R100S occasional sputter and idle issues. Coil?

[jjwithers]

77 R100S
Symptoms...

Bike runs great, but on occasion the idle looses its rhythm.
AND
the occasional sputter or hesitation upon giving it throttle.

It happened on the way home from work today, but not on the way there.
Then everything will be fine, and on an occasional trip, it will happen again.

This is all new (past month or two) and the infrequency has me scratching my head.
I cleaned the carbs about 6 months ago and the bike ran fine.
Fuel delivery and filters are OK.
New plugs gapped correctly...

I'm starting to wonder if the coils are going bad? How would I know if the coils were dead/dying?

It has a dyna ignition (with the booster box). Could the booster be the issue?

[SteveD]

Water in bowls or bad fuel from one particular fill up? Mobile crud in an orifice somewhere?

Cheap and easy to start at
1. the vacuum ports haven't lost their cover screws? 10 second look.
2. plug caps, HT leads. How old are they? Also check for good connection at the coil.
3. the mixture and pilot jets and their orifices, ensuring no crud. Idle and just off idle performance suggests this is a maybe.

then
4. maybe substitute the coils. Easy if you have some.

I bought a bike for a huge discount years back because the guy couldn't fix the idle and off idle stutter. He'd tried all sorts of things and just wanted it gone. I noticed the plug caps weren't right...took a punt. New ngk's the minute he left and puurfect! You might get lucky.

[George Ryals]

Coils and plug caps usually show themselves under load not so much at idle. I vote for idle mixture being on the edge of being too lean and perhaps not quite the right balance between mixture and throttle plate opening at closed throttle.

[SteveD]

Coils and plug caps usually show themselves under load not so much at idle. I vote for idle mixture being on the edge of being too lean and perhaps not quite the right balance between mixture and throttle plate opening at closed throttle.

You're right but in my example the bike wouldn't idle and was rough until underway. It ran nicely at speed. When it needed to idle, it wouldn't and needed the throttle kept open a tad more. The correct ngk plug caps fixed it. The seller had tried all sorts of carb things. The caps were nice and shiney and looked new, but I suspected they were from a /5 or /6. As I said, I got lucky, so I wouldn't discount it altogether

[jimmyg]

I'll vote for a bad or leaking condenser, assuming you have the stock points and condenser ignition?

Do you know how to check a condenser with a DVM?

jimmyg

[jjwithers]

I'll vote for a bad or leaking condenser, assuming you have the stock points and condenser ignition?

Do you know how to check a condenser with a DVM?

jimmyg

First post states i have the Dyna... so no condenser. I am thinking it is something electrical. it is too intermittent and occasional to be carbs. The problem is that I can't try one thing and test immediately. It might take 50 miles or so for the problem to occur again.
I have coils I can swap. Until then, I'm still scratching my head.

[barryh]

First post states i have the Dyna... so no condenser.

No external condenser perhaps but there might be one in the booster.

I have a points ignition booster, not a Dyna but how different can they be in terms of circuitry. The installation instructions explicitly stated to remove the condenser. The reason they will have said that is because there is a capacitor of the same value already built into the booster circuit across the points connection. That effectively substitutes for the condenser and presumably could also fail ?

You might consider trying reverting back to points ignition to test out if the booster has anything to do with the problem. It's usually just swapping a couple of wires and refitting the condenser.

[Ken in Oklahoma]

I have had coil issues on at least three occasions causing ignition problems. One is reluctant to go to new coils as the first suspect because they are so expensive. Finally, after at least two occasions when the coils were indeed the culprits, I resigned myself to the realization that if I run airheads I'm going to have coil problems--sooner or later. I bit the bullet and bought two spare coils. It's somewhat expensive, but the frustration of trying everything else but the coils has led me to stock the spares.

Oddly, with the spares in my stock of spare parts, my coil problems have gone way down.

By the way, bench testing the suspect coils, in my opinion, is a fruitless exercise. Coils that generate sparks on the bench at ambient air pressure will spark when coils trying to fire a plug at high air pressures likely won't.

With that in mind I have, in the past, narrowed the gap of the plug(s) to a few thousandths and tried them in the engine. (A smaller gap takes less voltage to jump the gap at low pressures.) The results of narrowing the gap has not proven to be a reliable diagnostic tool for me.

Ken

[jjwithers]

I'd make a terrible scientist because I had 3 potential issues that I corrected and they seemed to have solved the problem.
One was taking the carb tops off and making sure the diaphrams were seated properly. I re-seated them, reassembled carbs.
I checked the valves and re-torqued the heads. If memory serves, the exhaust valves needed adjusting. They were adjusted not that long ago but I figured I should check them anyway.
Lastly, the bottom screw came out of the front engine/timing cover and created a frightening sound when revving the engine. I thought there was an issue with the left piston/head until I realized the sound was the timing cover clacking. I popped it off and it was filthy in there, perhaps leading to a bad signal from my Dyna to the rest of the ignition system. I cleaned it up and re-assembled.
So to sum up, it was either the diaphragms, valves, or dirty electronic ignition.

[Seth]

I appreciate that you've posted the resolution to your problem. I think it helps to know what worked in addition to possible suggestions when you try to diagnose similar problems. Best of luck and thanks again.
Seth