Having a little misfire issue with the Valk. But only under acceleration and cruising (under load). Starts strong, idles perfectly, and you can hold the throttle open at any rev for as long as you want without a hiccup to be heard.
Anyway, decided to check out a few things over the last weekend.

Easy things first, cleaned up the spark plugs. Went for a test ride.
Little or no difference.

So, then I changed the plugs. Went for a test ride.
Initially, I thought that had cured it; but the misfire returned. Damn!

On turning around to head back, I found out that I was running out of petrol, so filled her up.
From the garage back to the house, the misfire was brutal and she had picked up some serious backfiring.

Back home again, I checked all the connections at the coils. Just pull and wiggle off to dislodge/remove any corrosion and reintall. Went for a test ride.
Aha, things had improved somewhat. Starting to run like the old girl used to, but with only a teeny tiny misfire now.

It was getting late, so for my final act of the day I thought I will extract the battery from it's bowels and give it a once over.
Imagine my surprise when I opened the battery box to find a date stamped into the top of the battery - 261004.
Then I got thinking. I bought the Valk in Jan '07, and I have NEVER replaced the battery in her; so I have no reason to doubt those numbers as an installation date.

Valk takes a YTX14-BS battery. Cheapest I could find was actually Halfords at 74. So planned to head out today to get one.
Had to go into Gorey first on an errand, and decided to go into Powers to see what they had. Shane had them alright - and he beat Halfords price - and I walked out of his shop with a new YTX14-BS battery for the very reasonable price of 65.

New battery is now charged, and I'll resume my investigations in the next couple of days.

I don't know if a deteriorating battery can cause misfiring, and my old battery had NEVER failed to start the Valk .. but at a minimum of 11 years (the time I've had her), to a possible 14 years old it was time for a battery change anyway.