Help! Big trip this weekend! Left cylinder misfire/backfire

Jmiller057

XS650 Enthusiast
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Location
Michigan
1976 xs650.
Stock points system

Left cylinder has always had this issue but I want it reliable for this weekends trip.
Left cylinder misfires and backfires a little when you rev it when let off. Right cylinder is dead on and never had an issue.

Cam chain good
Valves good
Points good
Carbs good

timing on the right cylinder is on. timing on left cylinder is weird. for the most part its on, but you can see the delay of the misfire every once in a while with the timing light. You can hear and feel the difference in pressure from the right cylinder muffler and the left cylinder muffler. Spark plug is obviously black.

less than 50 miles on top end rebuild.

Possible condenser issue? coil? how do I check the condenser?

PAMCO is not an option due to the departure time of the trip

Any help would be greatly appreciated!
 
You can watch the points in a darkened room, see if there's any significant arcing, which could indicate a bad condenser. If so, the associated contact point would need to be resurfaced or replaced...
 
1976 xs650.
Stock points system

Left cylinder has always had this issue but I want it reliable for this weekends trip.
Left cylinder misfires and backfires a little when you rev it when let off. Right cylinder is dead on and never had an issue.

Cam chain good
Valves good
Points good
Carbs good

timing on the right cylinder is on. timing on left cylinder is weird. for the most part its on, but you can see the delay of the misfire every once in a while with the timing light. You can hear and feel the difference in pressure from the right cylinder muffler and the left cylinder muffler. Spark plug is obviously black.

less than 50 miles on top end rebuild.

Possible condenser issue? coil? how do I check the condenser?

PAMCO is not an option due to the departure time of the trip

Any help would be greatly appreciated!

I would synchronize the carbs clean if needed and adjust floats. Symptoms of out of out of sync carbs unbalanced exhaust and backfire. The black plugs replace with new and check electrical connections sparkplug wires at coil and boots are a few places to look. :thumbsup:
 
I need help. I traced the ignition issue back to the points system. Left cylinder is not receiving a spark. What's causes this? Tested everything and traced it back to the points system. All wires are good with zero resistance.
 
I need help. I traced the ignition issue back to the points system. Left cylinder is not receiving a spark. What's causes this? Tested everything and traced it back to the points system. All wires are good with zero resistance.

You can watch the points in a darkened room, see if there's any significant arcing, which could indicate a bad condenser. If so, the associated contact point would need to be resurfaced or replaced...

Help!!!!!!
 
There are "brushes" that touch the lobe as it spins. One brush is not touching the lobe. Could this cause no spark?
 
Brushes are a negative. Brushes are supposed to come off the lobe just like points. Why isn't there a spark tho? Someone's gotta know
 
The left points ride on their own sub-plate. Might be a connectivity to ground issue there.

I'd recommend removal and disassembly of the entire points plate. All parts need to be clean/oil-free. The metal should be bright, not corroded. On reassembly, there should be no screw protrusions from the backside of the main plate, so note that the screws are different length and go in certain screwholes.

If screw protrusions have existed, there's a good chance that the points followers are worn crooked from the non-flat mounting of the main plate, requiring points replacement. The points gaps should be checked for arc/pitting, cleaned or replaced if so. The arc/pitting probably warrants condenser replacement.

Reassemble, install, gap-set, and time according to the manual.

Here's the scary part. An experienced mechanic can do this in less than an hour. If you've never serviced points systems before, depending on your skills and understanding, it could take days...
 
The left points ride on their own sub-plate. Might be a connectivity to ground issue there.

I'd recommend removal and disassembly of the entire points plate. All parts need to be clean/oil-free. The metal should be bright, not corroded. On reassembly, there should be no screw protrusions from the backside of the main plate, so note that the screws are different length and go in certain screwholes.

If screw protrusions have existed, there's a good chance that the points followers are worn crooked from the non-flat mounting of the main plate, requiring points replacement. The points gaps should be checked for arc/pitting, cleaned or replaced if so. The arc/pitting probably warrants condenser replacement.

Reassemble, install, gap-set, and time according to the manual.

Here's the scary part. An experienced mechanic can do this in less than an hour. If you've never serviced points systems before, depending on your skills and understanding, it could take days...

I took your advice and went into the points. Found out it was a grounding issue. I was a little to extreme when torquing down the subplate. Spark plug shows good spark now. But left cylinder is still acting up. Went back into valves and they are good to specs.

Compression tested the motor today. Both around 150. But the left cylinder takes 5+ more kicks than the right to get the compression all the way up? Thoughts? Valves? Fu*ked up cam? Poor carb tunes?

Cam chain is good btw.
 
The 'slow to come up to full compression' syndrome is often the rings. Squirting a little oil in the cylinder (to wet the rings), then do compression test, if compression improves, that confirms rings.

That could settle out by itself by running the bike, now that the left cylinder is firing...
 
I'm defeated and I've given up. Problem is in the carbs and I can't seem to figure it out. Left cylinder still back fires like a son of a bitch. I go over everything and when you first jump on it and ride, the bike is dead on with no issues. Once you ride for a mile. The bike just starts to back fire off the left cylinder and there is barely any pressure out of the left muffler. Just went to re-tune and fire it back up and the right cylinder back fired through the carbs so bad that it blew the right carb out of the boot. Both spark plugs are now fouled.
 
Well, that blows. Ruined weekend, and the more you work on it, the worse it gets.

Had a similar situation on my Toyota, long ago. Everytime I fixed or improved something on it, the worse it ran. Drove me nuts. Just kept at it. The fact that it kept getting worse was actually a clue to the real problem, just couldn't see or imagine it. It's still one of my favorite mechanical mystery stories.

That's the way it is with cantankerous machines. They're like a demanding girlfriend. Fun, if you love tinkering. Excruciating pain because they demand perfection, and if you don't have the patience, interest, or time due to schedule pressures.

Best thing I can recommend is a thorough and complete tune-up, avoiding frugality and replacing typical tune-up parts, and being meticulous about every detail.

The trap is in thinking that there's one single problem that, if solved, all problems disappear. While often true, it can lead to gyrating frustrations that take the fun out of this. Until you've acquired a lot of experience to recognize subtle tip-offs, it's best to acquire the experience of going thru everything.

That said, do your points 'arc' in the dark?
 
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