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Throttle cable repair, cleaning, and lubrication

by Had Robinson

Introduction

Here are the steps to getting your throttle working working smoothly and easily.  If it all looks daunting, take the whole thing to your 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 cable replacement a snap.  Use ONLY pure graphite powder to lubricate the Bowden cable 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 cable and the outer housing increases.  This is caused by wear and/or dirt.  Dirt is a particular problem if the cable was ever lubricated with anything other than pure graphite.  Over time, the Teflon coating of the cable (if it has it) wears off and the little pieces can jam the inner cable.

Sometimes, the throttle cable will stick completely.  Rather than buy an entire new throttle cable assembly (about $165), pilots can replace the inner cable, an item readily available from any bike repair shop for about $5.  The inner cable is what wears out or sticks.  A local shop can do the whole job which involves cleaning the inside of the sheath and replacing the inner cable.  They can also replace the entire Bowden cable itself.  Below I give the easy steps to get your throttle cable working smoothly.

The inner cable will last 50 - 100 hours.

If you buy a new cable, do NOT cut it BEFORE you re-insert it in the Bowden sheath.  If you have to insert a cut cable (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 cable in the sheath from the throttle end.  Use a rag to hold the hand throttle at about 1/2 and twist the cable so it tightens the weave as you gently push on it.  It is a tedious process but much better than breaking apart the throttle body and the specially sized shrink wrap.  If necessary, you may have to crack the throttle body just enough to see where to insert the cable in the sheath.  If you are careful, it will not disturb the shrink wrap.  Once it is fully in the outside sheath, 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.  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 cable itself 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 Bowden outer housing and the inner cable

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 cable can be easily removed without having to disassemble a complex device, like a gear shifter.  Miniplane should do the same because the outer Bowden cable rarely needs any attention.  It is just the inner cable 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.

Miniplane throttle body showing access hole for inner cable

With the access hole,  the inner cable can easily be removed.  Just undo the knarp that holds the cable at the carburetor and pull the cable free of the lower throttle lever.  Straighten the end of the cable as best you can.  The factory bends the cable through the throttle lever on the carburetor – something not recommended for prolonging the life of the cable near its end.  To fix this, see step #3 below.

If the cable 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 cable 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 cable 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 cable inside.  If you do not use the tape, the individual wires inside the cable 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 cables.  Bike shops can cut it for you.

Put the throttle lever in the idle position.  Carefully, push the cable out of the outer housing.   As you push the cable out of the housing at the carburetor end, look through the access hole for the round plug crimped on the cable 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 cable out of the housing about 30cm (12").  In order clean the cable, there must be some of the inner cable in the housing.  It does not matter if you plan to replace the inner cable or not.

DO NOT PULL THE CABLE COMPLETELY OUT OF THE SHEATH!

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 outer sheath.  Pull on the throttle lever and locate the hole in the lever with a  light.  Thread the cable through the hole.  Push gently or you will bend things!  Put the lever in the idle position.  While holding the cable, spin the throttle body and Bowden cable with the other.  As you turn the throttle body, push very gently on the cable.  It will usually find the hole in the Bowden cable and go right in.  This will NOT happen unless the end of the cable 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 cable 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 cable with an access hole:

  1. Affix the throttle body in some way (in a vice) so that the access hole is facing up with the inner cable coming up out of the hole.  Put the other end of the Bowden cable in a receptacle so that the cleaning solution will be caught.
  2. Put some mineral spirits (paint thinner) in a small glass and pour it on the exposed inner cable.  Make sure the cable does not touch the hole.  If you do it right, the mineral spirits will dribble down the inner cable and into the Bowden outer housing.  Pour enough mineral spirits onto the inner cable so that it runs out the other end of the Bowden cable.
  3. Push and pull the inner cable in and out.  This will help remove any grease, oil, or debris in the housing.  If the end of the inner cable is damaged, you might not be able to push the cable back and forth in the housing.
  4. Repeat step 2 until the solution coming out of the other end of the cable is clear.
  5. Remove the inner cable.
  6. Using compressed air, thoroughly blow the mineral spirits out of the throttle housing and cable sheath 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.
  7. Examine the inner cable.  If the coating is coming off or there are any frays in the cable, you should get a new one at your local bicycle shop.
  8. When performing the next step (#11) the Bowden cable must be straight!  If there are any bends, the inner cable may jam.
  9. Use pure graphite powder to sprinkle on the inner cable 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 cable in cable opposite the turns in the lay of the wires that make up the cable so that you do not unwind the twisted wires.  If you are using a new cable or the original (undamaged) cable, 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 cable wires together.
  10. 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 an outer housing (sheath) that has a lining which reduces friction between the inner cable and the housing.  In addition, a quality inner cable will be Teflon coated.  These cables 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 cable end attachment modification

The inner throttle cable will last much longer (100's of hours) if this modification is done where it attaches to the carburetor.  It also provides a much more convenient way to disconnect the throttle cable from the carburetor.  Instead of removing the knarp and losing the cable adjustment, the ends of the cotter pin are bent together and quickly removed from the the throttle lever.

Parts needed:

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 cable!

Make the small bracket out of sheet metal, as pictured here.  Drill two holes, one just bigger than the cable and the other just bigger than cotter pin (2MM).  Then bend the sheet metal into an "L".

Top 80 throttle cable attachment modification parts

The cotter pin must be shortened so that when it is pushed through the new bracket and throttle lever, it will not catch on other parts.  It is better to use a 2mm machine screw.  Put the washers under the head and nut, respectively.  Be sure to check for free movement of the throttle cable.  Failure to thoroughly check the throttle cable for free movement can result in a stuck throttle which can be EXTREMELY dangerous!  This modification can be used with the throttle spring modification.

Top 80 throttle cable modification parts installed

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.

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