In discussing Weber conversions with another member here the question of tuning a Weber for altitude came up. Toyota already has a fairly simple looking devise attached to their Aisan carb’d trucks – the High Altitude Compensator (HAC). It seems to be set up to do subtle - on the fly - adjustments.

I haven’t studied it but it looks like it advances and/or backs off timing via the vacuum advance. But, I believe, one vacuum hose goes to the charcoal canister and I’m not sure what that accomplishes. I also presume it somehow senses the air pressure changes, but I’m not sure how. It seems to effectively “tune” for altitude on the fly. But I strongly suspect it is NOT a substitute for initially tuning a Weber for the attitude you normally drive at.

Thinking out loud here, I also wonder if it senses air pressure changes due to weather conditions.

I think it is a pretty useful devise for those who travel between lower and higher altitudes. For example; I live at 3545 ft, but when I travel out of the valley I live in, no matter what direction I go, I have to go up at least 2000 ft. Also, Flagstaff, which is not far from me, is at nearly 7000 ft. My truck doesn’t feel like it has any problems when driving at those increased altitudes. I imagine the HAC would be equally useful on a properly set up/tuned Weber conversion.

So, my questions are:
1) Has anyone hooked up the HAC on their Weber conversion? Pictures please.
2) Has anyone taken one apart and have pictures of an HAC?

 

I think this question is way over every ones heads, including me. I think the only people to answer these questions are the Weber guys, if you were able to reach them.

 

Typically I will remove a vacuum line from the manifold. Just a small one. Maybe if you were to put a small 2 with a little filter If you think dirt is a huge issue, I don't). You can lean out the fuel mixture that way. It really helps out when up in the mountains. Most of these older trucks and cars with altitude compensators, the compensators don't function as they should. You could do the same by grabbing a normally closed VSV, then have a small signal wire open it when you want to.

 

I am still learning about carburetors, but here is how the High Altitude Compensation System is hooked up on a factory AISIN carburetor (From my 1987 Toyota Truck and 4Runner Repair Manual).

My truck does not have one, being I am only about 350-400 feet above sea level.




I don't have a Weber to compare to, but I suspect the original High Altitude Compensation System is not compatible with the Weber.

 

Probably for some here and I'm going to look further into it possibly with Redline. I still need to get up to speed on this HAC because it seems like a good thing to have vs. removing it because it has vacuum lines or the assumption that because it is part of the emissions system it must be evil. The way it works and what it does makes the engine run more efficiently, which equals better MPG and power. The charcoal canister seems to be a similar thing.


I came across that same info today on another website re about what it does and at what altitudes - thank you O87.
Does your manual have diagnostic / troubleshooting info on the HAC?

 

When researching carburetors on these trucks, I came across this ^^ a lot. If I went up into the mountains a lot, I would add this system to my truck. I don't care if it adds more vacuum lines if it makes the engine run better.

My manual does have the diagnostics section for the High Altitude Compensation System.

I will post those pictures up tomorrow.

I will try to scan them as my camera does not like taking pictures of paper very well.

 

In case anyone is interested I found a troubleshooting guide in my 1981 Corolla FSM. It's the same part so it should apply to all HACs.