Weak Ignition

[drcoopster]

It seems like I'm getting a weak spark in all four cylinders. The coils are original ('81 GS550L) and the battery is new. Is the correct remedy to replace the coils?

-Ian

[Dink]

The correct thing would be to clean ALL connections and trace all wiring to find any corroded wires, NO replacement OR stock coil will work correctly without a sufficient voltage supply.
A replacement coil may however mask it better as they usually have a higher amplification value than stock, meaning if both coils sets are firing a bit weakly the replacement coil will still be giving a "fatter" spark, hence somewhat "masking" the problem.
Dink

[chef1366]

I traced a weak spark recently to a corroded positive battery cable and a bad connection on the main fuse. Cleaned the fuse clamps with electrode cleaner and replaced the wire and good as new. On these old bikes 90 percent of electrical problems are bad connections.

[SqDancerLynn1]

If you suspect a low voltage CHECK the ign switch. I have seen them with overheated connections loosing 1-2 volts due to the resistance of the switch

[drcoopster]

Is there a way that I can measure the voltage that is getting to the primary windings of the coil? That would help me determine whether or not there's a fault elsewhere.

-Ian

[Dink]

Yes turn the ignition and probe the wires leading to the coils.
Dink

[drcoopster]

Thanks, Dink. I'd tried testing the voltage there before but I had neglected to flip on the ignition so I wasn't getting anything. So the results are in:

Coil primary resistance (same for both coils): 4.04 ohms
Voltage at battery with ignition on: 12.55 V
Voltage at coil with ignition on: 11.99 V

That all seems okay; maybe the coil resistance is a tad high but I don't know what the spec really is (I'm assuming around 3 ohms). A 0.55 V loss over the wiring seems like it wouldn't be much of a problem.

Here's the kicker though (I don't think this is right): when I run the bike, as the RPMs go up, the system voltage (measured at the battery) goes down. Last time I checked, the system voltage should stay constant around 14 V, especially with all of the accessories off (headlight, blinkers, brake/tail lights). I'm not killing batteries, though, I've ridden probably 500+ miles on the last battery (which was a flaky one to begin with) and it would usually start with the electric start, and always start if I tried to bump-start it.

Any ideas?

[nabrams]

You've got a R/R and/or stator and/or electrical connection (for charging system) problem. R/R problem is VERY common for the GS - see other posts regarding this. Go to the stator-papers article in the garage section of this site. If you're not familiar with the usage of a multi-meter, now's the time to learn.