1989 Yamaha YZ250WR – When Good CDI’s GO Bad!
It doesn’t look so great in this picture, but once the circumstances when it was taken are known, it makes more sense. Read on.
The WR was my first dirt bike. I brought it back from the dead hoping to save some money over a newer bike. I put a new top end in, replaced the pads and rotors, got new bars, re covered the seat, sandblasted the frame and had it powder coated, replaced the plastics, had the suspension rebuilt, etc, etc, and rode the heck out of it…then it blew a hole through the piston. I had downshifted to power through a turn, and the power was gone!?
Wiseco forged piston suffering from excessive detonation, excessive heat at the exhaust port, and…a hole!
I thought that the damage could have been caused by a number of factors: the top end was finished, and I hadn’t gone to the next oversize, the mixture may not have been perfect, etc, etc. I was determined to keep it going. And while I worked on the motor, why not improve it. I was going to replace the bottom end bearings, and it turned out to be cheaper to get an entirely new crank. I replaced the crank, mains, ported and polished the passages in the cylinder, had it bored out 0.75mm oversize, replaced all of the seals, and put almost a grand into it. I took great pains to rebuild it exactly right.
I rode it for about six hours, and it blew up again! It was a hard pill to swallow.
Yamaha cast piston – melted, dented, and pitted, but no hole this time!
The picture at the top of the page was taken after the second engine failure. It was the only one that I had that was of the entire bike.
At this point, I was done putting good money after bad. However, I still wanted to know why it happened. I was sure the fuel mixture was correct, the jetting was correct, there were no contaminants getting through the filter, the piston clearances were correct, I had rebuilt it correctly. I was nearly stumped.
The piston looked like a clear case of extreme detonation. I had made sure to properly replace the stator plate when I rebuilt the engine, and the static timing was at the proper setting. That left only one thing, the CDI. Everyone I talked to insisted that it was basically impossible for the CDI to cause that kind of damage. But I got to thinking about it. The CDI retards the timing, which is physically set to full advance. So what if the switching circuitry went bad, and didn’t retard it?
A function generator and ancient Tek scope that I borrowed from SMV answered the question. I put the scope on the bike and kicked it over to get the proper voltage and shape of the trigger and source coil outputs.
The clearly defined sine curve is the trigger signal, and the messy one behind it with the pulse is the ignition trigger. Note how the ignition trigger pulse is at the same point at every frequency, as is evident in the overlay.
As disappointing as this entire experience was, it wasn’t without its positive aspects. I learned a lot about motorcycle engines through the complete disassembly and rebuild. I also learned how to ride. The WR was scary fast in 5th, and I think its idiosyncrasies taught me to be a better rider. Compared to the CRF, the WR’s suspension was terrible, it was a bear to start, and the heavy flywheel made tight technical stuff interesting to say the least. From a financial viewpoint, the WR was a terrible waste of money, but from an educational viewpoint, it was extremely valuable.