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This has me scratching my head...the intermittent puncture.
Roval Rim - Tyre holds pressure when not being ridden. I've been over it with soapy bubbles, tyre off the rim, full check of tape (which looks fine), valve seems fine, grid casing tyre. No apparent leaks around the spoke holes.
Halfway around a ride, it goes flat! Pumped up mid ride, and sometimes stays up. Sometimes doesn't. Argh.
Should I just put new tape on and a new valve / start again? Anything I could be missing?
Sealant?
If it's holding pressure when not being ridden then you'll not find the issue with soapy bubbles etc.
Is it burping when you ride? You might see sealant witness marks on the tyre walls if so.
Is there any sealant at all in it? For it to go flat that quickly would need air to be escaping at a reasonable rate, so should be some hissing or squirting of the stuff going on.
Might also be worth pumping back up and riding the same bit of trail again if it goes mid ride. Then remember if you took a turn quick or rattled over rocky terrain etc. Try to find a pattern, which when combined with the pressure you run at, tyre size and your weight should give a start point for maybe more air in the first instance.
I'd start from scratch on this, so remove the tyre, sealant, valve and tape. Pick up the Roval specific rim tapes and then re set it back up.
I used to have issues like this but after using the roval rim strips and some bigger nuts for the valve I've been issue free for years.
It’s got fresh sealant and no evidence of it leaking at the beads.
I’ll go for new tape and valve nuts I think
I have seen a puncture that only leaked when the tyre was deformed a certain way, I was able to replicate it by pushing the tyre onto the edge of the workbench and it would leak, ease off and it sealed. I assume it leaked when that spot went over a root etc.
If you are using a fairly thick sealant, try using a thinner more liquid sealant. As other person has said flexing of tyre may open up a hole and it may bein a place where current sealant is not reaching.
£1 says your valve isn't sealing against the hope properly.