What my timing light revealed

[748cc]

First, although I'd done a static timing adjust to my bike, it was still running like dookie. So I hooked up my timing light (a Sears/Penske inductive unit-the best there is in my book) and discovered that the two firing banks weren't in sync, with 2/3 firing a bit tardy from that of 1/4. In order to sync them, I had to use up the entire plate travel of 2/3...but it was still a smidge off. Had to retard 1/4 by a hair in order for the bank to sync up with 2/3 (which was still a scoatch advanced). A twiddle & a tweak and they're in sync as best as possible.

Second, while strobing cylinder No. 3 I discovered that its' firing was sparatic-3 flashes on, 5 hits no flash, 4 flashes, 2 hits nothing, 2 strobes, 3 nothing, etc, etc.

Immediately began assuming points/condenser isssues, so to affirm my thoughts I moved the timing light's inductor to cylinder No. 2 - and it was rock solid with a strobe at every hit. Back to cylinder No.3-still sparatic.

Tore down the plug cap (which removed from lead far too easily...hmmm), spring & resistor core looked a bit heat stressed, otherwise OK. Cut 1/4" off lead & re-attached plug cap with affirmation (!). Start up, strobe still sparatic. Pulled plug, looked dry/sooty; replaced with known good plug...and then ran out of time.

Question: Has or would a failing coil present these symptoms or can I rule it out? The HT leads appear OK but are original to the bike ('78 GS750E), as are the plugs & caps (points/condensers look new-ish). If I find the symptoms still there I'll swap out the cap next, but I'm kinda hell-bent over true originality wherever I can be including common-wear components.

Your thoughts greatly appreciated. TIA.

[Grimly]

Daft to retain forty-year-old parts that degrade with time.
Put new leads and caps on.
You wouldn't keep plugs for four decades.

[chuck hahn]

Do away with resistors. Get some 1/8 inch rod and cut a little longer than a resistor. Use the bench grinder to size the rods same as the resistors. Insert rods and springs 2 and 3 have springs and 1 and 4 dont). That PERMANENTLY eliminates ant resistor fails. Stretch the springs a little to so they make good contact.

[fbody_mike]

I would move coil 1/4 to the 2/3 position and see if the problem follows the coil.

[Steve]

I would move coil 1/4 to the 2/3 position and see if the problem follows the coil.

Easier said than done, on some coils.

Some coils have terminal posts for the primary wires, and it would be easy enough to swap the wires from one coil to the other, but there are also many coils out there that have the primary wires molded into the coil housing. Since the connectors are specific from one side to the other, you will either have to swap the connectors or make some adapters.

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[fbody_mike]

An old GS should just have bullet connectors, swap the primary wires probably white and black, and swap the secondary wires from 1<->2 and 3<->4. It is definitely the next thing I would try.

[Steve]

I have not worked on a '78 for quite some time, but the '80-and-up bikes that I usually work on have two-pin connectors for the coils. One side will have the male pins on the coil, the other side will have the female pins on the coil, just so you don't mix them up.

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[Big T]

I'd be checking the vacuum line for the petcock to see if the diaphragm is bad and leaking additional fuel into cylinder #3
Just take the line off the petcock and plug it with a bolt or something, then run the bike on Prime

[850 Combat]

Easier said than done, on some coils.

Some coils have terminal posts for the primary wires, and it would be easy enough to swap the wires from one coil to the other, but there are also many coils out there that have the primary wires molded into the coil housing. Since the connectors are specific from one side to the other, you will either have to swap the connectors or make some adapters.

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For what its worth, I easily did this on a frr '79 GS 850 I got to determine where the problem was. Isolated it to a bad condenser, as the problem followed the points, not the coil. Plenty of slack to do it on that bike. I still like points.