High altitude jets

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Racer65

Racer65

10 Year
Joined
Jan 28, 2012
Threads
121
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3,199
Location
Palo Alto, CA
For those of you who run your rig at high altitudes, say 4000-5000 ft, what type of jet do you use in your carb? The assumption here is that you're running an Aisan, Trollhole or other aftermarket equivalent carbs. Also interested to know where did you buy your jets from. Thanks in advance.
 
I am interested in this as well, my rig lives at 5400 ft.................
 
I'm trying to remember (I don't think I wrote it down) what jets I used in rebuilding the stock carburetor on my '76 FJ40, which resides at 7,240 feet. I did run across an old post (IIRC from the Pirate4X4 site) stating that JimC recommends a 1 percent smaller diameter primary main jet size for each 1,000 feet elevation. Another old post from an individual with '70 and '71 FJ40s at 10,000 feet (Leadville, CO?) indicated that stock primary main jet was 1.16mm (116). The Toyota online parts website I use shows primary main jet as 90999-41118 (118) for 09/73-01/75 and a primary main jet spare (high altitude) as 90999-41114 for 09/71-01/75. From what I've read, the jet sizes varied from year to year, and tended to increase in later model years due to the added emissions. For instance, for FJ60s 08/80-08/87, primary main jet 90999-41147 (147).

For my older carburetors I was able to harvest jets from a number of spare carburetors and parts I've accumulated over the years.
 
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Too bad you don't have a Holley 350, their float bowl can be fully baffled for off road use, and you can run a 2 stage power valve that gives you a screaming hot rod above 4,000 feet.
 
I'm not at altitude (650') but I'm interested since I have a cabin Idaho at 6,150 and will want to have the right setup for when I finally take the 40 up for a summer (better quit my job first).

I converted back to stock a few years ago and matched a 75-76 carb off MUD onto my de-smogged 77 and it has the following jets:
Primary 150 Slow 65
Secondary 220 Low 85
Power Valve 90
I couldn't get it to run right with smaller jets (I had tried 144 / 180 and smaller slow jets)

With nothing to back my theory I was thinking I'd need to be at least 10% or so smaller. My trial and error plan would be to try 135ish primary and 200ish secondary. Do the power valve jet and/or the slow jets need to be adjusted, or just the mains?

I picked up a second carb a couple years ago (non-USA 10-1980) and to rebuild as a second carb so it may become my high altitude experiment
 
When I lived in Colorado, I re-jetted the stock Aisin carb on my '76 FJ40. At first, I went with 1.14 on the primary, but it was too lean IIRC. Seems like I ended up with a 1.28 primary jet and kept the 2.30 secondary. It ran great with that setup. Bought it from the dealership.

Back to stock 1.44/2.30 here at 600' elevation.
 
4Cruisers said:
Thanks for posting, I was thinking I had seen something here on MUD before.
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I think that source is now very critical regardless of whether you live on high altitude, since all those part #s you cited are now discontinued. For those of us in California, not having the right jets means not passing smog!
 
So the Mikuni jets fit Aisan '75 US spec. carbs? Do I order their, 'OEM equivalent' or, 'Genuine Mikuni?
Anyone ever lead-solder the stock jet closed, then drill it to size?
 
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Dizzy said:
Anyone ever lead-solder the stock jet closed, then drill it to size?
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I don't recommend this. It changes the entry & outlet shape of the jet, which has an unpredictable effect on flow.

Better is to harvest smaller jets & drill out to size. New jets, usually wrong sizes, come in the Keyster brand carb kits.
 
77mustard40 said:
Do the power valve jet and/or the slow jets need to be adjusted, or just the mains?
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In order of importance:
Pri main
Sec main
Power
Pri slow

The slow jets are not important, because there is an idle mix screw to trim the primary idle jet, and the carb rarely runs on the second slow jet.
 
The NAPCO carb kits available on ebay from SE Asia dealers have many OEM identical jets. I needed the 1.38 primary for my Biennial pilgrimage to Mr. Smog fascist Kalifornia-party-apparatchik since I was ONCE labeled as a gross polluter. My wife says the same thing after a dinner of over-cooked broccoli.
 
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I seem to recall reading that direct from the factory, you could buy a high altitude 40 with high altitude jets. LivinginthePast or Poser might know if this is true.
 
At 3700 ft, I just started to try the Mikuni (OEM) jets this past weekend. For sure, the threads are a slightly different pitch but I just worked the threads using the front plug that holds the spare primary. I have a '74 carb on my '78 2F.

My primary was a 120 for many years and had a slight hesitation when engine was "cold" (ambient in the 60s in the morning/80 at lunch) until it got to about 130 deg., then it ran great and had some stink to the exhaust. I finally tried a Toyota 136 I had a couple of weeks ago, hesitation when cold was gone, didn't have to use the choke, stink was gone and ran great with more power but, wow, used an extra 1/4 tank of fuel during 4 days compared to before. Very much like the pros though.

I ordered Mikuni sizes 125, 127.5, 130, 132.5 and 135. As of this past Sunday, running with the 127.5 with no hesitation when cold in the morning (60s), runs great and no stink (maybe, maybe very slight at times). Will probably try the 125 after a week of driving, maybe the 130 afterwards.

Jets were 2.90 each, highly recommended.
 
@algeeba have you figured out a good tool to change jets through the access holes?

Good idea using the spare jet location to massage the mikuni threads:beer:
 
There is no good tool that I know of to swap Jets thru the access hole. Plus, you run the risk of cross-threading the very soft alum thread in the float bowl.

I tried once with one of the locking-blade type screwdrivers, but I wouldn't do it again. That's my experience, anyway. Too easy to damage the carb.

Quick-Wedge Slotted Screwdrivers | Toolmonger
 
I just get a flat screwdriver bit that fits tight on the jet, unscrew it and hope it stays on the bit while getting it through the access hole. Sometimes the washer comes out with it sometimes it stays put. A lot of times, when the jet and/or washer falls onto the bowl, I just get a thin stiff wire, stick it in through the access hole, push it through the jet/washer hole, tilt the wire upwards to slide the jet towards the access hole, tilt the wire back to horizontal and fish the jet out. Works easy enough.

The Mikuni slot is wider than the Aisin slot so it would not fit tight against my flat bit. So I made a quick tool just to get it started on the threads. I got a piece of 1/4" copper tubing, smashed the end of it enough to fit tight on the Mikuni jet, trimmed the sides of the smashed end enough to fit the access hole and in she goes. Just need a light enough touch to feel it screwing in. Then I tightened with my flat bit.

I really hate having to unscrew and remove the top of the carb.
 
I had hunch you’ve worked on a solution

If you ever get a moment to post a picture of your copper tool, I’d appreciate it.

I too have an arsenal of Mikuni’s to test:cheers:
 
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