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I offer this as mere speculation, but I believe thta Toyota accomplished the mixture compensation via the HAC (high altitude compensation) valve.
Can't you just do the drill-bit removal method to pop out the "blank" on top of the mixture screw and go from there?
That's what the FSM suggests for idle mixture adjustments. The idle mix screw came capped to meet US emissions requirements, right?
Drilling the cap off the idle mixture screw will allow you to adjust only the idle mixture. While that is not a bad thing, it will have no effect on anything other than the idle mixture. They do have replaceable jets, you might try pulling them and looking for something smaller.
I appreciate the help. Just FYI, this issue first came up last summer when I moved to Colorado from Dallas. I was wheeling at about 11,000' feet and my engine died; it clearly was too rich where the air is thin up there. I had to roll the truck down the hill about 200' in elevation to get it to start back up. I then drove another route that was less steep up the mountain, and it died again at the exact same altitude. Other stock 2Fs here in Colorado can cross the Divide with no problem. I can't figure out if this is a jet issue or an idle mixture issue.
Has the carb ever been rebuilt? You don't have any vacuum leaks and your vacuum lines are in good shape?
I removed my stock carb shortly, VERY shortly after I purchased the rig. Puke and die was the typical MO when in the mountains on something steep. It probably could have been rebuilt, but I choose to replace it with a Weber which I ran for many, many years and it worked extremely well once jetted properly for my altitude. I'm pretty good at carb tuning on the simple stuff, but I'm just not a fan of Asian carbs or any carb with emissions crud on it; makes it too complicated for a simpleton like me. One thing you might check is the actuator for the secondary on your carb. Sounds like it isn't working. Find Jim C. and PM him, he'll know what to do.
Anymore info on this? I'm currently having the same issues now that I've moved to CO.