Help! Bike bogging down bad out of nowhere - ignition coil? carbs?


XS650 New Member
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richmond, va
Hey y'all! I have a stock 1975 xs650 that has run perfectly for the past few months. Synced perfectly, low mileage, no rust, beautiful bike. Can ride it all day long with no issues. The other day I rode it for a few hours, let it cool down and gave it its first oil change in a while, and then rode out again. About 20 minutes into the ride the bike just kept cutting out randomly - it would start, ride for about a block and then bog out and die at idle. It started sounding real dirty. Spark plugs looked good when I pulled them, and was getting spark from both cyls. When I checked the battery a few times (thought it was a bad battery at first), it sparked when my tools made contact with the frame while on the screw (the ignition was definitely off).

I let it sit for a few days and then went to go pick it up. I had taken the battery and charged it - battery was testing with the correct voltage both off and while revving (pretty much a brand new AGM battery). Bike was starting but bogging like crazy, sounding super rough, making it about a block or so at a time. Put my hands back at the exhausts and the left side was putting out way less pressure than the right - like way, way less. Spark plugs were then both looking bad - one carbon fouled and one oil. It's now still bogging and dying quickly, also now when I try to give it gas it kinda skips a beat and sometimes dies. Really hard for me to believe that it could have come out of sync somehow.

Does anybody have any idea what's going on here? I immediately thought that the coil fried, going to get to it later this week to test the coil and see what's going on there. It's acting like it's got massively dirty carbs but I'm really confused as to how it could go from running perfectly to like this out of nowhere. Also can't figure out how it would be possible for the oil change to be related but it was the only change that was made to the bike.

Any advice that you have would be great! I'm going to clean carbs and want to switch over to an electronic ignition anyway, but if you have any other advice I'd love to hear it. Thank you!
Last time I wrenched on my bike I must have knocked a carb loose from the boot. It started running on one cylinder. :)
Do your lights work? Hi and low beam. Do they dim a little at idle, then brighten up with revs. Turn signals work, brake light work? All a good indication that the system is mostly good. Swap plug wires. If the left is still dead it's carbs or an internal failure. Likely fuel related.
If the problem switches sides it's ignition.
Heard that! It's definitely still running off of both cyls, just weakly on the left side.

Lights work well, they brighten with revs, all signals work. Thank you for the tips! I'll look at those plug wires.
Long, long ago, when I was much younger.. had a problem just like you describe.
Cold start and first kilometres went fine, after that it started bogging as soon as it got more than just a hair of throttle.
In the end, the conclusion was, it was a weak coil (at the time, I had no idea those OEM coils were not the greatest), losing sparks once it warmed up.
Before I realised that, I already pulled out the engine and stripped it bare.
Do swap the plug wires, like tW suggested, before looking elsewhere.
Swapping the wire --please google how it is done being a points bike
I would first of all just to be sure measure charging voltage across battery mostly because it is simple to do
Check the fuse holder
Check the kill switch
Can be the ignition lock
One can also open the cover over the points look if there is sparking over the points while running
And measure Voltage to Points. if no voltmeter use a test lamp.
I don't believe it is carburetor problem ,, I would expect one side running if so.
I would first check if one only cylinder is bogging or both. Once the engine started, remove alternatively one plug cap and see what happens.
@townesvanzandt Welcome to forum, I'm a huge Townes Van Zandt fan and Townes is one my favorite Steve Earle albums. Your issue could be either carbs or ignition but it sounds like points to me. I would as recommended above swap the plug wires and see if that makes any difference. Could be a bad condenser acting up when hot.