Very Strange flat today.
 

I have a pair of Mavic MA40 wheels laced to Shimano 600 hubs that I thought I would put tires on and keep for spares. I had a pair of Vittoria Rubino Pro 23c tires that I had taken off my daughters bike so I mounted them with some used tubes. Everything looked good so I went for a ride. 3 miles out I heard the pop and felt the thump from the rear. I checked and the tire was still up. I rode a little further and the thump wasn't the road so I looked closer and the bead had popped out in a little area. Funny thing is the tire was still up. I rode to a safe spot and had my wife come get me. I got home and the tire was still inflated so I let most of the air out and pushed the bead back in. When I did the rest of the air wooshed out and there was a 1" split in the tube. I guess it was pinched and the rim was holding it together enough to keep the tire going going flat. In those 3 miles I didn't feel like the wheels were near as heavy as I thought. Picking them up they feel a lot heavier than my Araya CTL 370 rims do. I think I'll give them another try.

The Arayas are much easier to mount tires on. Otherwise I'm a fan of both those rims

Perhaps the tube got folded over on itself during installation. If the folded section was pinched off by tire pressure, it could pop and not deflate the rest of the tire.

I had a bead pop off on a modern carbon rim. I had a flat in the parking lot before a Club ride and did a hurry up change. Couple of miles down the road after a slow climb, on a 30 mph descent, thump thump thump. I stopped and my heart almost stopped, 5 or 6 inches of bead were outside the rim. Disc brakes, so no brake rub. The tube was still inflated, but I had to waste a CO2, to deflate and re-inflate after re-seating the rim. Don't know what happened, but never seen that happen again, thankfully. Sounds almost like yours. I recently acquired a barn find Legnano, with 20 year old tires that still held air, sort of. When I put newer used ones on (for the $100 clunker challenge), I found that of the two Araya rims, one was hook bead rim and one was not. My first time with an un-hooked bead rim. So far so good. I'm a big guy, so I routinely run around 100 psi. 🤞

Always, always add a bit of air to the innertube before installing it.

A "plumped" tube is actually quite hard to get folded, pinched, bunched or twisted as compared to installing a flattened tube, and I've seen more than enough cases of a trapped tube resulting in a dislodged tire bead.

I actually prefer to install the tube in the tire before even fitting it to the rim, but putting the tube after one bead is in the rim can work as well if some care is taken to prevent twisting it as it gets stuffed in.

Note that some air may need to be bled out of the tube as the last portion of the tire bead is being worked into the rim, this allows the tire beads to settle into the center of the rim where more slack in the beads can be created.

The valve should always be pushed into the rim just prior to inflation, and the tire moved about the rim sufficient to make the valve point to the hub center, again before inflation.

The Araya rims are somewhat notorious for not trapping the tire bead very effectively upon full inflation (made worse by aged tires that have less in the way of supple/grippy rubber covering the tire beads).

Get a Kool Stop bead jack if you don't already have one. Much easier to mount tight tires with less risk of pinching tubes or bead seating failure. I even tote my KS bead jack on rides. It's about the size of a mini pump but weighs little. I don't even notice it in my jersey pocket. Makes it much quicker and easier to fix flats with cold and/or wet hands too.

My flat repairs take longer than others because I've experienced one blowout at home when a poorly seated bead popped out. Now after mounting and partially inflating a tire I'll roll it, pressing down with my body weight a full rotation, then flip the wheel and repeat. Helps ensure the valve is centered, the bead is seated, and I can inspect the sidewalls during this ritual.

Probably unnecessarily cautious but so far, so good.

...the only Araya rims I've ever owned or worked on have had either no bead edge or an insubstantial bump that is inadequate to my needs for higher tyre pressures in modern clincher tyres. So I've gradually replaced them with more modern rims laced to the original hubs, or complete wheel replacements.

No problems here with tires on the Araya CTL-370 rims that came with some Ironman models. I've used Vittoria Zaffiro and older Specialized wire beads; and folders in Schwalbe One V-Guard, Continental Ultra Sport 2 in 700x23 and 700x25. The Schwalbes and Contis fit extremely tightly so the Kool Stop bead jack was a must.

Wolber Alpine rims look superficially similar but the fit isn't quite as tight as the Arayas. Ditto Alex S500 rims -- snug but not as tight as the Arayas.

^ All this, plus...

And after finally getting the last bit of bead snap-rolled on, before doing anything else, before even setting the wheel down for a rest, I go around the entire circumference of the tire, pinching in the sidewalls to make sure there is no little roll of tube protruding beyond the tire bead anywhere. I, too, find I usually had to let some air out of the "plumped" (great word!) tube in order to get the last bit of bead on, and that's typically when and where the tube gets trapped. It must be completely retracted up between the tire beads and therefore invisible all the way round. All you should be able to see is rim strip. It's most important to perform this check on the side of the tire you put on last, i.e., the same side you just finished. Someone asked me once if I spin the wheel over and check the other side, too. I had to think for a minute and then decided that No I don't. But my usual drill is to put one bead on, then insert the tube, then have at the second bead.

If I find even a sliver of tube visible, I first try to jostle and jiggle it back under the tire bead. This rarely works, because the bead is trapping the tube. So the next step is to carefully elevate the tire bead with two tire levers, one on either side of the visible tube portion, but not touching the tube. Two tire levers that close together may not fit under the bead so one or both may need to be a small flat screwdriver. If there is any air in the tube, this may be enough to allow it to retract under the bead. If not, leave the tools hooked under the bead and ask your assistant (that's the great thing about riding a tandem -- you always have an assistant :)) to pump a few slow strokes of air into the tube while you maintain pressure on the levers to keep the bead off the tube. This will do the trick. Then repeat the circumferential check. (It strikes me that it might be more efficient to re-plump the tube before doing this check, so that elevation of the bead will allow a protruding segment to snap back spontaneously....but air will make everything just a little tighter and it might be impossible to get a tool under the bead to elevate it in the first place. Dunno...)

My wife has the knack for using a tire lever to mount a tire without pinching the tube. I was much impressed when she showed it to me... and it was the inspiration for my Johnny Carson tag-line.

After partial inflation (to 15 psi or so) I wrestle with the bead as per Canklecat to make sure it is seating.