I've noticed on my '69 DT1 that if I fill the gas tank completely full before heading out on a bumpy off-road ride the gas sloshes out the original cap vent and runs all over the tank, occasionally spilling down onto the engine and exhaust. I need to fill the tank full in order to ensure that I'll make the 70 or so mile ride where no gas is available, and prefer not to have to carry an extra fuel canister along. I thought perhaps I could replace the cap with one designed for MX riding, the kind that has a vent hose coming out of the top that can be routed safely down and away from heat sources. Sounded like a simple solution but I haven't found one yet - the Yamaha YZ tanks have screw in filler necks, and a cap I ordered from 2FastMoto proves to be too large of a diameter. If anybody knows of such a cap that will fit properly please let me know how to find one, you might be helping to prevent a future California wildfire.

If it were me, I'd purchase an El-cheapo cap that fit and modify it with a hose and grommet.

Interesting. For a bit we couldn’t figure out why we couldn’t get the gas to flow from CT1C’s original Cap. We rebuilt the petcock, swapped out a perfectly good in-line gas filter etc. Then we took off the gas cap and poufff! Gas flow.
We ended up pulling the original cap apart and drilled the little vent hole bigger.
Is this a common issue with the original caps?
It’s a goofy vent system via restricted gas cap inners.

Had the same issue with a similar cap on my '72 Suzuki ts185. Only happened during extended WOT but took a while to figure out.