Coil Resistance

[Manfred]

Thanks, Duane. I've noticed that pre-1981 airheads use a 3 ohm coil; perhaps this accounts for the satisfied users of the HD coil. The only 1.5 ohm coil I've found is the $95 Dyna coil from Rick Jones. I am still diagnosing my starting situation. Fired right up this morning with full choke.

[mattcfish]

I've read Duane's advice about replacing the ignition coil in an old as a matter of preventive care. I've read Rick's notes here: http://www.motoelekt.com/ignition.htm and understand some of the problem with a coil with lower resistance than spec calls for. What is the risk with a coil that has higher resistance than called for? I have a coil rated at 3 ohms, my bike (1987 R80RT) calls for one at 1.5.

What type of ignition are you running? If it's a single coil with dual outlets, 3 ohms is pretty standard with a dyna III aftermarket ignition, two 1.5 ohm coils with a dual spark system. I'm running a single 3 ohm Dyna (green) coil on my R90. No issues with the Omega or Dyna III ignitions. From what I've read, more ohms (within reason) is not a problem. You risk damaging the ignition if you run too little resistance.

[Manfred]

What type of ignition are you running? If it's a single coil with dual outlets, 3 ohms is pretty standard with a dyna III aftermarket ignition, two 1.5 ohm coils with a dual spark system. I'm running a single 3 ohm Dyna (green) coil on my R90. No issues with the Omega or Dyna III ignitions. From what I've read, more ohms (within reason) is not a problem. You risk damaging the ignition if you run too little resistance.

My bike is an '87 R80RT with stock ignition, except new spark plug wires. From what I've gathered, '81 and later airheads with dual pole single coils (such as mine) use 1.5 ohm coils; older airheads use 3 ohm coils.

This is what Rick Jones says about the green Dyna coil: "Use ONE coil for ALL MODELS made from 1970-1980 that came from the factory with POINTS ignition." So it looks like you're doing right on your '75 R90. Accordingly, Jones recommends "BROWN 1.5 - Use ONE coil for ALL MODELS made from 1981-1995 that came from the factory with ELECTRONIC ignition(Hall effect system"bean can")." for my bike.