Voltage drops when rev'ing engine

I currently have a special guest in my garage. I successfully converted a co-worker over to the dark side, and he is now the proud owner of a vintage KZ (550-C3).

It is in rough, but servicable condition, and I offered to help him get it whipped into shape.

Last night when he brought it over, I started it up. The battery was dead, so I started it with a charger hooked up.

While it was running, I decided to check the charging system. While at idle, voltage at the battery reads 12.7v. When I rev'd up to 4000 rpms, the voltage dropped to about 8-9 volts.

This isn't normal, right? :blink:

Any idea what may cause this?

1) Make sure the volt reading is true. We had a long thread about a guy with a digital voltmeter getting wierd readings when he revved the engine (caused by EMI). If you have a needle type voltmeter, use that.

ALSO: check for a bad ground on the battery or alternator high current paths.

2) If the system voltage actually dips as stated, the charging system is clearly dead.

If true reading, the battery must be really weak since a good battery will not dip that much unless cranking the starter. Revving the engine is a tiny load compared to that current draw. I suspect the readings are not accurate.