The '93 XT225 I bought in Flagstaff turned out to have a broken intake manifold causing it to idle roughly and run lean.
Ordered a new OEM Yamaha molded rubber manifold for the bike. It was waiting for me at the Post Office today. Went home, wheeled the bike into the shop, stripped it down and removed the carburetor. A 4.5mm allen wrench worked to remove the old rubber manifold from the cilendar head.
This is what the bike's original intake manifold looked like after 24 years of service:
No wonder it was sucking excess air at idle..
Removed the original rubber gasket from the old intake manifold and found it in good shape. The replacement manifold was an exact duplicate of the original, however there was no new manifold gasket in the package it came in. Fortunately the original gasket fit the new manifold perfectly, which was a relief.
Here's the new intake manifold with the carburetor in place:
Put the tank back in place, hooked everything up, and once the carb filled with gas it started and ran perfectly. No more rough idle, lean running or sucking excess air. Runs like an XT225 is supposed to!