High Altitude Jets (1 Viewer)

[Jetlander]

Recently moved from Houston (sea level) to Idaho (4000’). Took the Cruiser out for its maiden Idaho drive today and haven’t re-tuned the carb. Could definitely smell it was running rich and there was some obvious protesting on the drive home. I do plan on leaning it out, but at what altitude should I consider putting in high altitude jets, and which ones? 98% of the time it’ll be at 4-5000’ elevation but I do plan on running her into the mountains come summertime.

Oh…and a couple glamor shots!

 

[mattressking]

First do a tune with existing jets, then check your plugs. What carb are you using? Ethanol has leaned out gas a bit since the 70s jetting so after doing a lean drop, you can then start to read your plugs. Also a good time to check vacuum and timing. If you haven't already, start by bumping up timing to 13-14 degrees for altitude prior to lean drop.

If you are living at about 4-5k feet, you don't want to rejet for short trips. The trick is to usually just raise idle/adjust mixture, use the choke or advance timing. Rejetting is too tedious for a weekend/week trip. These trucks will do just fine up to 13k with some minor tweaks.

 

[Jetlander]

First do a tune with existing jets, then check your plugs. What carb are you using? Ethanol has leaned out gas a bit since the 70s jetting so after doing a lean drop, you can then start to read your plugs. Also a good time to check vacuum and timing. If you haven't already, start by bumping up timing to 13-14 degrees for altitude prior to lean drop.

If you aren't living about 4-5k feet, you don't want to rejet for short trips. The trick is to usually just raise idle/adjust mixture, use the choke or advance timing. Rejetting is too tedious for a weekend/week trip. These trucks will do just fine up to 13k with some minor tweaks.

Timing was my next question. I have it set at 10° advance. My idle is set at 650, so what idle do you higher altitude guys run?

 

[mattressking]

Timing was my next question. I have it set at 10° advance. My idle is set at 650, so what idle do you higher altitude guys run?

You can keep the idle the same or bump it up a bit. Once you set the timing, mixture, and such, I usually set it to the stock rpm, so 690 then lean drop to ~650.

 

[cbmontgo]

Yep, advance timing 13-14 degrees and see what happens. That may be all you really need.

 

[Jetlander]

That was the trick I believe. Set timing to 13° and barely leaned it out. Made a quick trip and it seems to be much happier. Currently making 16.5” of vacuum.

 

[Dizzy]

Why does lack of street performance correlate to an overly rich mix - culprit is jets? The "protest," might just be its way of sending fuel, not burned, thru the exhaust. My guess is spark, dwell, stuck governor/centrifugal, points, rotor, etc. Or, vac leak.

Do you know what is the make of your carb?

 

[4Cruisers]

I installed smaller than stock Mikuni motorcycle jets in my '76 FJ40, but I live at 7,240 feet. Timing is advanced only to about 11 degrees BTDC, I could probably advance it more. It does fine, even at 13,114 feet.

 

[cbmontgo]

I installed smaller than stock Mikuni motorcycle jets in my '76 FJ40, but I live at 7,240 feet. Timing is advanced only to about 11 degrees BTDC, I could probably advance it more. It does fine, even at 13,114 feet.

View attachment 2844453

Interesting. So the Mikuni thread pitch matched the stock Aisin threads? Which sizes did you go with?

 

[4Cruisers]

Interesting. So the Mikuni thread pitch matched the stock Aisin threads? Which sizes did you go with?

Same size and thread pitch. Check out my Post #231 here:

Builds - Finally!!!!!

Seth u WILL want to get the heat shields in place for sure ............ Poor Cowboy , will sweat .........

forum.ih8mud.com

 

[4Cruisers]

Jet sizes I used in my latest (2019) carburetor rebuild:

primary main jet: 1.10, secondary main jet: 1.80, primary slow jet: 0.50, secondary slow jet: 0.50, power valve: 0.60

 

[1911]

I installed smaller than stock Mikuni motorcycle jets in my '76 FJ40, but I live at 7,240 feet. Timing is advanced only to about 11 degrees BTDC, I could probably advance it more. It does fine, even at 13,114 feet.

View attachment 2844453

My 1980 40 has the factory stock High Altitude Compensation system; it does well at high elevations also. It adds another 6 degrees of ignition advance, and bleeds more air into the primary low-speed and high-speed circuits in the carb to lean it out. A lot of people (mistakenly) delete the HAC when doing a de-smog, so it might be possible to find the components from folks selling or giving away deleted smog equipment. You would also need a late 40 or 60-series dual-diaphragm advance distributor to be able to use the HAC advance.

 

[4Cruisers]

You would also need a late 40 or 60-series dual-diaphragm advance distributor to be able to use the HAC advance.

In case anyone's interested, I happen to have a refurbished FJ60 distributor almost ready to go out the door, it includes a good used dual diaphragm advancer.

 

[Jetlander]

Why does lack of street performance correlate to an overly rich mix - culprit is jets? The "protest," might just be its way of sending fuel, not burned, thru the exhaust. My guess is spark, dwell, stuck governor/centrifugal, points, rotor, etc. Or, vac leak.

Do you know what is the make of your carb?

I have a Trollhole carb. Original desmogged 2F that makes 20” of vacuum @ sea level and 155-160psi compression on all cylinders. In other words, super solid.

By ‘protest’ I meant it started to stumble and exhibited several exhaust ‘pops’ if it was allowed to get anywhere near idle. I honestly don’t think it had anything to do with the mixture, but perhaps the timing. I was running at 10° advance but as soon as I advanced it to 13° it smoothed right out. I pulled 3 of the spark plugs before doing anything and they were as clean as could be.