Man, just as I thought I had everything fixed...

[ThrottleJunky]

...I took her down the road and she died on me. She had been running absolutely beautifully- the best ever. I couldn't tell if it was fuel or electrical, but my gut says electrical. She just sputtered and then went kaput. I switched to prime on the petcocks, then reserve, but none of that mattered. I got her started but she was sputtering and I barely made it back home.

After she sat for 10 minutes, she started right back up and was purring like a kitten. One thing that was weird - the headlight went off when she first died, then did not come on for the ride home until I turned into my driveway. The battery is brand new.

I still have a few loose wires for the front turn signals and horns, but I have the rear turn signals and brake light all hooked up. I've fixed so many problems on this bike I'm not sure what else could go wrong.

[Blank Slate]

your title is something i say on a daily basis

[3Phase]

If it does it again, pop open the fuel tank cap and if you hear a hiss from air rushing into the tank then fuel cap vent is plugged. The fuel can't get out of the tank and the engine runs out of gas until the pressure equalizes and fuel can get into the carburetors again, then the engine will start and run like nothing's wrong until it pulls vacuum again and runs out of gas.

If the headlight doesn't turn on, the headlight fuse, fuse clips in the fuse box, or the headlight relay might be bad but that shouldn't have anything to do with the ignition.

[ThrottleJunky]

If it does it again, pop open the fuel tank cap and if you hear a hiss from air rushing into the tank then fuel cap vent is plugged. The fuel can't get out of the tank and the engine runs out of gas until the pressure equalizes and fuel can get into the carburetors again, then the engine will start and run like nothing's wrong until it pulls vacuum again and runs out of gas.

If the headlight doesn't turn on, the headlight fuse, fuse clips in the fuse box, or the headlight relay might be bad but that shouldn't have anything to do with the ignition.

This is interesting. I have noticed a "hissing" sound from the tank cap area after the bike is shut down. I'll need to research how to unclog the vent.

[3Phase]

This is interesting. I have noticed a "hissing" sound from the tank cap area after the bike is shut down. I'll need to research how to unclog the vent.

Yeah, it's the same as holding your thumb on the end of a drinking straw and when you move your thumb, the liquid can run out of the straw.

The filler cap vents through a small hole in the bottom of the latch body. Inside the body, there's a small disk that's a one-way valve to keep vapor and fuel inside the tank but it unseats to allow air into the tank as the fuel level drops. You can disassemble the cap and clean/repair the vent or get some carburetor cleaner spray, then use the straw to blow it through the vent hole.

Some of the cap vents have a small spring to hold the vent disk in place but some use a piece of foam rubber. The spring-loaded vents can be cleaned pretty well with the carburetor cleaner. If the cap used foam rubber, the foam crumbles from old age. The unsupported vent disk will drop down and cover the vent hole and then it works backward: vapor pressure and fuel can go out of the tank but air can't in. Carburetor cleaner might help but it'll plug up again until you take apart the filler cap and replace the foam rubber.

[ThrottleJunky]

Yeah, it's the same as holding your thumb on the end of a drinking straw and when you move your thumb, the liquid can run out of the straw.

The filler cap vents through a small hole in the bottom of the latch body. Inside the body, there's a small disk that's a one-way valve to keep vapor and fuel inside the tank but it unseats to allow air into the tank as the fuel level drops. You can disassemble the cap and clean/repair the vent or get some carburetor cleaner spray, then use the straw to blow it through the vent hole.

Some of the cap vents have a small spring to hold the vent disk in place but some use a piece of foam rubber. The spring-loaded vents can be cleaned pretty well with the carburetor cleaner. If the cap used foam rubber, the foam crumbles from old age. The unsupported vent disk will drop down and cover the vent hole and then it works backward: vapor pressure and fuel can go out of the tank but air can't in. Carburetor cleaner might help but it'll plug up again until you take apart the filler cap and replace the foam rubber.

So, I took my cap all apart and there is a small disc in there but no spring or anything. On the rubber grommet there are two holes, a large and small. The small one was completely blocked. I cleaned it out. Like an idiot I didn't suck on the vent line from the tank before I cleaned it. Sure wish I would have.

As viewed upside down, the disc you speak of was sitting in a concave area which perfectly fits it, and the holes in the center match up perfectly. The only spring is the large spring which is what you feel when you close the cap.

At any rate, I put it all back together and it's running absolutely perfectly, again. I am hesitant to take it far, however, because I have a few hills around here that would not be pleasant to push a 500 lb bike up.

[3Phase]

As long as the vent works now -- great!

If you still have the stock fuse block, double check the Main 30A fuse and the 10A Ignition fuse to make sure they're good and the fuse clips aren't loose. Be careful, the metal clips get brittle from age and heat cycling and they'll break, then you have to replace the fuse block.

[ThrottleJunky]

Appreciate the help. It's running fantastically again. I'm a little nervous to go very far on it because I don't want to get stranded, but after unplugging that hole in the rubber grommet, all seems well.

[skids]

Appreciate the help. It's running fantastically again. I'm a little nervous to go very far on it because I don't want to get stranded, but after unplugging that hole in the rubber grommet, all seems well.

Well, if is running good, maybe you fixed it. I am wondering about the headlight going off. A charging issue can cause that. There are white wires (3 with and one "yellow" that looks white to me), located behind the fuse panel. They are from the alternator if you have an XS, not an XJ. The plastic connector can get damaged from arcing. The headlight runs from one of those wires. The voltage/current from the alternator, if shorted, may cause the bike to run off the battery until it drops below 10.2 volts or so, then the Black box ceases to provide ignition. At least this was a diagnosis on my 78 once, and I fixed it.