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Local folk lore has it around here that because of the altitude we should set our timing to around 10 degrees BTDC (factory is 7 degrees) on a F or 2F. To me, this doesn't really make a whole lot of sense, I would think that because there is less oxygen, you you tune it mildly retarded as it would take less time to combust the lesser amount of oxygen. A couple of weeks ago I retimed my '79 distributor (in my '75 2F) to 10 degrees and messed around with the vaccuum ports to get it as best I can with the distributor running the original smogged advance curve, but on a desmogged engine. It's one hose is plumbed directly to the carb port on the mail barrell. When warm it start and runs great, but on cold mornings until you are about 2 minutes into a drive it hesitates and misfires a little, as if it's too far advanced.

Any idea on timing for altitude? I'm still going to yank it for the HEI anyways (tomorrow most likely:rolleyes:), but I'd still be interested in what the jury says for timing it for altitude.

Also, anyone have any information on the OEM smogged versus desmogged advance curves? Is it really worth the time and energy to have it recurved? I hear the HEI's biggest advantage is that it combines a more accurate curve with hotter spark, so if I were to have the '79 dizzy recurved, should I set it to stock desmogged curves or something more similar to DUI's for example?
 

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If you are at a higher elevation, the air is less dense in the atmosphere and the cylinder nearing top dead center... because of this lower denisty, the heat propagation from one molecule to another is slower as they are essentially further apart. So, you get a slower burn, therefore advancing the timing partially makes up for the less dense air...

Part of the warming up issue is due to the abudance of large fuel droplets at warmup, as the intake tract is not warm enough to properly vaporize the fuel, and therefore it does not burn completely.

A stronger spark can help this situation, but its something to live with, as it cant be made perfect with a carb.
 

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My '84 FJ60 has the High Altitude Compensation (HAC) system. There's a diaphragm that's operated by the change in atmospheric pressure when you start to get above 3,000 feet. There's a secondary diaphragm on the vacuum advance distributor that advances the timing to 13 degrees BTDC. There's also lines (same material as the vacuum hoses) that introduce more air into the carburetor at high altitude. I was thinking of adding this set-up (carburetor, distributor, HAC valve/plumbing/etc.) to my '71 or '76 FJ40 using parts from my '84 FJ60 parts vehicle.

John
 
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