Intermittent Dead Cylinder...Switching Sides

freddy3

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Weird problem: Intermittently, RIGHT cylinder dead (i.e., pull RIGHT spark plug lead, NO change in idle....pull LEFT spark lead, engine DIES). Intermittently, LEFT cylinder dead (i.e., pull LEFT spark plug lead, NO change in idle....pull RIGHT spark lead, engine DIES). Intermittently, BOTH cylinders run normally.

Stock (OEM) 1972 XS2 650

MOST (not all!) of the time, regardless of whether starting from cold or starting the bike when warm, one cylinder won't run and the engine runs on the other cylinder (i.e., pulling the spark plug lead from the dead side has NO effect on idle, but pulling the spark lead from the running side kills the engine). Occasionally, if I let the bike sit idling for 15 minutes, all of a sudden, LIKE SOMEONE THREW A SWITCH, the idle speed will increase ~ 200 RPM, the dead cylinder will come to life and the engine'll run fine.....that is, until I turn the engine off. Then, it's like throwing dice to see what happens when I restart.

Anyone seen anything like this before?
 
I would check the points
And start and look at the sparking at them points if it looks normal
 
I would check the points
And start and look at the sparking at them points if it looks normal

Thanks. Points appear to be sparking normally. I did clean them anyway, but had no effect.
My gut says there's an electronic component shared by both sides that's on the cusp of failure. Does that make any sense?
 
Thanks. Points appear to be sparking normally. I did clean them anyway, but had no effect.
My gut says there's an electronic component shared by both sides that's on the cusp of failure. Does that make any sense?

I believe there are 2 separate sets on these early bikes
2 coils 2 condenser 2 points ( don't remember exactly )
So I don't believe there is a single common electronic part

The next step would be to shift over leads between Left and Right including spark plugs see if the problem follows to the other side
Please google how it is done
To check if it is an electrical fault at the same time service the connectors and spark plug caps and so on
 
I believe there are 2 separate sets on these early bikes
2 coils 2 condenser 2 points ( don't remember exactly )

I believe there's only one condenser unit, but two coils/points (I replaced the coils, but no change).


The next step would be to shift over leads between Left and Right including spark plugs see if the problem follows to the other side
To check if it is an electrical fault at the same time service the connectors and spark plug caps and so on

Because the problem is INTERMITTENT and affects BOTH cylinders, intermittently, I'm not sure how swapping one side to the other will tell us anything?
Today, I replaced the NGK B8ES plugs (gapped to factory spec) and removed/continuity-tested both spark leads (OK). Weird, huh?
 
I believe there's only one condenser unit, but two coils/points (I replaced the coils, but no change).
One condenser unit that contains two separate capacitors... so two of everything.
 
OK got it wrong my mistake bad reading Sorry
How is battery and charging ??
 
Under the tank on the right side of the bike is a rats nest of connectors where power is split off to the 2 separate ignition systems. Check and clean each of the connections one at a time. 50 year old bullet connectors can have problems. Do them 1 at a time very easy to get it wired back up wrong if you unhook them all at the same time. Some of the connectors are for power to the coils, some from coils to points and some for points to condensers.
 
Under the tank on the right side of the bike is a rats nest of connectors where power is split off to the 2 separate ignition systems. Check and clean each of the connections one at a time. 50 year old bullet connectors can have problems. Do them 1 at a time very easy to get it wired back up wrong if you unhook them all at the same time. Some of the connectors are for power to the coils, some from coils to points and some for points to condensers.

Thanks. Initially, I thought it was a carb problem (before I realized the problem intermittently switched sides), so I've had the tank off a number of times. I haven't taken any of the connectors apart (or cleaned them, which is probably a good idea), but I have jiggled the "nest"....along with the wires coming from the ignition switch (where you put the key in) and the wires inside the headlight case.

Lacking anything more specific, I'll try pulling apart (and cleaning) the nest of connectors under the tank. But, again, the intermittently switching sides nature of the dead cylinder makes me think it's a single electronic gizmo that's shared rather than a dirty connection, which I would think would affect only one side and stick to that side.
 
I don't know about charging (not really able to check it), but the battery's fully charged.

If you have a Voltmeter
Voltage --- everything off across the battery
Voltage ignition on --- across the battery
Start bike Voltage --- across battery idle and at 3.5 k
If no voltmeter turn lights on see if intensity increases when revving

Please check the fuse sitting OK
 
makes me think it's a single electronic gizmo that's shared rather than a dirty connection, which I would think would affect only one side and stick to that side.
The thing is a 72 does not have any electronic gizmos on them. It's most likely a connection problem to a non electronic part.
 
Does this sound like a failing condenser?
You don't have just 1 condenser, their are 2 with 2 separate connections going to them. The condensers just packaged into a single unit. 2 wires going to it, 1 for each condenser.
 
If you have a Voltmeter
Voltage --- everything off across the battery
Voltage ignition on --- across the battery
Start bike Voltage --- across battery idle and at 3.5 k
If no voltmeter turn lights on see if intensity increases when revving
Please check the fuse sitting OK

The bike's kept at a public storage (I live in an apt), but I'll try to get back there this week. I've got a voltmeter and I'll check battery volts.
The last time I checked it, I think the volts were ~14-ish at idle (when both cyls were firing) and, yes, the lights brightened when the engine's revved. It's ALWAYS done that (I've owned the bike since '75).
 
Weird problem: Intermittently, RIGHT cylinder dead (i.e., pull RIGHT spark plug lead, NO change in idle....pull LEFT spark lead, engine DIES). Intermittently, LEFT cylinder dead (i.e., pull LEFT spark plug lead, NO change in idle....pull RIGHT spark lead, engine DIES). Intermittently, BOTH cylinders run normally.

Stock (OEM) 1972 XS2 650

MOST (not all!) of the time, regardless of whether starting from cold or starting the bike when warm, one cylinder won't run and the engine runs on the other cylinder (i.e., pulling the spark plug lead from the dead side has NO effect on idle, but pulling the spark lead from the running side kills the engine). Occasionally, if I let the bike sit idling for 15 minutes, all of a sudden, LIKE SOMEONE THREW A SWITCH, the idle speed will increase ~ 200 RPM, the dead cylinder will come to life and the engine'll run fine.....that is, until I turn the engine off. Then, it's like throwing dice to see what happens when I restart.

Anyone seen anything like this before?
 
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