74 850 left me stranded

Went for a ride yesterday with intentions of doing about 150 miles with a mate. Lovely clear day in the 70's. The Norton ran flawlessly... until it didn't. At about 30 miles it gave me a pop-pop-pop and wouldn't stay running. Coasted to the side of the road and had a look-see.

This is a superbly maintained 74 850 MKII. Amal carbs, TriSpark ignition, TriSpark coils.

I had fueled up before the ride. Pressing ticklers made no difference. Both cylinders were affected. Thank God for e-start, as I was able to just push the button during troubleshooting.

Took the points cover off and did the TriSpark self test. The LED came on but it flickered, it was not steady as it should be. I carry a spare TS ignition and it's a simple swap even at the roadside. No difference, including the self test indication.

I checked the coil wires by tugging on them and everything was tight. Resigned to failure, I called a tow vehicle (a $375 ride, ouch).

When I got the bike home it sat in my garage for a half hour while I had a beer and sulked. Eventually I put the key in it, tickled the carbs, and she fired right up like nothing was wrong. Warmed up and idled steadily. Great. How do you fix something that's not broken? I'm not sure how I'm going to proceed here.

Sounds like a bad coil to me.

Had a couple of after market coils do just that before..... They were brand spanking new too... just sayin'... pop like a machinegun and then quit until cool getting progressively worse after each failure.

Sounds like it's heat or vibration-related - coil is certainly a suspect but EI could be as well - or even an intermittent primary ignition connection.

I troubleshoot intermittent failures every day.
First, be careful not to confuse what you know, with what you THINK you know. It's easy to go a long way in the wrong direction.
Good plan carrying spare EI.
Good suggestions already (coils)
The wires going into the engine where they move is a possibility.
Create a bypass wire to rule out key switch & kill switch.

concours said:

I troubleshoot intermittent failures every day.
First, be careful not to confuse what you know, with what you THINK you know. It's easy to go a long way in the wrong direction.
Good plan carrying spare EI.
Good suggestions already (coils)
The wires going into the engine where they move is a possibility.
Create a bypass wire to rule out key switch & kill switch.

Click to expand...

Kill switch is wired to power the EI, and does not supply coil current. Seeing that the self test at least lit the LED I'm inclined to think that connection is OK. But yes, I will double check.

I will look at the wires going through the engine to the EI module. Thanks for that.

Engine grounds are redundant - there's a 6 gauge wire from the battery to the engine for the e-start plus a super-flexible 14 gauge from the harness to the head. And yes, I will double check that as well.

"What you THINK you know", indeed. Words of wisdom.

The hard part of an intermittent failure is getting it to repeat so that you can identify root cause. One of my troubleshooting plans is to heat the coils with a heat gun and see if the problem returns.

You have two coils right?

If one coil breaks down, the other will still fire, resulting in a poor running bike rather than a dead bike.

Surely it’s too much of a coincidence for both coils to have failed simultaneously (especially new oil filled ones)?

Fast Eddie said:

You have two coils right?

If one coil breaks down, the other will still fire, resulting in a poor running bike rather than a dead bike.

Surely it’s too much of a coincidence for both coils to have failed simultaneously (especially new oil filled ones)?

Click to expand...

Coils are in series for wasted spark. If one of them opens both will have no current. If the 2nd one in line shorts to the case (ground) neither will fire.

maylar said:

Coils are in series for wasted spark. If one of them opens both will have no current. If the 2nd one in line shorts to the case (ground) neither will fire.

Click to expand...

Fairy nuff.

Are those the failure modes present in a coil that breaks down intermittently with heat?

Fast Eddie said:

Fairy nuff.

Are those the failure modes present in a coil that breaks down intermittently with heat?

Click to expand...

Beats the hell outta me. The only coil failure I've seen in the years I've been doing this is one that shorted to case internally because the mounting clamp was too tight and the case was deformed. Mine are TriSpark coils that I've had for over 10 years, and honestly a failure of one would surprise the shit out of me.

I was under the impression that heat related ‘breaking down’ of a coil wasn’t that kind of dead short type failure mode, but there’s no harm in trying I guess.

I think I’d be looking at possible dodgy connections first (just following the maxim of ruling out the simplest things first).

Talking of simple things… a blocked tank vent is not a possibility ?

Try your heating test but also do this on with your VOM hooked to the primary terminals to monitor Ohms, they should read the spec of the coil (typically 1.5-3 Ohms each primary circuit). There should also be an Ohm spec for the primary to HT terminals. If low or open circuit, that's a failed/failing coil.

Also, after you've done hte easy stuff, start thinking intermittant short someplace....as I think you read, my 5-way rubber multi block (new) was shorting out different circuits deep within the rubber separating the bullet connectors. No way to see it without pulling the connector metal joints out and inspecting the rubber dividers.

Something that concours mentioned has me hopeful - the wires that enter the points cavity can chaff against the engine and short. I had that happen with Boyer pickup wires a few times in the past. Mine are heat-shrink covered but if the coil wire is shorting there it could definitely cause my problems. It's been years since I looked at those wires.

It was the ignition switch on the 650ss. Full of crud.
Also had an Emgo replacement ignition switch fail on another bike.
You've probably checked this already, but if not, wiggle the ignition key while the bike is running.

Glen

The kill switch interrupting the power supply has been known to go funny.

My money is on a loose or intermittent wire connection. When yo say pop, pop, that indicates to me power going on and off.
Different ignition, but my Boyer had both types of know to occur loose connections. The first case caused a bang, bang, that
scared the crap out of me while exiting a freeway.

I always carry a few jumper wires with me when riding. A jump straight from the negative terminal ignoring all else. If it doesn't help you know what it isn't. EI does add a worry, so do
series coils or two headed coils. Much to be said for points I hate to admit.

Fast Eddie said:

You have two coils right?

If one coil breaks down, the other will still fire, resulting in a poor running bike rather than a dead bike.

Surely it’s too much of a coincidence for both coils to have failed simultaneously (especially new oil filled ones)?

Click to expand...

I have had a coil fail but still spark!
In my case the left coil had failed but was still sparking but not feeding the right hand coil

What we know: it started as normal once back at home in the garage.

What we THINK we know: it is heat related because it started once it cooled off.

concours said:

What we know: it started as normal once back at home in the garage.

What we THINK we know: it is heat related because it started once it cooled off.

Click to expand...

Quite right.

Correlation vs causation …