Firing on one cylinder

My 1974 850 Commando MK II is only getting spark to the left cylinder. This a recent development in the past few days. The bike has consistently been starting on the first kick up to two or three days ago. The last two starts have taken three or four kicks. Once started, the bike ran fine. Went for a ride yesterday with no issues. However, I figured I’d have a look at things to see if I could determine why the change had occurred. Partly trying to be proactive, partly paranoid. It’s usually never good when things change for no apparent reason. I cleaned the plugs (there wasn’t anything obviously wrong with the way they looked) and dressed the points with some fine paper. (Again, everything looked fine). Checked the battery (12.9v). Tried starting the bike and after five or six kicks she started but was obviously running on one cyclinder only. Pulled the plugs and tested them against the head. Left plug had spark, nothing on the right. Tried new plugs with no difference. Swapped plug wires, left to right, with no difference. Still no spark on right side. Checked the connections to the coils. Everything was still where it is supposed to be. All connections good. The only anomaly I noticed was the Charge Warning Light. It usually blinks twice and then stays on when the ignition is first turned on, indicating ignition system is good. The light was initially blinking two short blinks and one long blink which according to the manufacturer’s info indicates a dead battery. However, battery is reading 12.9v. For some reason, the CWL later started behaving normally again, two blinks and then steady. Not even sure if this would relate in any way to spark on one side only in any event. I’m thinking next step is to swap coils left to right to see if anything changes. I expect I’m probably going to have to work my way through the ignition system but any suggestions for things to check first would be greatly appreciated. Thanks.

Maybe just swap coil leads with coils in same place , if you had Boyer easy fix …. did you take good look points and all connectors …. guess you still run ballast resistor , make sure it all good … good luck !

Are both coils and points sets live 12V?

By all means swap the coils over.

WEM said:

I expect I’m probably going to have to work my way through the ignition system but any suggestions for things to check first would be greatly appreciated.

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Both coils are fed by the same (normally) white/purple from the ballast resistor so if there's a problem with the feed to the RH coil it has to be more or less right at the coil (-) connection or the (6V) coil itself, the coil (+) to points and condenser wiring (normally black/white for the RH cylinder) especially copper strands broken inside the insulation where the wire enters the case behind the points housing.

Thanks for the feedback. I’ll check and report back. In the meantime, a new piece of info; the contact breaker that controls the right cylinder (black/white wire as LAB noted) sparks occasionally when the points open as I turn the engine over via kickstart. Didn’t notice it before. I’m working in the driveway in full daylight. Might there be something going on with the condenser for that cylinder? But it appears to be intermittent. I’m not sure if a condenser can fail intermittently or if it either works or it doesn’t. I’m going to swap wires on the condensers first to see if the issue migrates to the points for the left cylinder. I say the issue is intermittent because it doesn’t happen all the time. On one occasion when the breaker wasn’t sparking, the bike fired right up and ran smoothly.
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RH coil failed? Check the resistance of the primary terminals...should be in the 3 Ohm range. Check the resistance from one of hte primary terminals (unsure if it matters +ve or -ve) to the HT terminal. Should be in the 20 kOhms or more range. Any open circuit (inifite resistance) or shorted circuit (0 Ohms) means the coil is done.

Not sure a failed coil would be intermittent , however if one coil has been over tightened at some time you can suffer from these symptoms once hot

Craig said:

Not sure a failed coil would be intermittent , however if one coil has been over tightened at some time you can suffer from these symptoms once hot

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I didn't see him state this is intermittent in nature?

I’ve had a coil test fine on the bench, start and idle fine, then fail when hot. Changing left to right will give you the clue.

WEM said:

Thanks for the feedback. I’ll check and report back. In the meantime, a new piece of info; the contact breaker that controls the right cylinder (black/white wire as LAB noted) sparks occasionally when the points open as I turn the engine over via kickstart. Didn’t notice it before. I’m working in the driveway in full daylight. Might there be something going on with the condenser for that cylinder? But it appears to be intermittent. I’m not sure if a condenser can fail intermittently or if it either works or it doesn’t. I’m going to swap wires on the condensers first to see if the issue migrates to the points for the left cylinder. I say the issue is intermittent because it doesn’t happen all the time. On one occasion when the breaker wasn’t sparking, the bike fired right up and ran smoothly.
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Arcing at the points indicates a failed condenser.
The sparks can be caused by other things as well, pitted points etc, but most often it is just due to a burnt out condenser.


Glen

worntorn said:

Arcing at the points indicates a failed condenser.
The sparks can be caused by other things as well, pitted points etc, but most often it is just due to a burnt out condenser.


Glen

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Yes , agree . Anyone still running points should be ready to fit new condensers in a moment's notice .

It can easy be a poor ground , in there .
While tightening down the screws'd help , the cure is remove & clean surfaces ( condensors ) .

a small screwdiver , lifing each contact breaker , should get a spark . GLARE at the CONTACTs while youre at it !
a tiny bit of grit or shard or chip can get you not going . SEEINg we are running POINTS -

Clean & lube the auto Adv. periodically ( 10.000 interval / 4 years . ) so it dosnt rattle & fall apart .
The CAM should be polished with 600 Wt then finer - or a polishing wheel . Mirror Finish ish .

If you put the ickey Nylon Points in UNLUBED youre DOOMED . Generally coat entire cam circumferance & scrape clean with matchstick or toothpick . or similar .
A small bead along each Nylon Point Heel . weeny one but coated ! . Start it run it then stop it - Clean the little excess at the point heel / cam interface . If you got it right , itll be a tiny one .
We wont maention Queenslanders & land cruisers & nylon ' NEw ' points . Take off the distributor cap . No itheyre new . 1 1/2 hours later . Take off the distributor cap . No theyre new .
We now assume near all qld.ers know everything about nothing . Particularly lubing new nylon poits heels and cams .

RIGHT . Try it at NIGHT . And dont forget to turn off the light . July 4 th etc . . OBSERVE RUNNING . And while Lucas Coils are comparable tecnology to a Binks Rat Trap carburator ,
theyre not as dependable . If theyre 6 Volt & past there 10th birthday , theyre senile . Often at birth . Must be female coils .

A 1/2 Inch blue & white spark isnt impressive , its mandatory minimum - from a plug lead . Or itll sound like a Honda SL 125 or worse .If it goes at all .
So , a bigger screwdiver in a plug cap - to earth starting at a 3/8 GAP , tweak the point with the widdle scewdriver , If It Caint jump free epthes , itll missn stuttern stagger under load .
Sometimes the coils have overheated and chared or de insulated there windings or chaffed from vibration , or been left on with a point whereit autnt , or leaked . Theyre Junk .

Torrry , WEM mentioned intermittent in his post just above your reply ….

I'd suspect a faulty/poor connection somewhere in the 'bad' set of points. As noted, the condenser's purpose is to reduce/eliminate arcing and subsequent wear on the points themselves. Arcing can cause a poor connection across the points over time. Carefully examine the points for pitting/clean them up with a small file...used to be "points files" specifically made for that purpose but a nail file was probably the more common tool. USUALLY, a bad condenser will not stop the plug from firing though the engine may run poorly, especially under load.

Craig said:

Torrry , WEM mentioned intermittent in his post just above your reply ….

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Got it, thanks.

Torry, we all guilty of skimming sometimes ,specially when an issue comes up we think we can help with ….

Sorry, I haven’t updated for a few days. I was waiting to see what, if anything, might happen. For the past three days, the bike has started first kick each morning and has run well on my daily drive. I have carefully inspected the ignition system wiring and connections. All seems OK visually. No signs of obvious issues. I spent considerable time closely examining the points mechanism. Again, no obvious signs of anything of concern. A mechanic friend suggested I re-dress the points again based on an experience he had one time with an old Triumph where a piece of sandpaper grit became embedded on the face of one of the contacts. I cleaned the points and blew compressed air on the assembly to remove debris, if any. My conundrum now is that the bike is running well again but I’m not sure if it was something I did or if the problem really is intermittent and the bike is now in the good bike phase of a good bike/bad bike scenario. It might be a mistake, but I think I’ll let sleeping dogs lie for now and follow advice my father always offered, “If it ain’t broke, don’t fix it.” The riding season here is short, so unless the situation recurs, to maximize riding time, I’ll probably wait for the off-season to pursue a more in depth investigation. I don’t usually stray too far from home in any event. Thanks to everyone for the advice. This forum is a great resource. People are willing to offer free advice based on their knowledge and experience, often hard won, without the drama and BS you often see on many sites. Kudos to the moderators and members for such a helpful and civilized site.

Looks like I might have jumped the gun with today’s earlier post. Bike started fine this morning and ran well initially on my daily ride but then started sputtering when accelerating after about 15 minutes of riding. I would ordinarily have suspected a fuel issue except for the recent history with one plug intermittently not having spark. I’m going to change the plugs again and see if that has any effect. The coils and condensers are four years old. Just wondering if a coil or a condenser can fail intermittently, and, if so, how to test them.

Coils can but normally its heat that gets to them and then they are ok when they cool before acting up again when hot. So compare the temperatures of the riding days and if the failure coincides with hot days then coils is a possibility. As you have mentioned sparks at the points then new condensers are a must.

Swap the HT leads so left coil fire right plug etc and then determine if the misfire stays put or migrates to other side. That will tell you if a coil is the issue. Even new coils can fail. If the clamp bracket is too tight it can crush the outer coil wall and lead to shorting internally.