one luke warm cylinder

1973 850, Boyer MK III analog ignition, was running great on a highway trip, then all of a sudden it starts to run like crap. I was thinking fuel starvation on one side, had to keep feathering the throttle to keep it going while I limped home. LH cylinder was farting and coughing and backfiring, like a misfire. I let the bike cool, checked fuel flow, it is good to both carbs. Changed the fuel in tank for fresh premium gas. Checked for air leaks, nothing apparent, but changed float bowl gaskets anyway. Still runs bad, and RH header is HOT, with LH header just warm after running for a minute or two. Checked valve clearances, and they are all good. Reversed HT sparkplug wires LH to RH, and RH to LH, same symptoms, and still isolated to LH cylinder. Changed sparkplugs ( NGK BP7ES) with new. Next did a compression test, with engine cold both cylinders were at 90 PSI exactly, not the greatest but both cylinders equal, and pressure held. Charged up the 1 year old AGM battery, still no improvement. I have done a thorough going over the ignition wiring, all seems good with no loose connections. I have removed one sparkplug, grounded it, run the engine on the opposite cylinder (did this for both sides), and I am getting equal spark on both, or so it appears. Engine is definately running crappy on that LH cylinder. I have previously replaced the wires on the Boyer stator plate, and bike has run smooth since then, two years ago. I will check the magnet coils on that plate for continuity, see if there is a fault there, but failing that, I have run out of ideas.
Would the Zenier diode be a possiblity, and if so how would I check that?
I appreciate any suggestions...thanks

I had this same exact thing happen to my Moto Guzzi V-50 Monza. Even mechanics tried & failed to locate the problem. I eventually put a new set of Dyna coil coils on it & all was well, actually better then well the Dyna coils kicked ass over the stock ones. I don't know how to test coils other then switching them nut my $'s on a bad coil. Good luck & let us know what happens. Glenn.

Single or duel carbs.
If single I'd not think carb problem but if duel carbs I'd do a clean out of LH one.
Even though you see a spark on both sides I'd swap coils to see if problem moves to RH side.
Zener won't be problem since it will affect whole system not just one side.

Bob

The reason I was surprized when it turned out to be a coil was that everytime I pulled the plug & tested it I thought I had good spark too.

gtsun said:

The reason I was surprized when it turned out to be a coil was that everytime I pulled the plug & tested it I thought I had good spark too.

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The pressure within the chamber can create more resistance to the spark making its jump so what you see outsde can be different inside.

Very common with carbs out of sync.

rx7171 said:

If single I'd not think carb problem but if duel carbs I'd do a clean out of LH one.
Even though you see a spark on both sides I'd swap coils to see if problem moves to RH side.

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+1 on that, for sure. Easy to swap the coils but I'd also want to see if richening the LS carb made any difference, just for diagnosis. Could use propane or just choke it a bit with your hand. Might tell you a lot. A cool pipe/miss can certainly be ignition but if it is a lean miss, you'll get the same symptom - no fire, no heat.

Been away from the site for a day, so I will reply now: engine is twin carbed, I have removed both carbs, gave them a thorough cleaning, all new internals were installed last summer, but I gave them a good cleaning anyway. Carbs are perfectly synched, and throttle movement is smooth. Carb needle clip has not slipped out of place, I have both needles clipped on the middle notch. Primary idle circuits are clean, and there is good flow. I can pull a plug and the opposite cylinder will run the engine (as long as I work the throttle) however that LH cylinder does not fire consistently like the RH cylinder. I will try and switch the coils again and see if that symptom moves to the RH cylinder, but I tried that last week, and the misfiring remainded on the LH side. This was a very sudden misfire,
all was very well and smooth, then instant loss of power & misfire on the LH side. I am suspecting an electrical issue, so I will go over the ignition wiring.

I wouldn't bother going all over the ignition system if you have one side running ok as the Boyer uses a wasted spark system, both sides fire at the same time. You only have power in power out. With the symptoms you have stated and the work you have so far done on the carbs I would try the coils next. Good luck.