One of the most common recommendations on forums is the ignition coil relay mod. Like virtually everything else on the 'net however, the recommendation is not well thought out. Putting a relay into the ignition circuit can indeed be effective. I did it for the first time maybe 40 years ago. Probably the most significant of Kettering ignition's weaknesses is that it is a voltage hog. The more you can get into it, the better it will work. This is how adjustments in dwell came about also, for example. So theoretically at least, the mod can't hurt and really only help.

That said however, there are three things to be said against it, One, doing this mod ignores the real issue, the buildup of resistance in the wiring harness, which needs to be attended to regardless. 1970s and 1980s Hondas all suffer from wiring issues that the wiring harness manufacturers themselves caused. There are in fact five areas in every vintage Honda wiring harness that need special attention. Two, it is not guaranteed the relay mod will offer any advantage over properly maintaining the wiring harness. It is typical of forums to blindly emphasize the wrong things. Instead of modification, do the needed maintenance! And, three, installing the ubiquitous Bosch style relay never seems to be done with any class. I have removed many in recent years. They sometimes present more problems than they fix, so poor is their installation.

Why can't vintage bike owners spend their energies on the things that really count? Careful engine tuning, proper clutch adjustment, avoiding Chinese replacement parts, and discovering and exercising factory assembly and adjustment ethic. So many are looking to get one up on the manufacturer and instead doing things that are worse than the manufacturer would ever have done. That's not high performance thinking. It's low performance thinking.

Back to that wiring harness. Most folks don't know there are brass crimps in the middle of them, called "buses". These electrical junctions give trouble. Even new bikes are exhibiting problems with their wiring harnesses. The condition of the harness needs to be on the troubleshoiter's checklist. Below is a picture of a voltage drop reading being made on the ignition circuit of a CB350 Four. Just 0.72 volts. This is from the battery positive terminal to the black/white ignition coil feed bullet connector. There are several connections in this current stream, including the key switch. The customer replaced his wiring harness with a new NOS part from CMSNL and his voltage drop decreased from 1.5v to half.

What this tells us is, if circuit voltage drops tempt you to install a relay, install a new wiring harness instead.

Recommended reading:
Relays