Single carb setup runs on one cylinder

kommando · Apr 11, 2026

Jacko said:
ut still I would assume if that is an issue, the problem should change to the other cylinder when switching the ignition cables, shouldn't it?

The 2 coils on the stator are not engine cylinder specific, the Boyer system is wasted spark and is a single switch unlike points which are cylinder specific ie two switches, the two stator coils signals are averaged to create the timing point so both sparks are timed the same. So when the wires fail or the box fails you lose both cylinders sparks. The one exception is the faulty earth route which limits the flow so there is only enough current for one spark, that is not a Boyer fault. I put a piece of foam over the top of the wires under the timing cover so the wires are not moving with vibration.

cliffa · Apr 11, 2026

Jacko said:
I replaced the connectors with flexible shrink tubes, but I left the tie in there (photo follows on monday). An ohm-test between the two cables looked good. I will have a look, if there is some damage to the cable already. But still I would assume if that is an issue, the problem should change to the other cylinder when switching the ignition cables, shouldn't it?

I changed the black box because I read of the low voltage problem and she was hard to start at all when not constantly charged by a battery charger. The starting is definitely better now but looking at the photo of the plate I regret that I didn't buy a set to be honest.

As I said I keep you updated with harder facts on monday.

Aha, gotcha, I re-read your post and realized you’d already changed the connectors, however, it’s definitely worth checking those wires as they may already be compromised. I think they are the single most common failure on Boyers in Commando’s due to the Isolastics. Have a look at one of my old posts here..

Thread 'Almost Stranded'

Dec 29, 2017

I was a bit bored today, as my kids have gone back home and my wife was at work. Bike riding is out at the moment as there is about 5 inches of snow outside. So I thought what can I quickly do in the freezing garage? Ideally I want to take a bit off the bike and check or refurb it in the house. Ah! The Boyer stator plate wires. As I’d had a mystery complete engine cutout at the top of the Klausen Pass in august so this was on my “things to check over the winter” list. Right, so out to the garage then. I opened the points cover and saw a sticky label with name “A. Woods” (a former owner...

I realize this is probably nothing to do with your original question, but still definitely worth checking.

lazyeye6 · Apr 11, 2026

Boyer is a lost spark ignition. Both plugs fire simultaneously every time triggered. A single carb rules out carburetion as a problem.
Switch coils from right to left. Does the problem follow to the opposite cylinder? Use new spark plugs. Even new out of the box
can be a bad plug. A visual “good spark” doesn’t mean good enough to fire the cylinder under compression. Test compression.
Check valve adjustment. Please do replace the pick up plate. Are both coils 6V? Use multimeter to check that their Ohms of resistance meet Boyers
suggested specification.

seattle##gs · Apr 11, 2026

If no improvement by Monday then wrap up the hole thing and pitch it in the bin. Buy a Tri-spark. It may not solve the problem but now you have a new Tri-spark. And always...have the battery in top condition.

Jacko · Apr 14, 2026

I'm currently working on the machine, so it's a work in progess.
The compression test seemed fine in my opinion (round about 9,3 to 9,7). Big relief.
I put in a brand new motobatt fully charged.
I checked the insulators at the manifold, they seem very much in place. No air leak.

I readjusted the timing, this time on the right TDC compression stroke minus 6mm (30,5°). Same result. Right ones too fat, left ones not firing at all.

I switched the cables again from left to right. Same problem.

My next step would be to look at the valves. What do you think?

P.S.: Currently there are two german brand 6V coils on it, but I bought a pair of new Lucas ones. The only thing is, that the cable boots are too big for the new coils. I have to fix that first to put them on.

Jacko · Apr 14, 2026

Ok. Very weird. Apparently I was cursed and must have bought two broken spark plugs. I replaced the one not firing with a new one last week, which apparently wasn't firing neither (eventhough there was a visible spark). With a third new one now it's running on both. Sorry to disappoint you with that very boring solution. My dad came by and just asked the simple question of "what about the spark plugs, maybe try it again?".
Hoping that there is not another issue, which kills spark plugs, I consider the problem as solved.

Thank you all for your time and thoughts on that!

concours · Apr 14, 2026

Jacko said:
Ok. Very weird. Apparently I was cursed and must have bought two broken spark plugs. I replaced the one not firing with a new one last week, which apparently wasn't firing neither (eventhough there was a visible spark). With a third new one now it's running on both. Sorry to disappoint you with that very boring solution. My dad came by and just asked the simple question of "what about the spark plugs, maybe try it again?".
Hoping that there is not another issue, which kills spark plugs, I consider the problem as solved.

Thank you all for your time and thoughts on that!

Thank you for closing the loop!
Glad you solved it.

Jacko · Apr 14, 2026

Seems like I didn't solve it. Something is actually killing my spark plugs. It ran great for a couple of minutes, then horrible. Turns out something bends the left spark plug contact in. It's the right one on the foto: LINK FOTO

Any experiences with too long spark plugs? I bought them from andover (Champion NY7C).

But since it's only on the left, I suggest one piston goes too far up? Not good.

concours · Apr 14, 2026

Jacko said:
Seems like I didn't solve it. Something is actually killing my spark plugs. It ran great for a couple of minutes, then horrible. Turns out something bends the left spark plug contact in. It's the right one on the foto: LINK FOTO

Any experiences with too long spark plugs? I bought them from andover (Champion NY7C).

But since it's only on the left, I suggest one piston goes too far up? Not good.

Tell us about your mechanical skill set. It looks as though you're on the precipice of having to go in really deep.

kommando · Apr 14, 2026

This sounds like a piston top has started to detach from the bottom, too much carbon on the top of the piston or the shell bearing has failed badly. The piston top detachment was a 72 issue but a bike this old could have anything fitted.

Tornado · Apr 14, 2026

Perhaps someone skimmed the head and removed cylinder block gasket in an attempt to raise compression and this put the plug in contact with the psiton?

Jacko · Apr 14, 2026

So far I couldn't detect any bearing issues. No clicking, to movement when pushed against the piston etc.

I measured both sides with the andover piston stop / TDC tool and got the exact same length (3,5). Very strange. I would conclude that means one piston only touches the spark plug when in motion. But why - I don't know.

@Tornado the idea of a skimmed head came to my mind too, but wouldn't that affect both sides / bend both spark plugs? Otherwise that must have been a very unskilled skimming job...

For testing we put a very slim washer under the spark in danger, that seems to work for know. We still have issues with the carb we will try to fix first. That makes the testing while running easier. But I keep you updated on the spark plug eating beast I have awakened.

P.S.: Do you have any experiences with shorter spark plugs? Like NGK B7ES for example? If it's coal, that could do the job. If it's something more serious - it will eventually blow the engine, I suppose. Still not sure if that is a good idea.

Thanks again and have a nice week!

baz · Apr 14, 2026

Jacko said:
So far I couldn't detect any bearing issues. No clicking, to movement when pushed against the piston etc.

I measured both sides with the andover piston stop / TDC tool and got the exact same length (3,5). Very strange. I would conclude that means one piston only touches the spark plug when in motion. But why - I don't know.

@Tornado the idea of a skimmed head came to my mind too, but wouldn't that affect both sides / bend both spark plugs? Otherwise that must have been a very unskilled skimming job...

For testing we put a very slim washer under the spark in danger, that seems to work for know. We still have issues with the carb we will try to fix first. That makes the testing while running easier. But I keep you updated on the spark plug eating beast I have awakened.

P.S.: Do you have any experiences with shorter spark plugs? Like NGK B7ES for example? If it's coal, that could do the job. If it's something more serious - it will eventually blow the engine, I suppose. Still not sure if that is a good idea.

Thanks again and have a nice week!

If the piston has managed to hit the spark plug do not under any circumstances run the engine
Strip the top end down and find out what is wrong

ashman · Apr 14, 2026

Pull the head and make sure what is going on, maybe something has sucked into the motor and is sitting on top of the piston to cause the plug to close, happened to a mate way back a fuel tank bolt fell off and got sucked into the carbie, the bolt was suck in the slide but the washer was sucked into the cylinder and sat on top of the piston which caused the plug gap to close, didn't find out till he removed the head, was on a 1947 AJS 500, left a few imprints on top for the piston but no other damaged.

Tornado · Apr 14, 2026

Jacko said:
So far I couldn't detect any bearing issues. No clicking, to movement when pushed against the piston etc.

I measured both sides with the andover piston stop / TDC tool and got the exact same length (3,5). Very strange. I would conclude that means one piston only touches the spark plug when in motion. But why - I don't know.

@Tornado the idea of a skimmed head came to my mind too, but wouldn't that affect both sides / bend both spark plugs? Otherwise that must have been a very unskilled skimming job...

For testing we put a very slim washer under the spark in danger, that seems to work for know. We still have issues with the carb we will try to fix first. That makes the testing while running easier. But I keep you updated on the spark plug eating beast I have awakened.

P.S.: Do you have any experiences with shorter spark plugs? Like NGK B7ES for example? If it's coal, that could do the job. If it's something more serious - it will eventually blow the engine, I suppose. Still not sure if that is a good idea.

Thanks again and have a nice week!

I believe the champions N7YC are not projected tip, unlike NGK BPR7ES (P Indicates projected tip)...so both seem to work well in a commando.

Mart UK · Apr 15, 2026

Do you have a thread insert on the offending plug thread? I've never done one, but one of the insert types requires a re-surfacing of the sealing face, where the crush washer compresses. Could be it was badly done and there was too much ground off that sealing face?

ilikevelos · Apr 15, 2026

Mart UK said:
Do you have a thread insert on the offending plug thread? I've never done one, but one of the insert types requires a re-surfacing of the sealing face, where the crush washer compresses. Could be it was badly done and there was too much ground off that sealing face?

I think overzealously installing a timecert might conceiveably do that.

Jacko · Apr 16, 2026

Mart UK said:
Do you have a thread insert on the offending plug thread? I've never done one, but one of the insert types requires a re-surfacing of the sealing face, where the crush washer compresses. Could be it was badly done and there was too much ground off that sealing face?

That's on the list of possible causes. From the outside it didn't really look as if someone put a thread insert in, but I will look deeper into that. I thought that might be outruled since the TDC tool showed the same distance on both sides, which means for my understanding that the thread stop to piston distance is equal on both sides. But the tool is not that exact. I might check that more precisely somehow.

We will start with an endoscope on monday and check for any intruders in the chamber and coal. If that doesn't help I will grow into a real mechanic by opening Pandora's box.

I subscribed for the Milano-Taranto Rallye in July with her and have to weeks to decide if we are ready to compete or not. I'm most certainly not skilled enough for a complete engine overhaul yet, so pray for me.

Carl H · Apr 16, 2026

Jacko said:
Seems like I didn't solve it. Something is actually killing my spark plugs. It ran great for a couple of minutes, then horrible. Turns out something bends the left spark plug contact in. It's the right one on the foto: LINK FOTO

Any experiences with too long spark plugs? I bought them from andover (Champion NY7C).

But since it's only on the left, I suggest one piston goes too far up? Not good.

The Commando uses the projected nose spark plugs Champion N7YC or NGK BP7ES

Champion N7YC - alternative spark plugs

www.sparkplug-crossreference.com

Even with an 1/8" skim off the head of a bike I built I didn't have spark plug clearance issues. You can see the tops of the pistons through the spark plug hole to see if you have a contact issue. Do you have stock pistons? with flat tops and have valve pockets in them. I would remove the carb and manifold and the exhaust pipes to let you see more of the areas where trouble could be. Are there any loose parts or missing screws / bolts in the exhaust or intake system. Do you have good and similar compression over 150 PSI in each cylinder??

Carl H · Apr 16, 2026

ilikevelos said:
I think overzealously installing a timecert might conceiveably do that.

In a normal bike the spark plug is nowhere close to the pistons. The valves can too close or hit with high lift cams or if the cams are out of correct cam timing.

Single carb setup runs on one cylinder

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