Stan Bradbury's Speedway Training Notes

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SPARK PLUGS

In a 60 to 80 second race, a spark plug rarely runs hot enough to deteriorate anything like it would in a car or road motorcycle. Even in a 20 minute motocross race the abuse that a spark plug takes is probably greater than that received by a plug throughout a speedway riders career.

Plugs which have been used for speedway for many years show little or no signs of wear and any deterioration is usually caused by improper storage or accidental damage while not in the machine. Occasionally you may come across a plug with a manufacturing defect or a cracked insulator due to external damage, but wear and tear on the plug for the short time it is in use is minimal. Don't be mislead by this information. Plug problems are by far the most common cause of engine failure, failure to start or poor performance due to misfire "oiling up" etc. These problems can nearly always be avoided by keeping the correct plug clean. Many riders use plugs which are far too "cold" and so increase the possibility of plugs becoming oiled up or wet with fuel. Plugs run relatively cool in a speedway engine, because of the enormous cooling effect of methanol fuel, consequently you need racing plugs from the "hot" end of the scale. Be sure they have the correct "reach" and are of the "recessed gap" type. Never use a plug with an external electrode, as the piston comes so near the electrode during normal operation that there is a real danger of the piston being marked by the electrode at high revs and, if the electrode should break off, serious damage could be done to your motor. Remember not to over tighten the plug as this will eventually strip the aluminum threads in the cylinder head and it also makes the plug difficult to remove in a hurry.

I used to change my plug after warm up and after each race to ensure an easy start. After the race meeting I had four or six slightly oily plugs that required superficial cleaning. This is far easier than having your motor fail to start when you have limited time available or, if its a hot night and you don't have a mechanic to change plugs for you. Changing the plug after every race may be extreme. It may depend on how cleanly your engine burns and how cool it gets between races. It's a good idea to cover the engine to keep it warm if there is to be a long delay between races. I have never ceased to be amazed at inexperienced (or even some experienced) riders who push their machine for dozens of yards and even if they are lucky enough to get the engine running, arrive at the start gasping for breath. Also they often push the machine further and further away from the pits and so reduce their chances of having a mechanic change their plug or changing it themselves under the two minute ruling. I always carried a spare clean plug in my leathers, wrapped in cloth to prevent injury, along with a flat, lightweight plug wrench tucked in my boot. That way, I could change a plug anywhere and so avoid even a short walk back to the pits which in 90 degree weather and all protective riding gear, can be exhausting in itself.

If the engine has flooded (which is easily done on a high compression methanol engine), be sure to spin the rear wheel by hand two or three times after you have removed the wet plug. This turns over the engine quite rapidly and dries out all the surplus methanol. Then, put in your clean plug and give the bike a short, quick push. As soon as the bike gains sufficient momentum, drop the clutch and bump the saddle with your backside then jump off again and push as hard as you can while opening the throttle just the tiniest bit and the engine should start. Don't do what many riders do and that is push the bike and jump on and expect the momentum or a single pusher to push the machine fast enough to generate a decent spark. With the weak spark generated at low revs on some electrical systems, it pays to have the machine pushed as fast as you can. If you want to sit on the machine and be pushed, to save your energy for the race, make sure you have two husky people to give you a good fast push.

As for cleaning your plugs, it is better to avoid sandblasting them unless they are very dirty (you should not allow them to get that bad) as sandblasting will cause wear if done more than two or three times. If the plug is slightly oily a spray with ether base plug cleaner or "Quick Start" will help or any good solvent and a small brush will do a good job. The solvent can be methanol, lighter fluid, acetone, etc. I prefer a 3" piece of fine Bowden (throttle) cable frayed at one end to form a brush. With this you can get down to the bottom of the gap between the insulator and the body of the plug and also you can get behind and under the recessed electrode. There is also a patent plug cleaner filled with long needles and solvent which you just screw your plug into and shake. This does a fairly good job but can't get at that area behind the side (recessed) electrode.

All in all, if you keep your plug clean, you will ride in more races and be better able to compete than if you neglect your plug on the basis that it has never given you trouble. One rider, after pushing his machine endlessly up and down the pits, stopped from sheer exhaustion and allowed me to take a look at his plug. It looked like the sump drain plug from an old car! After cleaning, the engine started immediately, and as I said earlier, I fail to understand why a rider puts up with so much trouble when the answer, in the majority of cases, can be so simple.

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