Leaning out fuel mixture when driving into higher altitude

Joined
Feb 6, 2019
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Oklahoma
I live in Oklahoma but try to get to Colorado a couple of times a year with my stock 1981 FJ43. My practice as been to adjust the carburetor and lean out the fuel mixture as I gain altitude. It has worked with some success (steep or moderate grades at altitude put me in the very slow lane) and I make notes of how many 1/4 turns in I make as well as the location (altitude) so that I can reverse the procedure when I return. Are there any tips or techniques I should use to ensure I don't run too lean. I read somewhere that you can advance the timing to address altitude issues? I have no idea how to do that and would probably end up in a shop in the mountains if I did.

Thanks
 

mattressking

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Since you have an Fj43, I'd take a look at the jets inside. I'd assume 138/200 but someone may have fiddled. Start with that, you can always bump to a 147 from the 60 series and lean it out some more so you have some wiggle room. The factory 40/60 series high altitude compensation (HAC) would do two things:

1. Advance timing 6* above 4,000 feet.
2. Pump air into the primary low and high speed circuits in the carb.

Now you can't do #2 with your carb, you'd have to swap to a 40/60 series carb with HAC and have a functioning dual vacuum advance, so we can assume to nix that.

If I were you, I'd just bump your timing 6*, mark where it is on the flywheel then kick it there when needed. If you do a plug chop and see what they look like at altitude, then you can start to see if you want to change jetting. Just know your baseline first.

Before you do any of that make sure you have the following:

1. No vacuum/exhaust leaks
2. 20+ inHg at idle with all accessories off.
3. Valves adjusted
4. Timing set at 7* (or up to 10* if you have advanced and adjusted mixture accordingly and verified no pinging)
5. Fresh, correctly gapped plugs
6. Points/weights/mechanical advance are in good shape
7. Carb has been went through and functioning correctly, most carbs have had a funky history and need to be run through to make sure it is good.
8. If you're this far, why not do a compression check for sanity.
 
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I know you may have already considered it but EFI conversion will take care of your issue. They automatically adjust for changes in altitude and usually make starts easier.
 
Joined
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You'll get the most bang for your buck by far, by just advancing the ignition timing once you get to Colorado. It makes a lot more difference in drive-ability than changing jets, and is easier to do.
Is it complicated? Would I still need to lean the fuel mix? Is there a video I could watch to see how to adv timing?
 
Joined
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Is it complicated? Would I still need to lean the fuel mix? Is there a video I could watch to see how to adv timing?
I forgot to mention one important factor. My ignition is not stock. I upgraded to a DUI HEI distributor. Not sure if that makes a difference. Should I assume I can still advance the timing to get the same higher altitude benefits?
 

devo

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I‘ve lived in the Colorado mountains for decades and was and will always be chasing the “more power” at altitude. Your best bet is more air in the mixture and timing advance. Cutting the fuel at the jet only decreases your power because you have less fuel to burn and that’s what make the HP. Get the fuel burning sooner on the compression stroke by upping the timing to at least 10*. Instal a Pertonix setup in your dizzy. Throw on a header. Make sure your secondary is opening. And if your running the carb that came with your 1981 FJ43 (I’m assuming Colombian vintage carb) upgrade to a 1985-87 FJ60 carb.
I also dropped in a big jet for the secondary because when it opens it suck a lot of air and leans out too much.
Also: get used to going a way lot slower above 6000 ft elevation and a lot slower again above 10,000 ft. Also get used to no matter what you do you won’t burn all of the fuel that your carb is throwing into the combustion chambers.

FIY: I’ve been passed by a vintage VW bus on a mountain pass!


No shame!



devo


PS: Sweet FJ43…… they are the best!

A151ACEF-D1BA-4120-BAE8-80A909010A04.jpeg
 
Joined
Feb 6, 2019
Messages
12
Location
Oklahoma
I‘ve lived in the Colorado mountains for decades and was and will always be chasing the “more power” at altitude. Your best bet is more air in the mixture and timing advance. Cutting the fuel at the jet only decreases your power because you have less fuel to burn and that’s what make the HP. Get the fuel burning sooner on the compression stroke by upping the timing to at least 10*. Instal a Pertonix setup in your dizzy. Throw on a header. Make sure your secondary is opening. And if your running the carb that came with your 1981 FJ43 (I’m assuming Colombian vintage carb) upgrade to a 1985-87 FJ60 carb.
I also dropped in a big jet for the secondary because when it opens it suck a lot of air and leans out too much.
Also: get used to going a way lot slower above 6000 ft elevation and a lot slower again above 10,000 ft. Also get used to no matter what you do you won’t burn all of the fuel that your carb is throwing into the combustion chambers.

FIY: I’ve been passed by a vintage VW bus on a mountain pass!


No shame!



devo


PS: Sweet FJ43…… they are the best!

View attachment 3017633
Thanks for the info. I’ll have to figure out the timing advance with the HEI distributor. I’ve never messed with timing.
 

1911

chupacabra
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Is it complicated? Would I still need to lean the fuel mix? Is there a video I could watch to see how to adv timing?

It doesn't have to be complicated. The distributor turns clockwise, so loosen the clamp at the bottom and turn it counterclockwise a little at a time. If you have a dial-back timing light, use that to advance the timing another 3 to 6 degrees BTDC. When I lived just west of Denver (6,000' elevation) I ran my trucks then about 10 degrees BTDC (7 degrees is the FSM setting). The higher you go into the mountains, the more you can advance it.

If you have a regular old timing light (not a dial-back), putting the timing BB at the bottom of the timing window in the flywheel cover will be about 10 degrees BTDC.

If you don't have a timing light of any kind, just advance the distributor (counterclockwise) to where you get the best throttle response and power, without any knocking or pinging. If you go too far and get pinging or knocking under load, then back it off to just where these disappear and set it there. To test it under load, drive up a steep hill or up an inclined freeway/highway ramp. This is how I time my trucks 98% of the time, and I own several timing lights.

It's the same procedure for any distributor, stock or otherwise.

You wouldn't have to mess with changing your jets, you'll get much more benefit just advancing the timing. If I was going to move back and live at high elevation I might change the jets, but not just for seasonal travel.
 
Joined
Feb 6, 2019
Messages
12
Location
Oklahoma
It doesn't have to be complicated. The distributor turns clockwise, so loosen the clamp at the bottom and turn it counterclockwise a little at a time. If you have a dial-back timing light, use that to advance the timing another 3 to 6 degrees BTDC. When I lived just west of Denver (6,000' elevation) I ran my trucks then about 10 degrees BTDC (7 degrees is the FSM setting). The higher you go into the mountains, the more you can advance it.

If you have a regular old timing light (not a dial-back), putting the timing BB at the bottom of the timing window in the flywheel cover will be about 10 degrees BTDC.

If you don't have a timing light of any kind, just advance the distributor (counterclockwise) to where you get the best throttle response and power, without any knocking or pinging. If you go too far and get pinging or knocking under load, then back it off to just where these disappear and set it there. To test it under load, drive up a steep hill or up an inclined freeway/highway ramp. This is how I time my trucks 98% of the time, and I own several timing lights.

It's the same procedure for any distributor, stock or otherwise.

You wouldn't have to mess with changing your jets, you'll get much more benefit just advancing the timing. If I was going to move back and live at high elevation I might change the jets, but not just for seasonal travel.
Thank you. Great explanation
 

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