be used to warm the cable and evaporate the spirits.
- Examine the inner wire. If the coating is coming off or there are any frays in the inner wire, you should get a new one at your local bicycle shop.
- When performing the next step (#11) the Bowden cable must be straight! If there are any bends, the inner wire may jam.
- Use pure graphite powder to sprinkle on the inner wire as you push it back into the Bowden housing. It does not matter if graphite gets all over the inside of the throttle body – it actually helps. Twist the inner wire in the housing opposite the turns in the lay of the wires that make up the wire so that you do not unwind the twisted wires. If you are using a new inner wire or the original (undamaged) inner wire, the end will have a weld so it will not unravel. If it does not have a weld, apply some 5 minute epoxy or superglue to hold the wires together.
- Reconnect the throttle cable to the paramotor, including the kill switch wire.
To thoroughly clean the cable without drilling an access hole in the throttle body it is the same as above but you will have to crack open the back of the throttle body. Start at step #1
Quality Bowden cables will have a housing that has a lining which reduces friction between the inner wire and the housing. In addition, a quality inner wire will be Teflon coated. These wires do not need any form of lubrication and will work smoothly until they are worn out.
Pilots who fly often will also appreciate this throttle spring modification which decreases the pull on the throttle, lessens carburetor throttle shaft bore wear, and provides a more positive return of the throttle to the idle position.
3. Throttle inner wire end attachment modification
The inner throttle wire will last much longer (100's of hours) if this modification is done where it attaches to the carburetor. It also provides a more convenient way to disconnect the throttle wire from the carburetor. Instead of removing the knarp and losing the wire adjustment, the ends of the cotter pin are bent together and quickly removed from the the throttle lever.
- (1) knarp – use the existing one
- (1) thin piece of sheet metal
- (1) small cotter pin [or a 2mm x 6mm machine screw, (2) washers, and locknut] It is better to use a machine screw than a cotter pin.
The correct size knarp a.k.a. "throttle clamp" is available from Miniplane-USA if yours is missing or damaged. The cotter pin (or machine screw) will wear out gradually so it is important to keep spares on hand. After 100 hours or so, the thin piece of sheet metal will wear out, as well. This is better than wearing out the inner wire!
Make the small bracket out of sheet metal, as pictured here. Drill two holes, one just bigger than the inner wire and the other just bigger than cotter pin (2MM). Then bend the sheet metal into an "L".
The cotter pin must be shortened so that when it is pushed through the new bracket and the lever on the throttle shaft, it will not catch on other parts.
Instead of a cotter pin, it is better to use a 2mm machine screw. The screw will last longer and give a tighter fit to everything. Put the washers under the head of the screw and the nut. Tighten things up just enough that everything moves easily.
Be sure to check for free movement of the inner wire. Failure to thoroughly check the throttle for free movement can result in a stuck throttle which can be EXTREMELY dangerous! This modification can be used with the throttle spring modification.
When the carburetor is serviced, pilots must also remove the throttle inner wire. With this easy modification of the throttle bracket, pilots can quickly remove the carburetor along with the entire throttle cable. All you have to do is unplug the kill switch wire. Use a fine hacksaw or a Dremel cut-off wheel to cut a slot in the throttle bracket, just like what is done on all bicycle brake mounts. The bracket below has a repair plate added. The original throttle bracket failed due to metal fatigue. With the slot cut, it is an easy job to remove the carburetor along the complete throttle cable.
4. Cruise control
A pilot in England asked me about modifying the stock throttle for cruise control. After thinking about it, I do not recommend it. Runaway engines and emergency shutdowns are conditions that require instant control of the throttle and a cruise control adds more complexity to a cascading event such as a stuck cruise control, a faulty kill switch, a broken throttle spring, or a glider deflation. The pilot who had the stuck cruise control rolled his trike and did a lot of damage (propeller, cage, etc.) What if his lines to the glider had gotten into the spinning propeller? Think about this sort of thing before making modifications to your throttle.
Anyone who has experienced a runaway engine knows what I am talking about. It is terrifying. However, if the above throttle spring modification is done (applies to most paramotors), throttle pressure is greatly lessened and will be much more comfortable when flying for many hours.
For those wishing for a way to construct a cruise control, I would modify the existing throttle pivot (drill it out) and insert a screw with a plastic knurled knob. Shortening the pivot will allow the throttle halves to be squeezed together by the knob and screw enough to lock it.
5. Kill switch
See the kill switch info section on how to troubleshoot kill switch problems related to the cable and the throttle assembly.