Technical Q and A for classic motorcycle maintenance and repair.
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1977 Yamaha XS750 Electrical Problems
Q: I own a 1977 Yamaha XS750 with about 25,000 miles on it. For the past two years the bike has gotten more and more stubborn about starting, and the idle is getting rougher, too. The starter turns the engine just fine, but it won’t start the bike. However, when I kick it, it starts right away. Until it is fully warm, it will idle at 500rpm, stuttering and stalling unless I keep the rpm’s up with the throttle. Once warm, it will idle at 1,100rpm, but if left alone for a couple of minutes it will slowly stumble and stall. In your July/August column, I read about an issue with the reserve lighting on a XS750, and it gave me an idea to try turning off the lights before starting my bike. Sure enough, with the lights off, the starter starts the bike, and it idles much better (cold and warm).
It still is not perfect. Kick starting the bike usually takes four to six kicks when cold, and one kick when warm (lights on or off). Even with the lights off, the starter takes a couple dozen rotations, with me feathering the throttle to coax it to start. It also takes forever to warm up, even on an 80 degree day. Lastly, with the lights off it does idle better when warm, but it will still slowly stumble and stall. It just takes longer for that process to run its course. It seems like whatever the issue is, the lights exacerbate it. The bike runs great on the road, and the battery seems to be getting charged. Whenever I stop at a light or stop sign I am always anxious about the bike stalling. Here is what I did over the past winter to try to improve this starting and idling issue:
1. Installed an original exhaust system, replacing an old aftermarket system
2. Rebuilt the carbs, installed factory-spec jets
3. Installed new points and condensers, plugs, wires and coils
4. Timed the ignition
5. Synchronized the carbs
6. Checked the compression
7. Cleaned the air filter
8. Replaced the battery
9. Replaced the starter button. Cleaned all the ground connections in the switch housing (the starter button grounds to the handlebars)
10. Cleaned the main ground to the frame
While these projects surely improved the bike’s overall health, they did not help my starting and idling issue. I suspect I am losing voltage somewhere, and I am very intimidated to start fussing with the electrical harness without knowing for certain that the problem is indeed electrical. Any suggestions would be most appreciated! - Joe Audia/Petoskey, Mich.
A: You’ve certainly covered most of the things I would suspect as the cause of your problems. Your Yamaha XS750 seems to be presenting a mix of fuel and electrical problems, and that can be very difficult to troubleshoot. About the only thing you haven’t changed on the electrical side is the starter solenoid. I’ve worked on several older bikes recently with starting problems, and the solenoid has been the common issue. The starter solenoid is a heavy-duty relay that uses a small current from the battery to control the very large current needed to spin the starter motor. The high current needs of the starter motor causes arcing across the contacts of the solenoid, building up carbon deposits. Those deposits cause resistance, reducing the voltage to the starter and causing it to draw down the voltage available to the ignition circuit. That might explain the difficulty starting on the button. The idle problem still seems to me to be fuel related, since you say the bike runs well at road speeds. Does your Yamaha XS750 have a vacuum-operated petcock? If it does, try running the bike in the “Prime” position and see if the behavior changes. Prime bypasses the vacuum diaphragm in the petcock and lets gas flow regardless of the engine vacuum. Engine vacuum should be highest at idle, but those vacuum petcocks find many ways to malfunction. If that doesn’t solve the problem I’m afraid you’ll need to tear into the carbs again, paying close attention to the idle circuit. MC