CB500K1: electrical gremlin

[AndyH]

Have just completed a fabulous 850km (530 mile) return trip from Perth to the SW of Western Australia to celebrate 50 years of the CB750.  My mate was riding my 500 (I was riding his 750K1) and it had no trouble keeping pace with the 22 750s....until late yesterday.


Roland (my mate) reported that it had instantaneously dropped to 2 cylinders for about 3 or 4 km and then straight back to 4.

When we arrived at our stay for the night we did some initial troubleshooting.  We were able to determine that cyl 1 and 4 went missing.
After removing the tank, we checked the connections to the coil in question but nothing was found.


Anyway, on the way home today it intermittently dropped 1 and 4 again and progressively got worse.  Roland managed to limp it home on variously 2 and 4 cyls.  No fluttering/fluffing, just straight to 2, then straight back to 4.

Tomorrow we plan to do more thorough troubleshooting to hopefully identify the problem.  The initial plan is to double check ALL connections and possible intermittent wire-shorts from the Dyna-S to the coils.  If that doesn't reveal anything, then we'll swap the coils over and see what that reveals.  If nothing again, then we may have to suspect the Dyna-S, which BTW has been running without issue since I 1st bought the bike, until this w/end.


For those with experience, does this sound like a good approach?  Are there any other things that we may need to look at?


Fuel issues have been ruled out.  (Someone recently remarked that 99% of fuel problems are electrical!  )


Cheers
Andy

[Nurse Julie]

Just proves a 4 cylinder bike will always get you home.....even if its only firing on 2 cylinders 
I bet you had a great time away, where was the gathering?

[K2-K6]

Sounds like a good plan to diagnose. You should find it with that.

Beforehand suspecting the Dyna,  check the volts + to the coils carefully.  Usually one supply from loom and split to the coil pairs.

If the connect there is flaky it'll do this.  I'm using silicone grease from plumbers supply to keep joints like this from getting any corrosion in them.

Edit:- and obviously the trigger wire coming from Dyna to ensure it has good continuity to 1and4 coil. 

[AndyH]

Just proves a 4 cylinder bike will always get you home.....even if its only firing on 2 cylinders 
I bet you had a great time away, where was the gathering?

It was a fabulous 3 days' riding Julie! We departed Perth around 9:30am and worked our way down thru some great twisties, finally arriving at Bridgetown on Friday afternoon.  On Saturday we took the "long way" to Nannup via Manjimup, Northcliff and Pemberton ...about 250km.  Sunday morning we left Nannup and worked our way back home thru a lot of great back roads.  Finally arrived home about 4:30pm.  30 riders all up, 22 of which were CB750s of various series.  The Classic Motorcycle Riders Club of WA (of which I'm a member) organised the run.  Weather was outstanding...temps in the hi 20s (Celsius) each day!

Sounds like a good plan to diagnose. You should find it with that.

Beforehand suspecting the Dyna,  check the volts + to the coils carefully.  Usually one supply from loom and split to the coil pairs.

If the connect there is flaky it'll do this.  I'm using silicone grease from plumbers supply to keep joints like this from getting any corrosion in them.

Edit:- and obviously the trigger wire coming from Dyna to ensure it has good continuity to 1and4 coil. 

Good points, we'll take on board.  As luck would have it, it's raining today so we'll delay troubleshooting until tomorrow.

[Nurse Julie]

I remember Manjimup well, huge logging area if I remember correctly. Was driving from Perth down to Albany and Esperance, via the black stump 😀😀😀 Had a key snap in the fuel filler cap there, this was on a car, and the chap in the fuel station managed to get the broken piece out and even cut us a new key. Fantastic roads and a beautiful area of WA.

[mike the bike]

Check the sensors are tightly secured to the backplate.   If the sensor is loose it will overheat.  I'd use threadlock.

[AndyH]

Check the sensors are tightly secured to the backplate.   If the sensor is loose it will overheat.  I'd use threadlock.

Another good point Mike!  I'll make sure both sensors are tight tomorrow.

We checked out the wiring today and found that the black/white connection in the double-female connector to the 1-4 coil appeared somewhat loose.  We fired up the bike on the test fuel tank and I felt that the connector was getting a little warm!
Out with the emery to polish up both male bullets, and a pair of long-nose pliers to squeeze up the female side.  Connected everything up again, and no noticeable warmth from the connector.
Rode home with all 4 pots firing as per normal.
Some longer rides are planned (weather permitting) to see if the issue returns, but I'm confident that we've identified the problem.
 

[mike the bike]

Result 👍