New Coils Going Bad

1976 KZ750B,

Coils keep going bad, and yes I bought them new from emgo, and z1 enterprises. They work for a while, then loss strength (spark becomes very weak, then when I put a another new coil on, spark is strong again) to where the bike wont run.
Im sure there is a simple explanation but I do not understand it.
(I dont use the starter, only kickstart.)

Thanks for your help!

What is the voltage to the battery while running?
Steve

At an idle of 1200rpms, it was putting out ~13-13.5 volts. Basically I know it wasnt high as 14 and not low as 12.5. And I was using a multi meter, I just cant remember exact decimal value. I replaced regulator and rectifier with nos ones if that helps explain any more.

There are a few things that come to mind.

Cheapie coils, coils under $50/£50 don't tend to live long. Secondhand coils are also fairly risky. Emgo/Z1 coils should be fairly reputable though.

If the coils are not rated for the ignition system, their lifespan is short. For a Kettering system (points), a coil primary resistance of 4 to 5 ohms is standard.
For a transistorised system, like kz750, Spectre, Dyna-S, usually 3 to 5 ohm is about right.
For capacitive discharge systems, the coils are normally under an ohm.

Next possibility is HT resistance. Kettering systems are designed to run with a few hundred ohms resistance in the circuit. Either plug cap resistors, spark plugs with resistors or graphite leads with resistance in the conductor. Run copper to the plugs without resistance will result in short duration powerful sparks and stressed coils.

Another possibility is shorting/dying ignition transistors. Run current continuously through a coil and it will overheat and die. Failing Dyna-S modules are usually the culprit.

A further potential issue is a poorly grounded engine block (pretty rare). Test the resistance between the cylinder head and the battery negative, it should only be a few ohms. If you never use an electric starter, you might not notice a bad grounding.

My KZ750 has points and a condenser. When you speak of Kettering, Transistor and Capacitor types, which type is mine? This is all new information to me, which is making me feel better about chances of success with the bike, so thank you for replying!

Are you getting a lot of blue sparking at the points?
Steve

Yes, more like white spark. I got new condenser (which got bike running again) and points (New as of weeks ago) and cleaned up the points plate grounding surfaces. Yet sparks still occur. Bike would run well, then sparks through the points would make it stumble.

Thanks again for help!

A lot of sparking at the points is a combination of gap and dwell. Ensure the dwell time is correct and the gap will take care of itself.
If the dwell is too long you are trying to overcharge the coils and the excess feeds back through the points causing the sparks. In simple terms excessive dwell will burn out the condenser, points and coils..

Charles 750B wrote: My KZ750 has points and a condenser. When you speak of Kettering, Transistor and Capacitor types, which type is mine? This is all new information to me, which is making me feel better about chances of success with the bike, so thank you for replying!

The 76 750B (points and condenser) is a Kettering ignition. That came with a coil that measures around 4 ohms on the primary windings (small wires). If you go a lot less than 3 ohms, you might shorten the life span of the coils, but maybe something else is happening.

Do you try cleaning and adjusting the points before replacing a coil? Points normally wear out much faster than coils and have to be adjusted periodically.
Do you use new plugs and wires when you get a new coil?

Yes so when the bike ran, I moved the points to where no spark was jumping across which helped tremendously in smooth idling. I use new plugs and wires. I also have the standard ngk plug caps. Its a very strange bike (harder to restore than my datsun 280z....). If coil is going bad, will it sometimes seem like a carb jetting issue ie, bad running at wot? Is there a list of things to check for coil operation without the bike running?

If this means anything, having headlight on, while using my turn signal would cause the bike to try and stall, so at traffic lights I would gently rev the bike before I made a turn. Can lots of resistance in the electrics screw with a good coil?

Sounds like a marginal battery.
Steve

Ive used two new batteries and it still did it just after I put a new one in.