What does a strong spark look like?

[cowboyup3371]

After getting my hopes up this past week, and subsequently shot down very hard, that we found and fixed the problems with Charmayne's bike (the 550L in my signature), I'm back at trying to determine why the engine dies after sitting through a long light (after about 3 minutes). When it does die, it can be very difficult to restart immediately especially if the bike is already hot. Previous work repairs have already included:

Steve and I both cleaned the carbs thoroughly at least twice each in the last two years
I put a different set of coils on provided to me by Witttom with no change in the bike's responses
Spark plugs are only a couple of years old
Valve clearances are at the upper end of the limits (.08mm)
Spark plug caps replaced last year and roughly 1/4" of each wire was removed at the same time

Advice given last night was to check the spark on each wire which I did see but to me it was a faint orange. I have heard of people describing sparks as fat and blue so I'm not sure if what I'm seeing is correct. Could someone describe a proper spark?

Also, to add something while testing - the number 2 spark plug wire felt warmer to the touch than the other three.

[1978GS750E]

Are you using resistor caps and....... resistor plugs? You only need one or the other. I had a simular issue on a bike once and swapped the plugs to non-resistor type and cured it!

[cowboyup3371]

No, only resistor caps

[Big T]

Spark should be blue/white
Yellow is low voltage
Red is almost dead

[cowboyup3371]

Here's a video of the spark taken this morning with the bike being cold. It is very hard to try holding the phone and the plug in the same hand but I didn't have any of my helpers available today. Also, the editing software I tried using to remove some of the fumbling around is harder to understand than it looks. First plug is number 4 then I move to 3, 2, and finally 1

[cowboyup3371]

To go with that video, I also did some looking around on the forum and in the service manual. I decided I would review this about the signal generator from the manual page 364

SIGNAL GENERATOR
The signal generator is mounted on the right hand side of the engine in the area commonly used for the
contact breaker points. It is comprised of a magnet embedded rotor attached to a mechanical advance
mechanism and two pick-up coils, with iron plates at their bases, affixed to a plate. Each pick-up coil consists
of a coil or wire and a yoke or coil and is mounted 180° apart on the plate.
As the rotor magnet is turned past the coils, AC current is produced and used for switching within the
transistor unit.
The transistor unit controls power to the ignition coils and causes the spark plugs to fire at the proper time.

INSPECTION
Measure the resistance between lead wires.
If the resistance noted to show infinity or too low
a resistance value must be replaced.

S.T.D. resistance
G/W-B/W 60 to 80 ohms

These are my results to the signal generator

Between B/W and G/W - 70 ohms
Between B/W and Brown - 70 ohms


Based on that, I think the signal generator is good. I'm just not sure what else I can do to troubleshoot this problem beyond what we have done. Any suggestions?

[Nessism]

You got strong spark.

Have you performed a compression check?

[Baatfam]

After getting my hopes up this past week, and subsequently shot down very hard, that we found and fixed the problems with Charmayne's bike (the 550L in my signature), I'm back at trying to determine why the engine dies after sitting through a long light (after about 3 minutes). When it does die, it can be very difficult to restart immediately especially if the bike is already hot. Previous work repairs have already included:

Steve and I both cleaned the carbs thoroughly at least twice each in the last two years
I put a different set of coils on provided to me by Witttom with no change in the bike's responses
Spark plugs are only a couple of years old
Valve clearances are at the upper end of the limits (.08mm)
Spark plug caps replaced last year and roughly 1/4" of each wire was removed at the same time

Advice given last night was to check the spark on each wire which I did see but to me it was a faint orange. I have heard of people describing sparks as fat and blue so I'm not sure if what I'm seeing is correct. Could someone describe a proper spark?

Also, to add something while testing - the number 2 spark plug wire felt warmer to the touch than the other three.

Probably already checked, but I didn't see it on your list. Gas cap vent?

[cowboyup3371]

You got strong spark.

Have you performed a compression check?

No I have not as I don't have a gauge. A friend bought one from Harbor Freight recently and I'll ask if I can borrow it. May I ask how compression will affect running when the engine gets hot versus when it is cold?

Also, that spark video was taken this morning when the engine was cold. Last night, while hot, the spark looked more yellow than that; I wish I had thought to video it then.

Probably already checked, but I didn't see it on your list. Gas cap vent?

We have looked a few times over the last couple of years and didn't see anything blocking it.

[rphillips]

Spark is probably good while bike is running OK. May be harder to do, but the spark should be checked should be checked when it dies & won't start.

[cowboyup3371]

Ok, compression check is done and if these numbers are any indication I think we found at least a contributing factor. If this Harbor Freight Gauge is correct:

1 - 78
2 - 70
3 - 80
4 - 75


That was done with the bike hot - 7-10 mile ride plus idling in the garage while I removed my gear
Throttle open full
All plugs were out

Although it didn’t die on me in that short ride, I could hear a definite change in idle almost as though a cylinder was cutting out while it idled.

Just based on these numbers and knowing the service manual limit is 92, I think I’ll go ahead and start the rebuild process.

[Big T]

Most auto parts stores will "rent" you a compression tester kit.

I'd double check those numbers with that better gauge before tearing the motor apart.

[Nessism]

I'm surprised the bike even runs with those compression numbers. Not good. I agree with Big T about trying another gauge. You can't un take apart the engine once you crack it open.

Also, try a tablespoon of motor oil in each cylinder too. If the numbers jump up that points at rings.

[cowboyup3371]

I'm going to borrow Steve's oil squirter tomorrow to do so but either way I'll go into the engine anyway especially since either way (numbers increase = rings, no increase then valves) it means having to get inside. I just need to talk to the owner first about how/when we are going to do it since ultimately it is her call.

And honestly, even if the numbers jump up 20psi with another tester they are still well below where they should be so I won't worry about finding another one.

Thank you for the help and I'm hoping this is the root cause of her problem but I'm also prepared to continue troubleshooting if need be.

[chuck hahn]

Those are horrible compression numbers. Sure all the valves are within spec? If yes i would do a leak test on each jug. Remove the oil fill cap and slide the airbox off the carbs. 3 places to leak...rings which youll hear through the oil hole. intake valves which youll hear from a carb. And if nothing heard from crankcase or intake side the leak is from an exhaust valve...the obvious third spot.