Motorcycle Repair: bad spark, vt 500, eratic

Question
85 vt 500 checked timing with light and when engine is reved timing light is eratic and cuts out. coils are good, new high and idle jets, carbs are synced when at idle runs good just on rev changed out spark unit with one from wrecking yard and same problem isthere something else or did i just get a bad used unit? thanks for the help. richard

Answer
Richard, there are three parts to the ignition system.  The pulse coils, the ignition modules and the coils.  The coils have to be well grounded to get them to work.  You can check the pulse coils with a VOM.  They should have about 3.4 ohms of resistance.  If those check out it has to be the coils or the ignition modules.  There aren't any checks you can make at home so it's more or less trial and error.

Regards
Rich