Motorcycle Repair: no start 1980 cb900c, voltage circuit, voltage output

Question
My bike seemed to run aright but as far as the rumors I'm not super impressed. Wile riding last summer it started missing to bad to stay running. So I changed the plugs but only get week spark on all 4 cylinders and still won't start. I have a prepare manual and can figure out most tests but for spark-unit it says substitute with known good part. Any suggestions appreciated.

Answer
Scott, I assume the battery is fully charged.  There are only two things that can cause this.  The coils could be internally shorted or have a bad ground or the CDI units could be going bad.  If the coils don't have a good ground it increases the resistance across the low voltage circuit reducing the high voltage output.

Regards
Rich