Service
SERVICE
Car coils, again
I owned a 1973 RD350 back in 1976. A few different people told me then about installing automotive-type ignition coils on the RD. Supposedly, these coils would produce a hotter spark and make the plugs last longer.
Just recently I purchased a 1974 RD350 for racing use, and my interest in car coils has been renewed. I’ve found there are basically two types of 12-volt coils: 1) coils with an internal resistor; and 2) coils using an external resistor. After doing more research and talking to people and mechanics at bike shops and even people who had installed car coils on their own RD’s, I can-not find anyone who can tell me for sure which coils to use. Can you? And what are the advantages of the different types?
Gary Stumm
Bradenton, Florida
Either type of coil will work; to understand why, you must understand why the resistor is used in the first place. When starting a car, particularly in cold weather, a starter motor imposes such a large load on the battery that line voltage can drop substantially. That drop includes the voltage that powers the coil, producing a weak spark just when a strong one is most demanded—during a cold start.
A simple cure for this (used on some cars) is a coil designed to work best at this low voltage. Then, to prevent the coil from overheating during normal running when the starter motor isn 7 in
operation, two different circuits are used to supply the coil: one that draws from the decreased line voltage that is present only when the starter motor is running; and a second that draws from the full line voltage, but that uses a resistor to drop the voltage to a level the coil can live with during normal operation.
Because a motorcycle's voltage remains fairly constant, you must use a coil designed to operate with an external resistor with such a resistor. If the coil is designed to work directi y from normal line current, because its internal resistance has the proper value, no external resistor is required.
In either case, almost any car coil will provide a far better spark than the coils used on pre-CDI RDs.
Time, speed and distance
Will you please explain how to read the graphs on time and speed and time and distance used in your tests. There are 5280 feet in a mile, a quarter mile is 1320 feet, and the graph shows it as being about 2200 feet. Or is it two separate graphs? If so, it should be done differently.
I was looking through your April, 1982, issue at the Seca 650 test—I still take CYCLE WORLD and still can’t read the graphs.
Ike Woolf Natchez, Mississippi
Yes, two different quantities are plotted on the graph f ound on our
specifications page. The solid curve is time versus speed, and mph intervals are shown on the left-hand axis. The dashed curve is time versus distance, and intervals of 250 feet are shown on the right. Either of these curves is meaningless if you try to use it with the wrong units; notice, too, the quarter-mile point is shown twice: at the intersection of the quarter-mile time and the terminal mph on the speed curve, and at the intersection of that same time with 1320Jeet on the distance curve.
As for doing it differently, now that you understand the graph, do you still want us to change it?
Slow leak
I am the owner of a 75cc trailbike that has a rear knobby tire gradually losing air. I have not found any leaks in the inner tube, but after about two days after pumping the tire up to 20 psi, the pressure drops down into the 5to 10-psi range. What do you think the problem is and what can I do to get rid of it. Bogdan Oproescu Vancouver, B.C., Canada
A leak as gradual as you describe can be ver y difficult to trace. You might try replacing the valve core of the tube, and if that doesn 7 cure the seepage, the next step is to replace the tube. As an alternative, you might try putting a sealing liquid such as AC P's Balance Plus in the tube; we've had good results with this product for sealing leaks in the tires of offroad bikes.
More gas, please
I would like to buy a 1 986 Honda Shadow 700, but the small gas tank stops me. Can the tank off the 1 100 Shadow be fitted to the 700 easily? How about Canadian tanks; are they larger?
A. Carte
Milpitas, California
No, the 1100 tank is not an easy retro-fit. and, as far as we know, there are no other Shadow models (intended for other markets) that use a larger tank. If any readers have enlarged the fuel capacity of their Shadow (or that of an y other motorcycle), we would appreciate hearing of their experiences, and will run their suggestions in a later Service Column. 0
