Many of the engines that are included with RC vehicles feature simple pull-start systems that are used to crank the engine over. Pull starters are practical and generally reliable, but they have been known to fail. By far the two biggest issues are breaking the pull-start cord and a one-way bearing with an oil build-up that causes it to slip on the crank shaft.
Remove the engine from the vehicle. This entails loosening the engine mounting screws under the chassis, disconnecting the throttle linkage and removing the muffler or tuned pipe from its mount.
Remove the pull-starter from the back of the engine. The pull start mechanism is usually secured to the engine with 3 2mm hex head screws or Philips head machine screws. Carefully pull the pull-start mechanism away from the crank shaft. Use a flat head screwdriver to pry it away and to insure that pull-start mechanism doesn't come apart as you remove it.
Remove the one-way bearing from the pull-start pulley by prying it out with a flat head screwdriver. Exercise extreme caution while performing this step because there is a tightly wound coil residing behind the pulley. If this coil unwinds, it will shoot out with velocity right towards your face, so wear safety glasses as an extra precaution. Although the coil can be rewound and successfully reinstalled behind the pulley, you'll be better off buying a new pull-start mechanism than trying to fix it.
Pour a couple ounces of denatured alcohol, electric motor cleaner or some other kind of solvent into a jar and then toss in the one-way bearing. Allow the one-way bearing to soak in the degreaser for 45 minutes.
Remove the one-way bearing and wipe it dry with a rag.
Place the one-way bearing inside the pull-start mechanism, and then apply a couple of drops of machine oil, or WD-40 on the crankshaft. Reinstall the pull starter back on the engine. The one-way bearing should be able to "grab" the crankshaft and turn the engine over. If it continues to slip, the one-way bearing will need to be replaced. Refer to your vehicle's instruction manual for the part number and order a replacement from your hobby shop or directly from the manufacturer.
The pull-start cords in most pull-start mechanisms measure 14 inches or longer, which is about twice the length that's required to crank the engine over. In fact, most manufacturers recommend short quick pulls to start the engine rather than pulling the cord all the way out of the mechanism, which takes too long to retract. In most cases the cord will break a few inches bellow the handle, which makes repairing the pull start cord easy. Refer to Steps 1 and 2 from the previous Section to remove the pull-start mechanism from the engine.
The cord will be wound around a pulley inside the pull-start housing. Turn the pulley counterclockwise with your fingers until you could see the broken end of the cord. Use Needle nose pliers to grab the end of the cord and feed it through the hole in the pull-start housing. Pull the cord through the hole and secure it with pliers so that it does not retract back into the housing.
Remove the other broken end of the cord from the pull-start handle. Some pull-start handles have a small lug that the cord attaches to while others just have a hole where the cord passes through and a knot on the end of the cord and glue prevents it from pulling through the hole. Remove the lug by prying it loose with the tip of a flat blade screwdriver. You can use a flat blade screwdriver to remove the knot and glue as well. Throw away the broken piece of cord.
Cut any loose strands from the end of the pull-start cord that's attached to the pulley with sharp scissors. Next install the cord to the handle. Tie the cord to the lug or just simply pull the cord through the hole in the handle and make a couple of knots to prevent it from slipping through the hole when you yank on the pull-start handle. Apply a few drops of CA glue on the knot and let it dry completely.
Reinstall the pull-start mechanism on the engine, and then install the engine back on your vehicle.
•Do not yank on the pull-start cord at an angle. You want to pull the cord out as straight as possible. •Be careful when yanking on the pull start cord with the vehicle's body on. Do not let the cord scrape on the body because the chafing will damage the cord. •Never pull the cord all the way out of the pull-start mechanism. Use short and quick yanks to start the engine. •Do not run your engine overly rich. Oil will build up inside the one-way bearing and cause it to slip on the crankshaft.