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Partly because my two 710M's travel between 8,750 ft. and 13,200 ft. in altitude every day, they tend to run rich. Especially on the descent.
Besides changing out the carb jets, what else can I do to "lean them out" a bit?

That's about all you can do, since the jets are the metering valves for the gas that runs through the engine. I have no advice as to what sizes to install (besides smaller), but I'm sure someone has that information.

In the bike world, it's common for guys to cut out openings in their airboxes and cover them up at low altitudes and then open up at high altitudes. If you are travelling over big altitude changes often, you could do the opposite of this. Jet your Pinz lean and run an air restrictor (like a very subtle choke) at low altitudes, then remove the restrictor at high altitudes.

A valve would be the nicest setup, but for a very simple solution just find a rubber or plastic cap that fits over the inlet for your filter housing. PVC cap would work too. Cut a hole in it and see what yields the best results at 'low' altitude. Then when you get to high altitude, pop it off. As you descend, pop it back on.

If you can't adjust fuel, adjust air

pinzinator wrote:That's about all you can do, since the jets are the metering valves for the gas... I have no advice as to what sizes to install (besides smaller), but I'm sure someone has that information.

Ok then carb gurus! Who has the info.?

I'm confused as to which of the jets get tweaked for high altitude. Apparently there are idle jets, idle air jets, main jets, and air correction jets. Which version of each jet would be the ideal setup for high elevations?( 9,000-13,000 ft.)

P.S. - I'm currently running whatever jets came with the trucks from the Swiss Army and the trucks run strong at altitude, it's just that they run SUPER rich, puff smoke, and end up fouling my plugs.

Thanks!!

My 710k came with the 140 mains and 60 idle fuel jets when I got it from the PO in California. Those are the largest jets (most fuel) available to us. Im at about 5000 feet at home and travel to over 9000 ft for camping. I am switching to the 125 mains and 55 idles which are the smallest (least amount of fuel) offered. Haven't checked my air jets yet so I cant help there. I would assume that the larger the air jet and smaller the fuel jet would help for the extreame altitudes.

Hope this helps.

I’m not sure how to view this problem, and here’s why. At a higher altitude there is less air pressure, therefore less vacuum in the venturi, correct? With less vacuum, would a larger jet be needed since there is less pressure/less vacuum? With less vacuum, I would think the fuel should be allowed to flow with less resistance, since a jet is basically a resistor. In other words, make it easier to let fuel in since the force drawing it into the carb is less? Or, since there is less air, should the jet be smaller to let less fuel in to maintain the same air/fuel ratio? I don’t know.
As for the air correction jets, I see them as a vent which allows fuel to flow in the carb chambers much like the vent in the spare gas can that comes with the Pinzgauer. The correction jet also allows air to mix with fuel, which is called emulsion. With a smaller correction jet, the vacuum would have to be higher in order to maintain the correct air/fuel ratio under the same conditions, since a correction jet resists air flow instead of fuel. The main jet and air correction jet would have to be matched in order for the ratio to stay correct, therefore allowing maximum efficiency at the drive wheels.
I hope Jim steps in with is 2 cents, because I’m just not sure. The repair manual gives a good explanation as to how the Zenith carb works, so that is where I got my basic information.

Ok now I'm REALLY confused!
Your theories seem to make logical sense Pinzinator but Mr.Fixit's reply also makes sense.
I figure that somebody is bound to know for certain which combo of jets is ideal for operation at the highest elevations. Before I go and buy a bunch of expensive new carbs jets, I want to know for sure.
Argh...

I don't know if this will help or further confuse the issue but a 710M I bought a few years back that had the carbs set for higher altitudes (Idaho) were as follows:

Idle Jet 55
Main Jet 125
Air Correction Jet 230
Idle Air Jet 130

I had to change the Idle Jet to 60 and the Main Jet to 140 to get them to run properly for me at my lower altitude.

When we drove from home (Madison, Wi) to tour the Rockies in Colorado, I only changed out the main jets, and IIRC went from 140 to 125, somewhere around Denver. These are the jets located under the access cover on the driver's side of the carbs, with one bolt holding them on.

There is less oxygen in the air up there, so you are running fat - little oxygen and normal amount of gas. Smaller jets reduces the amt of fuel to be closer to what you need with the reduced oxygen. Might another solution be a supercharger (or similar) that would get more air and thus oxygen to the engine????

Find the guy that the local old Porsche collectors go to, some of the early Porsche's also used zenith carbs, he would likely have the test equipment and knowledge to properly adjust your carbs. The hard part will be convincing him that working on a Pinz isn't beneath him. IIRC all of the test information he'll need is in the manuals.

pcolette wrote:I don't know if this will help or further confuse the issue but a 710M I bought a few years back that had the carbs set for higher altitudes (Idaho) were as follows:

Idle Jet 55
Main Jet 125
Air Correction Jet 230
Idle Air Jet 130

I had to change the Idle Jet to 60 and the Main Jet to 140 to get them to run properly for me at my lower altitude.

Turns out, my trucks are jetted for sea level. I've been running them up to 2.5 miles above sea level with almost no problems (besides lots of fouled spark plugs) for the last ten years.
Just ordered all new idle and main jets. Let's see how this works...

Wow, what a difference!
I think that I solved it. With the new (high elevation) jets, my trucks now start quicker and no longer belch smoke out of the exhaust as much. I suspect that my chronic plug fouling problem might also be remedied.

Twin Pinzies, what jets did you end up using?

landy wrote:Twin Pinzies, what jets did you end up using?

I swapped out the stock factory jets (#140,#60) with high elevation jets (#125,#55).
The trucks run smoother, idle better, and don't smoke nearly as bad as before but they still foul plugs. (WTF?!)
I'm now fussing with my float levels (they were high) to try to solve the continued fouling problem.

Twin Pinzies wrote:...but they still foul plugs. (WTF?!)
I'm now fussing with my float levels (they were high) to try to solve the continued fouling problem.

Have you converted to civilian ignition ? What make and model of spark plugs do you use ?
I have a civilian setup and use NGK BKR6EKC in the Alps betwees sea level and 2700 meters altitude.