Boxerworks.com

1984 R100S

I had a spark plug cap fail.

It was the original.

I replace with OEM parts.

My idle has gone up from 1000 rpm to 1500 rpm.

Your thought on why this happened, with no other changes to the bike, other than an oil change.

I had NGK resistor caps go open circuit when measured with an ohm meter. They still passed a spark but I suspected it was a little weak.

I dismantled them and found corrosion on the internal contact with the resistor which when cleaned up resolved the problem.

To get them apart grab the bit that screws into the ignition lead and pull. It's only held in by a dished serrated washer. be careful not to lose the tiny spring and resistor when it pops open.

Other reasons why idle rpm might rise. I've seen it when adjusting valve clearances if they had closed up. No surprise in that effect at all as opening up the clearances reduces valve overlap and improves the idle efficiency by reducing the amount of incoming mixture that goes straight out of the exhaust. I also noticed an increased idle speed when changing oil from 20W50 to 15W40.

I would suggest that the old cap has been becoming 'weak' over a long period and you have been adjusting the idle to take this into account. The idle speed is, after all, a delicate balance involving a lot of factors and even a small variation in any of those factors will lead to a change in idle speed. On the road, you would simply open the throttle a bit wider to compensate without noticing but the idle is 'fixed' at the throttle stop. Unless there is any other symptom, I would suggest you can simply sdjust the idele back to spec.

Rob