Boxerworks.com

Hi Everyone,

Forum and airhead newbie here. I'm gradually going over my '95 R100R Mystic and finding a lot to keep me busy in the garage!

One of the last things on my servicing list was to find two black, sooty spark plugs on removal.

They're Bosch R6 and I could find no reference to these being used or recommended on my model.

Snowbaum does not recommend resistor type spark plugs and says that "... If you install both resistor plugs & resistor caps, your spark can become too weak to reliably fire the mixture 100% of the time, particularly in later leaner-running Airheads."

I believe resistor caps are standard fitment and I'm not sure if I have resistor spark plug caps or not. I will try to check this, but on looking for replacement plugs, quite a few options came up. NGK BP6ES seems a favourite although some recommend going cooler to a BP7ES. Then there's an Iridium option.

Which has worked well for you?

The recommended Bosch plug for all later airheads is a W7DC. This is a copper electrode non-resistor plug. The only reference I can see to a Bosch R6 plug is in an advert for Enfield Himalayas but that is a 10mm plug so it can't be the right one. No matter... Bosch have always used the same heat ratings throughout the range and the letter R in a plug designation has always meant a 'resistor' plug.

You are right, it is not a good idea to have both a resistor lead and a resistor plug. One or the other, not both. Stock BMW leads are resistor leads but if your leads are aftermarket, it would be a good idea to check whether they are resistor or not.

In essence, I think the plugs you have are one step too cool and are resistor so they probably aren't ideal for the engine. This may contribute to the sootiness... but... that being said, sooty plugs are generally a symptom of overfuelling so don't rule out some carb adjustment being required.

I would suggest that you need to replace the plugs with non resistor ones the right heat range then carry out a plug chop (In the absence of more sophisticated mixture testing equipoment) to see what is happening under load. Remember, a cold engine or one that has been idling is unlikely to give a good indication of mixture from plug condition.

Rob

Thank you Rob. I found that Himalayan link too and that was about it!

Yes, I'm resigned to investigating the fueling but thought I'd start by getting the plug type correct.

A plug chop is a great idea. I've not done it before, only aware of the basic steps so I will look into it.