Honda-SOHC

Hi Kev,

Welcome aboard.
Finish the bike! With the help from the experts here will enjoy the build and the ride!

If there's response by swapping the leads over (within one  oil pack) then analysis of why that could happen may give answer.

There's specific performance in the condenser relating to this (if the original system is in place) with it's ability to fire the mixture.

The opposite ends of the coil have slightly different spark characteristics,  with also the condition in cylinder from the live one to the wasted spark one maybe causing a bias to disadvantage the live cylinder on compression and give a shift in performance to that cylinder.

Swapping the leads checks for that, also the insulation of the HT too.

Interesting thread. This fouling at idle - is this when the revs drop after running steadily for 20 seconds and can be cleared by revving the throttle to restablish the steady idle?  Mine does have that tendency on the denso plugs but also one cylinder which is cooler than the others so I’m guessing that’s the one that fouls.

I have found on many different bikes cars  lawnmowers-that iridium plugs give better starting, smoother idle  and avoid fouling on two strokes.  It seems there isn’t one available for the 550 - why not ?

Think I'd initially swap the plug lead with it's paired one to see if the cool running transfers with that lead. If it stays where it was, then looking more at that cylinder condition, compression analysis to count that out, idle jet system to ascertain if there's any impairment.

Matt Hull the editor of Classic Bike Guide has spent the last year or two restoring a RS100. Every month there's an update and a how to type feature. He got it back on the road but after a few hundred miles he had the death knock and has had to strip it down again. Problem turned out to be a failed main/big end bearing, something he didn't attend to in the first rebuild as he was told these never give problems!
He's made some home made tools along the way such as puller's etc.
I know it's not the R80 but probably transferable information between the two. Well worth a read & subscription 👍.

I have a thought, the joints and the  splines on the shaft drive,

I know a friend that had a K brick had issues

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