Killing a Run-A-Way Engine
by Alan Buehner
Originally printed in the 2008 issue #40 of Still….Keeping Track
Ted Landers called me a couple of months ago to order a new throttle cable for one of his KTM race bikes. During a race, his "bargain" throttle cable became frayed and caused the carb slide to be stuck in the wide open position. This caused the engine to high rev beyond the "red line". He instinctively hit the kill button, but it did nothing to kill the spark. His bike wound up exiting the track and taking off through the woods. The spectators found him laying on the ground and his bike laying on it's side a little further into the woods. Fortunately he and the bike came through that experience unscathed.
After talking to Ted, I thought about my own experience where I was at one of my trail riding areas in 1974. I had just kick-started my 72 Kawasaki 350 Bighorn when it started high-reeving all by itself. I turned the ignition key off but it just kept running. I panicked when the key didn't work and knew I had to make a quick decision as to what to do before the engine blew. So, I kicked it into l51 gear. Lucky for me, it pulled a big wheely and stalled out.
I always advise everyone to install a kill button on their bikes as a safety precaution. But there are situations where when an engine is red-lining that the current being produced by the magneto and being sent to the coil is so strong it cannot be grounded out by the kill buttons.
What would you do in this type of situation? Become superman and grab the spark plug wire and pull it off the spark plug? Every situation is different and it all depends upon what size engine it is you are dealing with. Is it inside a garage where you are starting your newly restored pride and joy - and failed to notice that the throttle slide was resting up too high because the new cable was too short or the set screw was turned in too much. Is it outside while you are riding, like in Ted Lander experience. Or is it like my case where you are outside and have restarted your bike. No matter what the situation is or where you are at, you need to know what to do and you need to do it fast. So, I contacted a few of our Penton experts and see what solutions they had to offer.
Al Born - Turn the choke on, up shift to a higher gear, and hit the brakes.
Doug Wilford - Turn the bike upside down (lay it down on the ignition side to prevent hitting the shifter). Put your foot or something over the end of the exhaust pipe. If you are riding, put on the brakes. Put the front wheel up against a large tree.
Do not pull the spark plug wire - this can knock you on your butt - it can ruin your ignition - the engine may still keep running after pulling the wire - it could cause a fire if the loose plug wire creates an arc.
Dane Leimbach - Put the bike into a high gear and hit the brakes to stall it out. The other option would be to pull the spark plug cap - this could hurt the ignition but would be better than having the engine self-destruct. There is the risk that in pulling the spark plug cap that the engine could still keep running if it develops a dieseling condition.
Bobby Lucas - Put the bike in a higher gear and get on the brakes to stall it. Put a shop rag into the exhaust end and turn off the gas. The last resort if you are riding would be to pull the clutch in and let the motor blow.
The following are tips from Dane Leimbach to prevent a throttle from sticking:
There are lots of issues that can cause throttle sticking, and I always make sure that it isn't going to happen. I can't remember the last time that I've had the stuck throttle.
The very first thing that you need to do before you start your engine, no matter if it is a brand new bike or one that is 40 years old, is to turn the throttle and close it, to see if the carburetor slide hits the bottom.If it doesn't, then don't even start it before you figure out what is wrong.
The second thing that I always do when I turn the throttle before starting the engine, is to make sure that there is a very small amount of slack in the cable. You need to be sure that there is some slack between the throttle and the carb slide, because when you turn the bars from one side to the other, and the cable gets bent a little more, it will tighten up the cable. Without some slack, you will be moving the slide up and down just by turning the bars from one side to the other.
If the slide doesn't hit the bottom of the carb when you turn the throttle close, then you need to find out why the cable isn't moving back easily. If you have a broken cable wire, that can cause some sticking. Sometimes, it's just a little kink in the cable and that needs to be straightened out so the cable moved smoothly in each direction.
All of this being said, you don't normally find the throttle sticking. However, in some carburetor designs, you can have an issue. Some of the older carburetor's idle screw, used to hit the bottom of the slide on the side of the slide. And over time, the idle screw can create a burr on the side. of the slide. A file can take the burr off the slide and fix the sticking problem.
The throttle position on the bars, can cause the throttle to stick too. If you don't have the throttle/grip just off the end of the bars, Then it can bind up the throttle and cause sticking.
I never put any lubrication on the bars under the throttle tube unless it is a silicone spray that actually dries out. If you have a moist lubrication, it can attract dust to the throttle which will cause dragging in the throttle. If you're in a muddy situation, as long as it's actually wet, then it won't stick. But when it starts drying out, then it will cause dragging.