Technical Q and A for classic motorcycle maintenance and repair.
Editor’s note: If you’re having trouble with that old Suzuki, BSA or BMW, Keith Fellenstein is your guy. From motorcycle tuning tips to detailed motorcycle engine repair, he can draw from a wealth of experience to help guide you to success. Send questions to: Keith’s Garage, Motorcycle Classics, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as the subject.
Q: I’m having trouble getting spark at the plugs on my 1980 Suzuki GS850. At first I wasn’t getting any spark, so I replaced the igniter unit. It then had spark, but when I installed the carburetors and tried again, there was no spark. What should I do? — Tyler Hubenthal/via email
A: We’ll consider the igniter a good part. Get your volt-ohm meter (VOM) and check from the battery to the igniter for voltage. From the battery, there should be two wires: one to the starter relay and a second, smaller lead to the fuse box. Follow the smaller lead to the first junction, disconnect and test for 12 volts from the battery. If good, reconnect and follow that lead to the fuse box. Test the 15-amp circuit on both sides of the fuse for 12 volts. That leads to a multi-pin connector that leads to the ignition switch. If I can trust the colors on the wiring diagram I have, the 12-volt lead into the switch is red, out is orange. Orange leads back to the fuse box where it branches to feed three 10-amp circuits. The one we’re interested in looks to me like orange/white. Test both sides of the fuse for 12 volts with the ignition switch on. Next is the emergency stop, or kill switch. Test for 12 volts on the orange/white wire leading in and out of the switch. Now check the orange/white leads at the coils for 12 volts with the ignition switch on. Now we end up at the igniter. Test the orange/white lead there for 12 volts with the ignition switch on. Assuming you find 12 volts or slightly less at each of these test points, switch ranges on the VOM and test the signal generator that triggers the igniter module. You’ll find the connector under the left side cover. Put the VOM in resistance testing mode, set to 100-ohm range if not auto ranging and put one probe on the blue lead and the other on the green lead after disconnecting them from the igniter module. Look for a reading of between 250-350 ohms. If very low (0) or higher than 350, the signal generator is bad and needs to be replaced. Finally, pull number 1 and 2 spark plugs and rest them, connected to their high-voltage leads, on the cylinder to ground them. Set your VOM in resistance test mode to the 1 ohm range. With the ignition switch on, touch the plus probe to the blue lead, and the minus probe to the green lead on the harness side of the interconnect for the igniter. Number 1 should spark when you connect the second lead, and number 2 should spark when you disconnect it.