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Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.

BSA Firebird Scrambler Diagnosis

Motorcycle Classics tech expert Keith Fellenstein

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, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as the subject.

BSA diagnosis

Q: I recently purchased a 1968 BSA Firebird Scrambler. It took many tries to finally get it started. When it finally did start, there was a loud backfire through the mufflers. The next kick got it started for good, but it wouldn’t idle down very far. Thinking it was probably stale gasoline, I rode it for a few blocks to check out the clutch, brakes and transmission. All seemed fine, except for a tendency to die when the throttle was released. Long story short, it died right after I returned.

The next day, I tried starting it again. It wouldn’t fire. Thinking it was the battery, which was very weak, I installed a new battery with a full charge. Still, there was no spark at either plug when cranking it with the ignition switch on. I removed the points cover to see if there was anything obvious that would cause this. Much to my surprise, it had an electronic ignition. I don’t know the brand, but I’m guessing it’s a Boyer. Now I’m really stumped. I know it must have been properly installed as the engine did start and run. Where would you suggest I start in diagnosing the problem? For what it’s worth, the white wire to the Zener diode is disconnected. The diode is mounted to the frame under the fuel tank, but the wiring schematic shows the white wire is not used on the 1968 Firebird Scrambler model. The polarity is correct, with a positive ground, unlike many that have been wired incorrectly. Any help you might be able to give will be greatly appreciated. There aren’t any BSA mechanics in my area! — John Botts/Ponca City, Oklahoma

A: You can easily test for spark with a Boyer. First, pull a spark plug and lay it on the cylinder head to ground it. Next, disconnect the trigger wires from the unit under the points plate. With the ignition on, touch the ends together. Every time you touch the trigger wires together you should get a spark. As for the wire to the Zener being disconnected, look around under the tank or seat to see if someone installed a different regulator/rectifier like a Podtronics. If you have one of those installed you disconnect the Zener diode, as the Podtronics device does the work of both the rectifier and the voltage regulator. I’d also check to make sure the rotor under the pickup plate is tight on the camshaft end. If it is loose your timing becomes erratic at best and your bike becomes unstartable at worst. MC