Leaning fuel mixture for altitide

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O

osamenor

10 Year
Joined
Apr 28, 2013
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4
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42
Location
Southern CA
Do any of you recommend leaning the fuel mixture via the carburetor (I've got a mixture screw on my carb) when going from low altitude to high altitude? I am going from sea level up to about 10,000' for the first time in my rig and haven't messed with the fuel mixture on a road trip. I don't know if it is more trouble than it's worth. I understand that I'll have to re-enrich the mixture on the way back down the hill.

I tried searching the forum and it seems like most people set their fuel mixture for their local area and leave it at that.

Thanks!
 
Pighead said:
I just advance my timing a bit as I climb in altitude, I don't usually fiddle with the idle air screw. Couldn't hurt to try it though.
Click to expand...

^^^^^^^this^^^^^^^

I've been out west and number of times from sea-level. On my sbc, I usually just advance the timing back to proper idle. It needs to breathe. Sometimes I may advance it and bump the idle a little bit. Keep in mind the idle mixture adjustment is for idle. It doesn't really effect anything off idle.
 
Yup.. above. The idle mix screw really only helps at idle and your’e probably not sitting at idle much (if so you could dial back by ear). The rest of the circuits would require re jetting and then re-re jetting when back down so not worth it. (I played the game when I used to run a Holly 350cfm 2 barrel back in the day.. easier to re-jet, but not really worth it)
 
Agreed with @cbmontgo , bump your idle speed as well. You lose 3% power for every 1,000' gained. Timing plus idle speed should do it. I've been up and over 13K elevation with just those two tweaks. With that much power loss and depending on the health of your motor, you may need to get a little farther into it than you are used towhen making climbs or getting over certain obstacles. Also, when way up there don't shut down after making a climb, let the motor run for a bit so it doesn't boil over. Don't ask me the physics, just good advice given to me on a trip with some pros.
 
cstav86 said:
Agreed with @cbmontgo , bump your idle speed as well. You lose 3% power for every 1,000' gained. Timing plus idle speed should do it. I've been up and over 13K elevation with just those two tweaks. With that much power loss and depending on the health of your motor, you may need to get a little farther into it than you are used towhen making climbs or getting over certain obstacles. Also, when way up there don't shut down after making a climb, let the motor run for a bit so it doesn't boil over. Don't ask me the physics, just good advice given to me on a trip with some pros.
Click to expand...
True about vapor lock, as it gets much worse with elevation since the atmospheric pressure/boiling point is significantly lower. One thing that can help is running non-ethanol gas. If you shut the engine down briefly, you'll be fine. If you shut it down more than 5 minutes, you'll immediately start to feel the effects of vapor lock. Not a big deal as long as you plan for it. Takes about 45 minutes for an engine to fully recover from it if you let it sit. Just be careful when parking on steep inclines on a trail and expecting it to fire up and go.

The horsepower difference is very noticeable. I'm only running at 70% horsepower at my house with +/- 3% per thousand feet.

Be ready for an afternoon thunderstorm to cool things down 30-40 degrees and make shorts weather turn into coat weather in an instant.

All part of the fun, I reckon.
 
Yeah, take everything you own for weather.
It could be sunny and 80 and be snowing around the bend. It's awesome.
 
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