high altitude

[tbrannon]

I am planning on towing my 81 CJ7 to Colorado and need advice on tuning the carb for high altitude operation. Any help will be greatly appreciated. thanx

[Dan-H]

I don't have any advice for your engine specifically.

I used to live up in Lake Tahoe -- lake level is 6200 feet, passes are 7K to 8K, the rest is somwhere in between.

I drove a few different cars and trucks, and all had carbs.

The advice I was given was advance the timing one degree for each 1000 feet of elevation increase. This seemed to help abit. I found six degrees worked well for the 6-8K range with an occaional trip to Reno which was 5,000 ft.

I didn't adjust anything else.

The only things on the carb I could change was the choke (how fast it would lean out) and the idle mixture. I experimented with these but found neither were worth the effort to change.

YMMV

[Iceman]

would a fuel pressure regulator also work in that case?
Just wondering since some of my wheeling buddies swear by adjusting it for different type of terrain (we're at 4500 ft) and now I'm wondering if adjusting the fuel pressure would also have an effect on the way it runs at high(er) altitude?

Any comments are welcome, since I'll be a little more challenged fuel wise, once I get the carbed CJ7 to TX

[Matt Pascoe]

I'm not sure how early they started this on CJ's, but the later models with the Carter carb had a black ground wire with a ring terminal zip tied to the harness on the firewall behind the distributer. If you drive in high altitudes you were supposed to attach this wire to a bolt on your firewall.

Was supposed to change your timing etc for altitude.

[Dan-H]

I don't see how a fuel pressure regulator can affect a carb.

fuel pressure needs to be enough to keep the bowl full.

the float stops the flow of fuel. end of story.

If there's too much fuel pressure its gonna leak past the needle valve. I don't see how altitude changes this.

the height of the fuel in the bowl can affect the flow of fuel out of the ventures, which in turn affects the mixture.

also the vacuum difference sucks fuel in via idle circuits which depend on the level of fuel.

if the carb has an externally adjustable float, then you can change the fuel level without tearing the carb apart. Again without knowing which carb, and even then I've only got limited experience with a couple of carbs none of which I believe came on CJs.

[DC]

Ideally, what you want to address is the air/fuel mixture with the lessening oxygen content per cubic volume of air as you go up in altitude (otherwise, you'll be running richer and richer).

Don't know if this is helpful for you or not, but when I had a '75 Scout II XLC with a 345cid V8 equipped with a Carter AFB 4bbl carb, I bought a Carter "Strip Kit" with a variety of needles, jets, metering rods, and seats. It took a little bit of experimentation, however, coming from sea level in California to upwards of 13,000 feet in Colorado, I had three combinations that worked well (i.e., for sea level, for ~5000 ft, and for above 8000 or 9000 ft). The swap-out did not take that much time at all, however, I am sure it depends on the specific make/model of carb.

Good luck,
Don