Elevation jet size?

[Beedunkin710]

Hello new to the nissan/datsun community And I currently live in, Colorado. And with a good amount of elevation change depending on where you drive. 5000-11000 ft. so I was wondering what size Jets should I use? What do you use and what's your elevation range?

 

  I have 1985 king cab 4x4 with the altitude compensator but the previous owner replaced the carb with I federal emissions model. 

[datzenmike]

The carburetor will run slightly rich as you gain altitude which is better than going lean. There is a loss of performance but this is from lack of air not going richer which is simply a waste of gas.

 

All federal carburetors use the same jets and the altitude compensator simply leaks in air to the carburetor to lean the mixture. Does your federal carburetor have pipes on it that are capped off?

[Beedunkin710]

No they do not exist on the carburetor. It's the same in every way except the ports have brass plugs and no hose barbs

[datzenmike]

My car is from Nevada average elevation 5,500 ft. I always got good mileage, I'm at sea level, but on long vacations I got consistently better mileage in the mountains inland. Not a lot but better. My primary jet was a 99. I picked up an engine locally and the jet was a 112. I put it in and my mileage went up at sea level as I had been running too lean and loosing power. Heavier denser air needs more gas. The 99 was good for higher altitudes.

 

You could fiddle with jet sizes if you like. I don't know what jet size is in there right now, you would have to take it out and look. The '84 720 factory manual says the California primary jet should be a 94. (but they like everything lean) Elsewhere it's a 107. In very general terms, every increase of 2,000 feet you can reduce the jet size 1 or 2 sizes. This would suggest that if you have a 107 you could  run 2.5 to 5 sizes lower or 104 down to 102.

 

 First it's going to run just fine, perhaps a light rich if you do nothing at all. The end result of smaller jets is to save the gas you are wasting. I would run 6 or more tank fulls through it and keep meticulous records of your gas mileage. If you have a 107, drop to 103 and see if your mileage improves. Don't expect much. If it gets worse go back to the 107s.

[DIY 1985]

On my Weber, I couldn't get it jetted small enough for the proper tune at idle.  I had to open the throttle too much and wasn't running on just the idle circuit, but rather the first progression hole (it was also too low an rpm to be running vac to the canister purge and spark advance).  I drilled a 1mm (ish) hole, opposite the progression holes, on the primary throttle plate, and am basically using the jets in the Weber kit for Z24.  It makes sense, because I'm not trying to deliver less fuel, just more air.  Drove to Colorado last summer, got 30+ mpg on US 285.  Plugs, and digital O2 sensor readout, show no lean condition.  Frequently drive to 10,300' and get good economy there too.

Edited June 15, 2020 by DIY 1985