28mm tubeless wheels are not sealing on 18mm ID/25mm OD carbon wheelset

[noodleshop]

Hey there everyone, I recently picked up a pair of hooked carbon wheels with an 18mm internal/25mm external width rim. I've been trying to set them up tubeless with 28mm Continental GP5000 TLs but for the life of me they will not seal, and keep leaking air from the tyre around the valve area.

Lightcarbon have told me these rims are tubeless ready but the LBS is suggesting that this is not the case. They believe that due to the narrow internal diameter, when the tyre is inflated, it is lifting the valve out of place, causing the leak. Has anyone had this issue before? If so can you comment on how to fix this? I am going crazy trying to fix this, any help is appreciated! Thank you very much!!

[carbonazza]

Did you try with a valve with a smaller end ?
I use mainly these https://www.aliexpress.com/item/32989510427.html
Maybe your sealant is not the best.

Check as well your tape around the valve hole, as sometimes it is cut somewhere.
In doubt, clean and retape 10cm around the valve and try again.

[svanimpe]

I had the exact same problem a few weeks ago. In my case, it occurred after swapping valves, after which the hole in the rim tape was probably too large or too loose, and it leaked air around the valve. Fresh rim tape solved the problem.

For what it's worth, I usually puncture the rim tape with a needle first, then push the valve through, so the hole is round, and as small as possible.

[noodleshop]

Thank you both for the advice, I really appreciate it The LBS was telling me the problem was with the rim, which I had a hard time believing.

[carbonazza]

... I usually puncture the rim tape with a needle first, then push the valve through, so the hole is round, and as small as possible.

This ! I do exactly the same.
Using a torch light from inside through the valve hole, to pierce it right in the center.
Then with the valve core srewed