Absolutely mystifying failure, 850 Commando Mk III

Lannis · 2026-06-29T18:50:49-0600

All -
Something weird is going on with my MkIII.
I was 60 miles into a ride, summer day, cruising at 55, when suddenly the right side drops out, just pops and bangs occasionally, the left side running normally. Trailered it home.
Bike has original Amals and Zeners, with a Pazon ignition. First guess was that it had dropped a needle; pulled the carb top and found the needle in its proper place. Then thought it was a loose wire on a coil, but everything was tight.
So I started methodically with Compression, Ignition, Fuel.
All valves opening and closing normally with proper clearance. Compression check (throttle wide open) pumps to 175 psi on each side.
Swapped plug wires side to side, no change. Swapped coils side to side, no change. Coils are wired in series, so both spark on every stroke. New plugs, no change. Lay plugs on head; strong spark, both sides. Reinsert plugs, put a "spark detector" in line in each high voltage lead, both are flashing in unison indicating spark under compression.
Swapped carburetors; no change, the carb formerly on the dead side runs the left side strongly, the carb formerly on the left side will not run the right cylinder.
What am I missing? It quit like I'd thrown a switch or dropped a needle. Right side just pops and bangs as the left side pulls the engine around.
Help me, I think I'm falling ....

Lannis

Torontonian · 2026-06-29T20:04:07-0600

Swapped carbs and the siuation reversed ?
Perhaps the ill carb has a blocked fuel filter ?
Perhaps the float punctured with age ?

L.A.B. · 2026-06-29T20:57:47-0600

Lannis said:
Swapped carburetors;

How did you manage that as the ticklers would clash as the unused tickler bosses and carb flanges on Commandos are shaved so the carbs will sit side by side surely you would end up with an air leak at the flanges.

Adam Lynam · 2026-06-30T00:22:11-0600

L.A.B. said:
How did you manage that as the ticklers would clash as the unused tickler bosses and carb flanges on Commandos are shaved so the carbs will sit side by side surely you would end up with an air leak at the flanges.

I swapped mine over as a process of elimination for a blocked pilot jet. It worked for testing purposes. There was just enough movement through the flange bolt holes to orientate the carburettors to sit flush.

hybridracer · 2026-06-30T01:31:19-0600

Unless something is blocking the air filter outlet to the rh carb, my money is on a Pazon ignition unit failure, your tests haven’t covered the possibility that it is only firing every two revolutions instead of every revolution.

kommando · 2026-06-30T02:14:22-0600

When I had the same the cure was a dedicated earth wire from head back to battery. This was on a MK2a, a Mk3 with a starter should already have a good path or the starter would not work. Took two weeks of changing every component several times to then notice that if I left the working sides HT lead off the non working side would finally fire. That got me thinking that the earth path was compromised and would only support one coil's current back to earth and I added the dedicated earth wire and got 2 sparks back.

dobba99 · 2026-06-30T04:05:34-0600

Had similar when called to look at a commando that displayed similar symptoms. The owner had sleeved the coils with a thin rubber sleeve so the earth path was compromised.

concours · 2026-06-30T04:22:43-0600

Lannis said:
All -
Something weird is going on with my MkIII.
I was 60 miles into a ride, summer day, cruising at 55, when suddenly the right side drops out, just pops and bangs occasionally, the left side running normally. Trailered it home.
Bike has original Amals and Zeners, with a Pazon ignition. First guess was that it had dropped a needle; pulled the carb top and found the needle in its proper place. Then thought it was a loose wire on a coil, but everything was tight.
So I started methodically with Compression, Ignition, Fuel.
All valves opening and closing normally with proper clearance. Compression check (throttle wide open) pumps to 175 psi on each side.
Swapped plug wires side to side, no change. Swapped coils side to side, no change. Coils are wired in series, so both spark on every stroke. New plugs, no change. Lay plugs on head; strong spark, both sides. Reinsert plugs, put a "spark detector" in line in each high voltage lead, both are flashing in unison indicating spark under compression.
Swapped carburetors; no change, the carb formerly on the dead side runs the left side strongly, the carb formerly on the left side will not run the right cylinder.
What am I missing? It quit like I'd thrown a switch or dropped a needle. Right side just pops and bangs as the left side pulls the engine around.
Help me, I think I'm falling ....

Lannis

Intermittent broken wire into the timing chest.

If you don't want to wait for a new wire test, use any two conductor, salvaged lamp cord etc.

JimNH · 2026-06-30T04:33:58-0600

Wasted spark system skipping every other spark?

Steve W. · 2026-06-30T04:35:43-0600

I had an issue on my Pazon equipped mkiii. One of the tiny wires at the tiny terminal screw connector on the pick up board was almost broken. The holding ty rap had been omitted by the PO. I found it by running the bike with the cover off and messing with the wires. I repaired the connection and added a tiny tywrap and a blob of rtv to prevent movement of the wires at the connection.
Cheers

MexicoMike · 2026-06-30T08:50:50-0600

Ignition issue!

trappnman · 2026-06-30T08:56:48-0600

You have eliminated anything to do with ignition.
You have eliminated mechanical issues inside the head.
You have eliminated the carbs themselves.
I suppose it could be timing someway, but don't see how.

Seems all that's left is fuel to that side. Could something inside the intake be blocking free fuel flow?

Hammerski · 2026-06-30T09:12:10-0600

“right side drops out, just pops and bangs occasionally”

It’s a long shot, but are your intake balance pipes connected or capped properly? “Pops and bangs” is how mine ran when I forgot to reinstall the hose.

Onder · 2026-06-30T10:02:03-0600

Did you do an ohm test on the coils?

MexicoMike · 2026-06-30T10:20:18-0600

Is this misfiring occurring now with the bike sitting/reving in neutral? IOW, does it seem OK revving in the driveway? If so...

There is no load at all on an ignition system at any RPM in neutral compared to accelerating on the road. A coil, a wire, or a plug can appear/function perfectly fine in the garage but not under load. If it is running OK "in the driveway" I would specifically look for carbon tracking on a coil/sparkplug wire. Of course there could be a problem with the E-ignition itself - maybe a poor connection that is affected by vibration or a faulty coil under load.

Absolutely mystifying failure, 850 Commando Mk III

Lannis

Torontonian

L.A.B.

Moderator

Adam Lynam

hybridracer

kommando

dobba99

concours

JimNH

Steve W.

MexicoMike

trappnman

Hammerski

Onder

MexicoMike

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