Boxerworks.com

R90S.
The bike stranded me again. Thought I had things corrected, but guess not. While on the road, it stumbled once, then continued. A couple minutes later, it stumbled again. Five minutes later it quit. Fuel is ok. I pull the plugs. Weak spark in both cylinders. If you continue cranking, the spark goes to nothing. The points are new. I'm thinking either the condenser or one of the coils is shorting. The coils (oem) are in series, right? So 1 bad coil could impact both cylinders? I have a new pair of Bosch blue coils at home I was thinking about trying.
Your thoughts?
thx.

I would never wait to replace coils only after they quit. It just isn't worth it. After about 20-25 years, just replace them, they have done the job.

You don't need to crank it with the battery, just open the points by hand and it should spark.

Replace the coils.

Coils age and get weaker, so if you already have new ones, you should put them in regardless. If they cure the problem, which is entirely possible, all to the better.



Edit: As you can see, Duane and I were thinking the same thing at the same time.


Okay, I'm pretty creeped-out now....

Seth wrote:So 1 bad coil could impact both cylinders?

Yes, the coils are in series with the jumper between the two coils. The voltage path is through the coils to ground at the points. If any part of that path is broken, no spark.

Kurt in S.A.

Kurt in S.A. wrote:Yes, the coils are in series with the jumper between the two coils. The voltage path is through the coils to ground at the points. If any part of that path is broken, no spark.

I agree with Kurt, but I would add a note that he is speaking to the primary windings of the coils. The coil secondaries can break down without affecting the other cylinder. That is why we often see advice to swap plug leads side to side and see if the problem moves.

My advice is the same as Duane and the Major. Buy a new set of coils. It's time. On two occasions I have wasted much effort and incurred a bad disposition chasing my tail on coil problems.

One very typical problem with coils is that they will work OK when cold, but problems appear when they warm up. This is because the coil secondaries are made of very fine wire, and that wire "squirms" around inside the coil body with heating and cooling cycles. Among other things this can make or break the electrical connection of a broken winding.


Ken

I'm still running original coils on my R75/5.

However, I've had this problem. The connection from the points to the condenser was slightly loose. Clean and tighten fixed it.

Let us know what you find out.

Kurt in S.A. wrote:

Seth wrote:So 1 bad coil could impact both cylinders?

Yes, the coils are in series with the jumper between the two coils. The voltage path is through the coils to ground at the points. If any part of that path is broken, no spark.

Kurt in S.A.

Some might find this misleading. The whole purpose of the points is to break the path to the primary side of the coils. When the points open up, that is when the spark happens.

I would put it this way, if anything except the points break the path, then the system won't function. There are other things that will cause the system to not work, such as any short, as in a shorted condenser. There are a variety of ways for the short to occur. When the system gets shorted put, then when the points open, the short isn't removed and there will be no spark.

This system is super simple and I never understood why people must make it seem complicated.

The old wives tale of always replace the points and condenser as a set is just marketing.

I have an article on the care of the points system, but haven't looked at it in years. I hope to covers everything.

The R90S is now running well again with the replacement of the condenser.
I took off the tank and removed the plugs. Cranked it over and there was decent spark.
Checked all the connections on the coils and checked voltages at each connection and then at the condenser.
Used my multimeter to check amperage draw at the wire into the condenser, figuring if the coils had a problem with the primary winding, the heat from the draw might make it apparent. Everything looked normal. Reinstalled the plugs and tank - started the bike. Ran well, adjusted the timing. Reinstalled the front cover. Tried to start again - no start. Pulled the plugs - a weak spark. Swapped the condenser, borrowing one from my 77RS. The bike started right up and ran great. Took it out for a 40 minute ride with no stumbles or problems.

Thanks for the support.

Great. Glad you had success.

BUT, if those coils are original, you should still put those new ones in. You'll get a lot hotter spark than 40 year-old coils can possibly provide.