Heading to Colorado - Carb Question

[Hangin]

Hey all,

First I sure am watching the wildfire issues going on in Colorado, kinda scary.... My question is I have a 1980 CJ5 with all stock AMC 304 v8. I currently live at around 1500 Feet. Would like to take the jeep with us to Grand Lake, CO. Elevation of about 8500 and wanting to run a few trails around there upwards of 11000 feed. My question is will the jeep run as is or will it be mandatory to have it rejeted and timing tweaked.

Thanks

 

[IOPort51]

by my way of thinking the worst that can happen is you run a little rich. Other than the smell and a marginal loss of power you should be OK for a trip. If I was going permanently it would be a different question.

If your running OK at home you should be OK for the mountains for that short a time.

Hey all,

First I sure am watching the wildfire issues going on in Colorado, kinda scary.... My question is I have a 1980 CJ5 with all stock AMC 304 v8. I currently live at around 1500 Feet. Would like to take the jeep with us to Grand Lake, CO. Elevation of about 8500 and wanting to run a few trails around there upwards of 11000 feed. My question is will the jeep run as is or will it be mandatory to have it rejeted and timing tweaked.

Thanks

 

[Purple77CJ7]

I agree with IO. But I think you may have to adjust timing a bit. I have a 401 and am running about 8 degrees advanced, for the elevations your talking about. Just my thoughts.

 

[IOPort51]

which direstion? I am running about 10 BTDC down here on the flat.

I agree with IO. But I think you may have to adjust timing a bit. I have a 401 and am running about 8 degrees advanced, for the elevations your talking about. Just my thoughts.

 

[Purple77CJ7]

Sorry, I meant 8 degrees BTDC. Hangin will just have to see how it runs and go from there. On vehicles here at home that did not go to the mountains we used to run about 5 degrees BTDC.

 

[bondo258]

I'm curious, too.

which direction?

Advance or retard timing for elevation?

 

[Purple77CJ7]

From my experience, with out re-jetting the carb, you will be running rich as stated above from thinner air. So you would advance the timeing a bit more to fire earlier to burn extra fuel.

 

[Hangin]

Hey all thanks for the replies. We got back last night from our little trip to Colorado. The jeep ran like a champ. It did smell a little gassy and a very very slight reduction in power but other than that we had a great time. I did not advance the timing but did have a light in case I needed it.

The only problem I did have was a severe loss of power at 10000 feet due to a faulty gas gauge.....

Lucky for me both my kids were on quads so back to the trailhead I went to fetch some gas. Ya I know... "why didn't you have extra gas with you? you idiot..." Anyway gas in the tank and a little coaxing we were back in business.

 

[Purple77CJ7]

Glad you had a good time and everything worked!

 

[GRONK Performance]

Quick rules in regards to jetting/timing for televation:

Assuming a correct base setof jets and timing for sea-level, you will decrease jetting 1% for every 1000' increase in elevation. At the same time, you will advance timing 1deg for every 1000' increase in elevation. This is true for 2bbls,l 4bbls, side-draft, down-feed, etc carburetors.

Example, a AMC 258 i6 / 4.2l inline-6, non-computer controlled, Motorcraft 2150 with 1.08 venturis at sea-level will run #48 jets at 9deg BTDC at 650 RPM.

At 6000' elevation, the same exact carburetor will run # 45 Jets with a 15deg advance.

BOTH setting will provide equal power/performance through the powerband and will burn clean at the exhaust.

 

[csuengr]

BOTH setting will provide equal power/performance through the powerband and will burn clean at the exhaust.

Advancing the timing, which is all you need to do since you won't be here long, will make it run better, but still a bit rich. Fuel injected engines lose power as well.

No matter what one does, one will lose power with altitude. That is why WWII fighters and such ran superchargers, turbo superchargers, compound systems and such. They still lost power at altitude. Even jet engines will lose thrust. Their thrust ratings are at sea level.