Topic: intermittent coil failure?

[the architect]

1975 CB750K.  Odometer shows ~20k miles.  As far as I know, coils are original.  I custom re-wired the bike when doing my cafe work on the bike.

For the last few weeks, at random times, the bike goes noticibly low on power, dies if left to idle and sounds half gutless.  When feeling the exhaust, I can tell that cylinders 2 and 3 are cold.  So, it's obviously running on 2 cylinders when this happens.  This sounds like a coil problem, to me.  I have one coil accidentally come disconnected a while back, and it felt just like this and experienced all the same characteristics.  Right now, all my connections appear sound. 

So, here's an odd wrinkle to all this (at least for me).  The issue comes and goes with no obvious pattern or frequency.  Sometimes it doesn't show itself for days (or even a week).  Last time it happened, I noticed that if I down shifted and got the rpm's up a bit (4-5k, maybe), it kind of "snapped" out of it.

Ideas?    Can coil failure actually begin as an intermittent problem like this, before it goes completely belly-up?

[scondon]

Sounds like a problem either at the coil or the condenser(points plate). When you rev the bike the stator starts putting out maximum voltage, punching through whatever resistance you have in the troubled circuit. Clean all wire connections going to your coils and condensers, and see if things get better. When condensers start to fail you might see excessive sparking on your points while the engine is running.

[tlbranth]

Sounds more like a fuel problem. Which 2 cylinders stop firing?

Oops. Just re-read the original post. If 2 & 3 gave up it does point more to spark. Try reversing the primary coil connectors. If the problem moves to 1& 4, check points/condensors. If not, try switching plug wires (if they'll reach). If problem moves to 1 & 4, bad 2/3 coil. Still has a fuel ring to it though. Heat can make a weak coil pack up.

[Duke McDukiedook]

Yeah, sounds more like a carb/fuel problem with that kind of frequency. I would look at that first.

[scondon]

Since it's a problem that effects 2-3, which fire off the same coil and condenser, and the problem clears itself when the bike is revved, can either of you expand on why you think it's more likely a fuel problem?

[tlbranth]

see amendment above

[scondon]

see amendment above

Nice. I like the wire swapping diagnostic technique since I can never seem to remember the procedures for testing with a multimeter. Plus you can do it on the side of the road with few tools, which is where most of my failures occur 

[Duke McDukiedook]

Since it's a problem that effects 2-3, which fire off the same coil and condenser, and the problem clears itself when the bike is revved, can either of you expand on why you think it's more likely a fuel problem?

I would say a coil if it was constant and frequent.
It still could be, he hasn't elaborated on all the tests he ran on his electrical/fuel delivery system so I can't really do much more than blindly guess on the problem at this point.
 

[DHolbrook]

Architect,

I had an identical problem to yours.  My 2/3 would stop working whenever they felt like it.  If I continued to ride around, I'd have extreme popping backfire out the exhaust.  She would die under 2k rpm.  High revs could pick 2/3 back up temporarily.  I had an Dyna elec. ignition, so I replaced it with the stock points plate.  She has ran fine for about 2 days/30 miles.  Maybe you can learn from my experience, I'd check your condensers, since my bike smoothed out without touching the coils or carbs.

[camelman]

I literally just went through this with my 350F about 4 hours away from home.  I mucked around with the points and timing before I finally realized what I was seeing.  My 1/4 points were sparking like mad.  I had a spare condenser with me, so I swapped it in and the issue was fixed.

So, before going any further into that thing, why don't you pull your points cover off to see if there is excessive sparking at the points while it is running.  You really shouldn't see any sparks from either set of points in full daylight.  If you do, then it is time for a new condenser for whichever pair is sparking.  Symptoms are irregular power loss, popping through the exhaust, intermittent poor idle, but somewhat normal operation at higher rpms.

Good luck,
Camelman

[Henning]

Check the solder joints on the coils themselves - where the black/white, blue and yellow wires meet the coils. I had one of them go bad and cause intermittent misfiring.

I got lucky; it was the first thing I checked when tracking down the problem. No beating of head against nearest brick wall.

[the architect]

Great advice, guys -- very much appreciated.

I hesitate thinking it's a carb issue, as I recently completed a full dismantle, clean and rebuild of said carbs.  I won't rule it out, but they are clean and set to spec, at this point.  During my build, I also de-rusted and cleaned the fuel tank.  It may have some evidence of minor flash rust, but no rusticles that should be breaking off into my fuel lines.  I did have inline fuel filters for a while, but temporarily removed them when chasing some carb tuning issues.  Probably not related to that, though.

Last night, I did swap the connections at the coils (1-4 swapped with 2-3).  I forgot to swap the corresponding connections at the points, so I got some fireballs before I realized my error.  Swapped those quickly   I'll ride it, weather permitting, and see what happens.

I literally just went through this with my 350F about 4 hours away from home.  I mucked around with the points and timing before I finally realized what I was seeing.  My 1/4 points were sparking like mad.  I had a spare condenser with me, so I swapped it in and the issue was fixed.

So, before going any further into that thing, why don't you pull your points cover off to see if there is excessive sparking at the points while it is running.  You really shouldn't see any sparks from either set of points in full daylight.  If you do, then it is time for a new condenser for whichever pair is sparking.  Symptoms are irregular power loss, popping through the exhaust, intermittent poor idle, but somewhat normal operation at higher rpms.

Good luck,
Camelman

This really sounds spot-on.  I have the intermittent power loss.  Idle seems good most of the time, but when it's "acting up", it won'd idle for more than 30 seconds before it dies.  I do have popping through the exhaust.  Since I'm currently running no mufflers, I figured this was just normal for that type of open setup, but now I'm wondering if it could be related to points/condensers.  As for operating temps, this is my first one, so I don't have a good frame of reference for normal temp range.

I did see some sparking on the points.  I'm not sure how much is too much, though.  I wasn't in full daylight, but I was in my relatively well-lit garage.

[Duke McDukiedook]

Sooo.... let me get this straight, you are running headers without mufflers and you didn't rejet or change needle height?

[camelman]

Great advice, guys -- very much appreciated.

I hesitate thinking it's a carb issue, as I recently completed a full dismantle, clean and rebuild of said carbs.  I won't rule it out, but they are clean and set to spec, at this point.  During my build, I also de-rusted and cleaned the fuel tank.  It may have some evidence of minor flash rust, but no rusticles that should be breaking off into my fuel lines.  I did have inline fuel filters for a while, but temporarily removed them when chasing some carb tuning issues.  Probably not related to that, though.

Last night, I did swap the connections at the coils (1-4 swapped with 2-3).  I forgot to swap the corresponding connections at the points, so I got some fireballs before I realized my error.  Swapped those quickly   I'll ride it, weather permitting, and see what happens.

I literally just went through this with my 350F about 4 hours away from home.  I mucked around with the points and timing before I finally realized what I was seeing.  My 1/4 points were sparking like mad.  I had a spare condenser with me, so I swapped it in and the issue was fixed.

So, before going any further into that thing, why don't you pull your points cover off to see if there is excessive sparking at the points while it is running.  You really shouldn't see any sparks from either set of points in full daylight.  If you do, then it is time for a new condenser for whichever pair is sparking.  Symptoms are irregular power loss, popping through the exhaust, intermittent poor idle, but somewhat normal operation at higher rpms.

Good luck,
Camelman

This really sounds spot-on.  I have the intermittent power loss.  Idle seems good most of the time, but when it's "acting up", it won'd idle for more than 30 seconds before it dies.  I do have popping through the exhaust.  Since I'm currently running no mufflers, I figured this was just normal for that type of open setup, but now I'm wondering if it could be related to points/condensers.  As for operating temps, this is my first one, so I don't have a good frame of reference for normal temp range.

I did see some sparking on the points.  I'm not sure how much is too much, though.  I wasn't in full daylight, but I was in my relatively well-lit garage.

A couple sparks here and there isn't an issue.  However, if the sparks are regular, then you need a new condenser.  If the points are sparking every time they open, then you might want to clean the faces off too.

Running without mufflers will cause some other issues, and you might need to rejet to solve them.  However, your intermittent issues sound like something else.

What is like without mufflers?  It must be horribly loud.

Camelman

[TwoTired]

I hesitate thinking it's a carb issue, as I recently completed a full dismantle, clean and rebuild of said carbs.  I won't rule it out, but they are clean and set to spec, at this point. 

Um. There is no spec for carbs not used with the stock air box and exhaust.  FYI

Check the ignition leads for cracks, and/or arcing in the dark.  If none, check points and condensers, and the wire connection integrity between coils and points. ... And coil then coil to power supply.

[the architect]

I hesitate thinking it's a carb issue, as I recently completed a full dismantle, clean and rebuild of said carbs.  I won't rule it out, but they are clean and set to spec, at this point. 

Um. There is no spec for carbs not used with the stock air box and exhaust.  FYI

Check the ignition leads for cracks, and/or arcing in the dark.  If none, check points and condensers, and the wire connection integrity between coils and points. ... And coil then coil to power supply.

Thanks for the tips!  I'll try it tonight.  When I said "set to spec", I was mainly referring to float heights. 

Great advice, guys -- very much appreciated.

I hesitate thinking it's a carb issue, as I recently completed a full dismantle, clean and rebuild of said carbs.  I won't rule it out, but they are clean and set to spec, at this point.  During my build, I also de-rusted and cleaned the fuel tank.  It may have some evidence of minor flash rust, but no rusticles that should be breaking off into my fuel lines.  I did have inline fuel filters for a while, but temporarily removed them when chasing some carb tuning issues.  Probably not related to that, though.

Last night, I did swap the connections at the coils (1-4 swapped with 2-3).  I forgot to swap the corresponding connections at the points, so I got some fireballs before I realized my error.  Swapped those quickly   I'll ride it, weather permitting, and see what happens.

A couple sparks here and there isn't an issue.  However, if the sparks are regular, then you need a new condenser.  If the points are sparking every time they open, then you might want to clean the faces off too.

Running without mufflers will cause some other issues, and you might need to rejet to solve them.  However, your intermittent issues sound like something else.

What is like without mufflers?  It must be horribly loud.

Camelman

Horribly loud, indded!  I can't wait to get something quieter, but I'm still deciding what direction to go with it.  In the meantime . . .  at least the cars hear me coming.