Greetings, salutations, and troubleshooting.

[CaptCardboard]

Hey kids!
Been lurking on this forum for a couple months now soaking up knowledge about these sexy machines. I've been wanting to dig my toes back into the murky waters of vintage bike ownership now that I'm no longer depending on my bike for daily commuting and grocery acquisition. Several years ago I endured a failed love affair with an old '73 CL350 that never ran right for more than a week at a time and left me stranded for work on numerous occasions. I never got to the bottom of getting that bike even remotely reliable. My inexperience and shallowness of pocketbook didn't help matters. I'm in a better place now, and after nights of extensive drunken googling I determined an XS650 would be a most excellent bike to tinker, toil, and maybe even enjoy a few extended periods of riding between catastrophic repairs.

A few weeks ago I picked up this running '75 for a fair price. I've been wanting to introduce myself and my new machine to this group, but couldn't find the words until developed my first problem!
I've just been riding to work and back since I got it. Roughly 12 miles a day, and maybe only half a dozen trips since I bought it. Two days ago on my way to work, the bike stalled and wouldn't start until I flipped the petcocks to reserve. I immediately went to get refill my tank, but only filled two gallons. I assumed maybe that's just where the reserve starts on these bikes? I rode the rest of the way to work and home that evening without issue. I rode to work again this morning without a hint of distress, but after just a couple blocks of riding along on my way home it was like my power was cut in half. Decelerating incurred a lot of loud popping backfiring. I pulled over and the bike idled super low, maybe 6-700rpm, for a few seconds until it stalled. Repeated kicks yielded little reward and wouldn't fire up again, until I set my petcocks back to reserve. Then the bike fired right up and I rode home just like nothing happened.
I question if my two little bits of trouble are related to the same issue, and of course what that issue is. Could I have a petcock issue that's stopping fuel from getting to one or both cylinders? It looks like the interior of my fuel tank has been sealed at one point. Could that sealant be failing and intermittently clogging up my petcocks? How could I know? Am I chasing the wrong tail?
I have ideas of possible solutions, but I would greatly appreciate an order of operations of a sort to check and diagnose such an issue.
I look forward to learning what you folks have to say about this, and hopefully contributing my own wisdom beyond star trek facts and opinions on cheap bourbon.
Thanks!
Also. Look at it. Ain't it grand?

 

[Downeaster]

I'd say you're on the right track. There are several threads here on tank sealant failure. Your symptoms are consistent with plugged gas lines.

Chances are the carbs will need to be disassembled and cleaned as well. Once that crud gets in the system, it gets everywhere.

 

[CaptCardboard]

I'd say you're on the right track. There are several threads here on tank sealant failure. Your symptoms are consistent with plugged gas lines.

Chances are the carbs will need to be disassembled and cleaned as well. Once that crud gets in the system, it gets everywhere.

I had assumed a carb rebuild was in my future regardless, but figured it wasn't a huge issue since it runs quite well between hiccups.

 

[Adamc]

Very nice bike @CaptCardboard; and welcome to the forum.
I chased a similar set of symptoms last year, following a full rebuild and rewire.
Took the carbs off umpteen times cleaning and resetting everything.
Then the wise ones here said check electrics first.
Bike would start and idle nicely; then would run for a few miles really well; then the left cylinder would backfire / half power/ stall.
Checked Spark / timing / compression / valves, all good; until a few miles and Pop Bang Wallop again.
Changed the new spark plugs for another set..... Problem seems to have gone away.
The moral is: Check ignition / sparks and timing first. Then look at fueling once all is good on the electrical side.

 

[Lou_TX]

Howdy from Texas and welcome to the forum.
Plenty of good information and great folks here. I myself am a newcomer but feel welcome by everyone. Good club.
And a very nice bike!

 

[CaptCardboard]

Thanks for that. Tomorrow is my day off and I plan to give this beast a spa day. I'll pull plugs and see how they look. Hopefully I can find a good YouTube video explaining points adjustment. It's been a long time since I've messed with points.

I've done some searching about tank liners, but am still unsure how to determine if the tank liner is failing. There's a small piece of liner that's a little loose at the filler neck, but it's quite rubbery and doesn't seem to pull away with much effort. Of course I'm not trying to pull it off either though!

 

[Jim]

Hopefully I can find a good YouTube video explaining points adjustment.