ignition coils.

[wheelsrus]

i am working on a 75 B, i would like to know how to check the coils for proper resistance. the engine is running ok left plug seems to carbon up quicker than right i assume beacuse this is the side with the choke. Or should i go with new coils because (a) 50 years old (b) weak even when new? everything is stock carbs,points ect. I plan keeping it pretty close to stock just for ease of working on it. running B8ES plugs switch to 7's ? are any of the vendors replacement coils hotter thank stock without going to ones for electronic ignition? thanks in advance

 

[Jim]

That's a lot of questions...
Testing the coil: To test the primary, put the meter on the two wires.... meter on the lowest RESISTANCE scale. Pri resistance should be about 4Ω.
Secondary resistance is about 8KΩ. (that's eight thousand). Use a scale large enough to suit. Touch the meter leads to the HT lead and the other to either the ground wire or the coil frame. I think your plug cap is supposed to be a resistor type? If it is, add 5KΩ to the total secondary resistance.

 

[Its been a long time]

The thread touches on a subject I've been thinking about. Back in the 1970's these bikes didn't have outstanding points ignition systems. Fifty years later and they haven't got any better.

I have wondered if there's anything to be gained by breaking into the switched live to the ignition and using it to switch a relay. In turn the relay would connect (via an in line fuse) the battery positive terminal to the ignition system. My thinking being that by using the switched live to switch a relay it'll ensure as much as of the battery voltage gets to the ignition system as possible. Thus bypassing all the potential voltage drops in the ignition system from 50 year old switches, crimp bullet terminals etc... That can only be a good thing. And standard operation of the bike is unaffected.

Has anyone done such a modification?

 

[Jim]

The thread touches on a subject I've been thinking about. Back in the 1970's these bikes didn't have outstanding points ignition systems. Fifty years later and they haven't got any better.

I have wondered if there's anything to be gained by breaking into the switched live to the ignition and using it to switch a relay. In turn the relay would connect (via an in line fuse) the battery positive terminal to the ignition system. My thinking being that by using the switched live to switch a relay it'll ensure as much as of the battery voltage gets to the ignition system as possible. Thus bypassing all the potential voltage drops in the ignition system from 50 year old switches, crimp bullet terminals etc... That can only be a good thing. And standard operation of the bike is unaffected.

Has anyone done such a modification?

Some folk have done that... for the reason you stated.

 

[Its been a long time]

Some folk have done that... for the reason you stated.

Thanks, I guessed so. Makes a lot of sense in order to maximise the spark from a set of not especially wonderful ignition coils.

 

[ckahleer]

Adding a relay will get more power to the coils, but will also add a componentthat could fail. Do it in a way that you can bypass the relay on the side of the road.

 

[5twins]

Yes, low power to the coils, resulting in a weak spark, is apparently a common problem on old bikes. Although I've never done it, here's a write-up on adding that relay .....

https://wgcarbs.com/index.php/using...content-component/article-categories/89-coils

I have considered it, just haven't done it yet. I may try it on my '83 though. It doesn't cold start as easily or quickly as my '78. I thought it might be the different carbs (BS34s vs. BS38s) but now I have swapped on BS38s (exact same set as my '78) and the issue still persists.

 

[40north]

Ah recollect that as a stocker ma 82 specuial hated rain... Poor igniter location ah tink... Resistence input DC happens, so do HV arcs.

 

[jetmechmarty]

Yes, low power to the coils, resulting in a weak spark, is apparently a common problem on old bikes. Although I've never done it, here's a write-up on adding that relay .....

https://wgcarbs.com/index.php/using...content-component/article-categories/89-coils

I have considered it, just haven't done it yet. I may try it on my '83 though. It doesn't cold start as easily or quickly as my '78. I thought it might be the different carbs (BS34s vs. BS38s) but now I have swapped on BS38s (exact same set as my '78) and the issue still persists.

Same situation with my ‘83 v. ‘81. They’re identical bikes. One has miles. The other doesn’t.

 

[5twins]

Which one cold starts easier?

 

[mrtwowheel]

I did this mod on a '81 CB750C. I would say it is more consistent starting.