Search the Community

Howdy All, Just returned from the shortest outing ever, a single over-nighter of about 150 miles just to gain 5,000 feet of altitude and check a Toyota "option only" (per factory manual) altitude compensation system I assembled out of used parts. (diagram below) Those who are as old as I, remember that carbs run richer at altitude because of lack of oxygen and this system is designed to,beginning at about about 4,000 feet, advance the distributor timing and lean out the carburetor fuel mixture to offset that. (For those who have a fuel injected motor, you can quit here, this is all handled by the computer). Anyway, I was foiled in my attempt by a bad / used HAC valve that seems to be stuck shut and didn't open to allow the carb to pull more air via 3 air-mixture ports at the top of the throat of the carburetor. (other below) BTW, a new HAC valve is about a hundered an fifteen weasle hides. I stopped at the nearest mountain town of Placerville, and talked to a mechanic who deals with this kind of thing much more than the sea-level mechanics I use. He was kind enough to inspect the part and concurred it was defective and said they cannot be reliably repaired. He also kind of inferred these systems can be a can of worms. Has anyone out there had any experience with this particular system, and can they speak to any change in performance at altitude? It seems to me reasonable that advancing the spark 7 degrees could help with horsepower loss at altitude and it sure doesn't make sense to me to run the motor richer than required for hours at a time. Just asking.... BR, TG