Try our new info resource - "Aladdin's Cave" (Main menu)Just added a separate link to Ash's Dropbox thread (shortcut)
We use Demi-John rubber bungs in the exhaust, drilled for a 6mm air pipe to shove in.https://www.homebrewcentre.co.uk/demijohn-solid-rubber-bung?gclid=CjwKCAjw0qLOBRBUEiwAMG5xMP5pIvm2OhcZyPo22W-0ttFRXA7E-yym9FFghhEiilg7_44bhWLPkRoCTZkQAvD_BwEHTH
I've just snapped a plug off in the head on an XS1100 Yam, head off job too so feel your pain On the upside I'll lap the valves and fit new guide seals whilst its off.
Going back to the original problem, I had a 500 that did almost the same, it was in 1972 and TBH even the Honda dealers didn't know what it was. Ran like stink when cold but as soon as it warmed up it wouldn't go above 5000 rpm, turned out to be a duff spark plug cap, fitted a new one (with those god awful metal shrouds which soon got binned) and the bike ran like a dream. The bike was less than a year old so I didn't feel like tampering with it myself as it was under warranty, still took the Honda dealers almost 2 months to figure it out.
If the problem does not move when you swap leads, you can rule out the boyer as rule, are you getting 12volts at the coils, i would be looking at valves and carburation.
Can you test the compression as it is? I know you won't get a hot/absolute figure for it, but it would makes a valid comparison from one cylinder to the others and see if you have any major discrepancies.
The Boyer ignition fires all 4 plugs at once,its called wasted spark system, the plug No2 is its pair,,can you swap leads 2 and 3 over?