This is why I enjoy this forum so much. A great write up on the topic and excellent advice based on solid experience.

MarkT, I'll follow your advice and order the cap-off plug and carry a new spare. And, like you say, the compression chamber volume will be reverted back to stock spec. Very likely to help performance and at the least won't degrade it. I did wonder if that second plug might disrupt intake charge flow but never even considered the volume issue.

Lesson learned - don't believe everything you read unless it's from a trusted source. And maybe even someone here besides me will learn something new and benefit from this information. Cheers and happy Labor Day to all.

Jumping on this thread quite a bit later than the original post.  I just picked up an AT1-C though and am looking for plugs.  I know B8ES is the correct plug, however I've come to find out that NGK no longer produces this plug and they are impossible to find.  I took a peak at the plug currently in the bike (from PO) and it is a BR8ES.  The bike appears to run fine with it, but I know that the BR8ES plugs aren't supposed to be used since the AT1 cap already has a resistor in it.

Can anyone help me figure out what to do?  What plugs I should seek out and if it's okay to run the engine with the BR8ES in the meantime?

TIA!

Just what i do--I tend to leave well alone if its happy as it is. I'm guessing yours is electric start so 12 volt electrics. Won't be a problem with a resistor plug. If it's magneto then i think i'm running a resistor type in my AT1 & no problem. Someone will tell me but is a resistor to stop electrical interferance on tv's around the village? as when we bucket races --"100cc roadracing--at the local airforce base we had to run resistor everything, caps & maybe plugs too i think. Had to each do a run past the tower before practice to check for interferance or go away & fix it.

I purchased a bunch of NGK B8ES before the stores ran out. Recently' I believe that I sent five or six of those plugs to New York to a customer that was looking for them specifically.

I think they're worth the purchase price plus actual shipping?

a

I bought a bunch of those coveted plugs in Honda packaging

I stocked up on plugs too...  but if I hadn't I'd probably just swap to a non-resistor plug cap and run the BR plugs.  Caps just thread on the end of the wire.

www.mikesxs.net/yamaha-xs650-ngk-plug-ca...fh-non-resistor.html