Why is it running rich? or, all good things come to an end

My relationship with Honda motorcycles began way back in 1963. As a 15 year old part time apprentice mechanic I learned a couple of absolute truths. Number one of those was that "it is never an ignition coil component failure" and number 2 was "leave the carburetor jets Honda installed alone". These "truths" were proven over and over again, especially when I saw many English bike owners switching to Honda ignition coils. Yeah, they were bulletproof, but maybe not for ever.

Several years ago, at the request of my nephew, I picked up a couple of derelict old bikes from the local salvage yard for a grand total of $50. The plan was to put together a good trail bike from a complete but with frozen engine CL175 and rusted out CB200 with what appeared to be a usable engine. This worked out fine and my nephew got what he wanted. A couple of year later my wife wanted to learn to ride so I reacquired the bike from my nephew. In the course of making it street legal, I ended up buying 3 more derelicts for $85. One of these was a low mileage CB200 that had been relegated to the back of the owner's garage after a minor but painful front end collision. This was the bike I chose to fix up for her. She learned to ride, bought a new bike, and the CB200 became my "old reliable".

A couple of years passed then, shortly after I had installed a new transistorized ignition for test, "old reliable" started missing and running rough. When I got home I pulled the plugs and they were black, fouled by too rich a mixture. I went through a complete tune up process and removed the prototype TI system, but nothing improved. I cleaned and rebuilt the carbs but again nothing changed. I checked the compression and swapped out those "bullet proof" ignition coils but still the problem persisted. Out of frustration I gave up and parked the bike in back of my shed.

About a year later I decided to get rid of some my smaller bike parts by putting together a complete street legal bike out of the original CL175 (with a CB200 engine) and selling it. When I started it up it had exactly the same problem as the CB200 yet none of the CB200 parts were on this bike. I went through a similar process with this bike as I had done with the CB200 without success. I was baffled. A friend suggested the coil but I had already tried a couple of spare coils without success. Out of desperation, I tested the resistance of the coils and it was within specifications. I then set up a bench test by hooking up the coil to 12v and simulated the points by making and breaking contact to ground while observing the spark on a new plug. The spark was a thin yellow thread, not good. I tried the spare coil and it was the same. I tried a third spare coil and it was also the same. I then hooked up a coil from a CB360 and got a nice thick blue white spark. Mmmh, I thought, something is not right here.

So, I wired up the CB360 coils in parallel to run them on the CL175. I know this will decrease the coil resistance and therefore increase the current through the points but it is for short test and I'm clutching at straws here. Bingo! The bike runs better than it ever has before, lots of pep, and the plugs look much better. I ordered new after market replacements from two different sources and both bikes ran better than ever. I reinstalled the TI system and was pleasantly surprised at the improvement I felt.

What have I learned from this? The most important thing is that the saying "All good things must come to an end" is true. Those coils that were great 35 years ago still work but no longer provide an adequate strength spark. And, it doesn't appear to be just one coil but all three of the coils I have of that model. I have also recently read about a similar problem involving several used CB160 coils. The bottom line - If your 70's vintage 175/200 is running rich and the carbs are good, replace the coil with a new aftermarket one.