ToyoGuy Posted September 21, 2018 Share Posted September 21, 2018 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 Quote Link to comment Share on other sites More sharing options...
Maineah Posted September 21, 2018 Share Posted September 21, 2018 I would leave the timing alone you will probably end up with spark knock particularly if you try it lean the mix. Quote Link to comment Share on other sites More sharing options...
ToyoGuy Posted September 22, 2018 Author Share Posted September 22, 2018 Thanks maineah, It's true that most of the input I've gotten from mechanics on trips reflect the attitude that "If you're not in the neighborhood long enough to re-jet it, don't worry about it". TG Quote Link to comment Share on other sites More sharing options...
Maineah Posted September 22, 2018 Share Posted September 22, 2018 Yeah it won't do any damage you will lose some power but that's about it carburetors are pretty crude any way so without an analyzer you really don't know where it is to begin with. Quote Link to comment Share on other sites More sharing options...
linda s Posted September 22, 2018 Share Posted September 22, 2018 Looks like Toyota still has the part if that helps. https://www.toyotapartsdeal.com/oem/toyota~valve~sub~assy~high~altitude~compensator~25709-38010.html?Make=Toyota&Model=Pickup&Year=1984&Submodel=&Filter=()&Location=vacuum-piping,,25709 LindaS Quote Link to comment Share on other sites More sharing options...
ToyoGuy Posted September 23, 2018 Author Share Posted September 23, 2018 Thanks Linda, That's the cheapest I've seen them. TG Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.