Single carb setup runs on one cylinder

Jacko · 2026-04-09T03:03:49-0600

Hey there, I'm new to the scene and bought quite a messed up 1971 commando. It came with a single carb amal setup, I just replaced the 932 concentric with a premier, keeping its 260 main jet, needle in middle position. We had ignition issues, so I put in a new boyer mk 4 black box and replaced all cables and spark plugs. The pick up is still the old boyer mk 3. Since then it seems to have quite a good spark on both sides, but it runs very bad. Black smoke and too much pressure coming out of the right exhaust (way too fat?), while the left is very weak, maybe not firing at all. When driving it doesn't rev above 3000 and there was unburned gasoline leaking out of the left outlet, dripping from the lockring. I switched the ignition cables from left to right, but that didn't change anything, which as far as I understand rules out the ignition as the problem. My next steps would be a compression test and a valve check. But what are your ideas? Thank you!

kommando · 2026-04-09T03:15:01-0600

Its very rare but one issue that can get a Boyer to only spark one plug is a compromised earth path from engine back to the positive terminal of the battery. Its a quick and easy test, run a wire directly from the head back to the battery positive terminal and then see if the left fires up. Modern spark plugs can foul at the drop of a hat so have a few new ones to hand, iridium seem to be less prone to fouling,

Jacko · 2026-04-09T04:25:09-0600

kommando said:
Its very rare but one issue that can get a Boyer to only spark one plug is a compromised earth path from engine back to the positive terminal of the battery. Its a quick and easy test, run a wire directly from the head back to the battery positive terminal and then see if the left fires up. Modern spark plugs can foul at the drop of a hat so have a few new ones to hand, iridium seem to be less prone to fouling,

I tested the earth of the engine with a multimeter and could'nt find any problem. But I will try the wire shortcut technqiue too. Thank you!

kommando · 2026-04-09T04:33:45-0600

The multimeter may show a connection but on mine it was down to a few strands so did not have the capability of carrying the amps despite a connection showing, it would pass enough A for one spark but not two. If you are using a single twin spark coil then the earth is not used, but it is for two single coils.

concours · 2026-04-09T05:20:49-0600

OP, have you performed a compression test?

Jacko · 2026-04-09T05:41:29-0600

concours said:
OP, have you performed a compression test?

Not yet, but a friend is going to bring a tester on saturday. The weird thing is that we got her going before she had the ignition issues (I think caused by bad kill switch wiring + bad battery + old boyer) and she ran way better without any differences between left and right. There were all sorts of other problems, like that she wouldn't really start, rev and that the spark would stop occasionally, but the cylinder compression seemed to be equal. So I hope it's not a compression issue.

concours · 2026-04-09T06:08:49-0600

Jacko said:
Not yet, but a friend is going to bring a tester on saturday. The weird thing is that we got her going before she had the ignition issues (I think caused by bad kill switch wiring + bad battery + old boyer) and she ran way better without any differences between left and right. There were all sorts of other problems, like that she wouldn't really start, rev and that the spark would stop occasionally, but the cylinder compression seemed to be equal. So I hope it's not a compression issue.

Typical rusty bore scenario.
Bike started after long neglect. Iron oxide gobbles up the rings.
Hopefully I'm wrong.

Good luck.

Test, don't guess.

Please report back after compression testing...

Deets55 · 2026-04-09T07:10:14-0600

Jacko said:
Hey there, I'm new to the scene and bought quite a messed up 1971 commando. It came with a single carb amal setup, I just replaced the 932 concentric with a premier, keeping its 260 main jet, needle in middle position. We had ignition issues, so I put in a new boyer mk 4 black box and replaced all cables and spark plugs. The pick up is still the old boyer mk 3. Since then it seems to have quite a good spark on both sides, but it runs very bad. Black smoke and too much pressure coming out of the right exhaust (way too fat?), while the left is very weak, maybe not firing at all. When driving it doesn't rev above 3000 and there was unburned gasoline leaking out of the left outlet, dripping from the lockring. I switched the ignition cables from left to right, but that didn't change anything, which as far as I understand rules out the ignition as the problem. My next steps would be a compression test and a valve check. But what are your ideas? Thank you!

Also , probably not the issue, but the timing will change when swapping the black box. I went from a MKIII to a MKIV and kept the pickup. I think there was 3 degrees difference.

kommando · 2026-04-09T07:37:33-0600

Deets55 said:
I think there was 3 degrees difference.

8 degrees for me, black box to red box with rotor and stator unchanged. Exhaust pipe went blue before I could even move from kickstart to strobe light I already had in place.

cliffa · 2026-04-09T09:42:45-0600

If it's a very early Boyer pickup stator, the coils are lower resistance @ 33ohms ( later are 66) I don't know what effect that would have on the MKIV box, but maybe drop a line to help@boyerbransden.com They are normally very helpful and responsive.

Deets55 · 2026-04-09T10:17:57-0600

cliffa said:
If it's a very early Boyer pickup stator, the coils are lower resistance @ 33ohms ( later are 66) I don't know what effect that would have on the MKIV box, but maybe drop a line to help@boyerbransden.com They are normally very helpful and responsive.

I think it's the electrical components in the box. Different tolerances, maybe different manufactures and even slightly different design. My understanding its a pretty simple/basic system.

Tornado · 2026-04-09T10:25:50-0600

Single carb setup and seemingly running on right side only. So not a carb blockage or jetting issue. Teporting good spark both plugs so not likely ign setup. No balance pipe leakage issue, but perhaps an air leak at the manifold to inlet port on the left? I like to use a locktite gasket maker product at these flanges.
Compression check may reveal a ring or valve issue.

baz · 2026-04-09T11:36:16-0600

What manifold is it?
Is it a 4 bolt or 2 bolt type?
I'm wondering if an internal bolt has come out although the ports are fairly close together it could possibly affect the side closest more than the other?

cliffa · 2026-04-09T12:09:16-0600

Deets55 said:
I think it's the electrical components in the box. Different tolerances, maybe different manufactures and even slightly different design. My understanding its a pretty simple/basic system.

Agreed, but dropping an email to Boyer would do no harm, and be another thing to cross off the checklist. (sharing their answer would be good for future reference too).

Tornado · 2026-04-09T15:06:07-0600

baz said:
What manifold is it?
Is it a 4 bolt or 2 bolt type?
I'm wondering if an internal bolt has come out although the ports are fairly close together it could possibly affect the side closest more than the other?

It could be a central bolt has come off and fallen into the lnlet port...jamming up the valve on the left side.

Single carb setup runs on one cylinder

Similar threads