Looks like one of the cheapo kinds that come with new toilets. They seem to fail pretty frequently. I wouldn’t bother tinkering with it, just replace it with a new Fluidmaster or Korky brand valve ($10-15 at most hardware stores).
I just don’t understand why Toto felt they had to redesign the Drake. They have the Drake II and the G-Max, both of which work perfectly. If it ain’t broke, don’t fix it. Just my two cents.
Another thing to check is the siphon jet at the bottom of the bowl. Over time, it can become plugged with mineral deposits and slow the flush down. The majority of newer toilets have them, some older ones don't.
I recently saw a video of a Kohler toilet from Japan which uses somewhat of the same idea as the old Wellworth Lite with the exposed siphon jet pointing up into the trap. Hopefully this time they executed it a little better.
Lamosa toilets are known to be very low quality and cheap, FWIW I’d get something better. I’d say 99% of the toilets on sale today are better than a Lamosa.
Cool, an American Standard Yorkville! American Standard does still make the Yorkville, although it’s a pressure-assisted model so it’s a little louder, but performs pretty well. Kohler also makes a rear outlet toilet called the Barrington, which like the Yorkville is pressure assisted. Either...
I personally have a Champion 4 and it’s been fine for me. However, there have been numerous complaints about the flush valve gasket leaking and the toilet itself being defective.