High altitude jets

[FJ40Jim]

I seem to recall reading that direct from the factory, you could buy a high altitude 40 with high altitude jets.

Yes, 75-78 2F could be optioned with hi-altitude carb & emissions.
Earlier models came with the hi-altitude jets stored in the carb, 79-newer were equipped with Hi Altitude Compensation system to change mixture automatically.

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.

Free advice: The fact that it starts cold with no choke indicates it's very rich. A carb or injection that is tuned right at normal operating temp, needs cold enrichment. Related, there should be a hesitation without choke when cold. Finally, consider that starting & idling & tip-in throttle are mostly the slow circuit. Rather than drowning the main circuit to hide a lean idle jet, just change up the idle jet. That won't kill cruising fuel economy.

 

[algeeba]

I was afraid you were going to say that Jim, but it does make sense. I did read somewhere before where, obviously, when cold the engine needs more fuel hence the choke and that once warmed up the engine needs less fuel, hence able to run a leaner main jet. I kept thinking during this this time that the 120 behaved exactly like that and that maybe it was just right for my setup. Oh well, it was still fun to experiment.

I do have a 60 primary idle jet that I can swap for the 50 in there currently. If I try the 125, maybe a 122 that my son has, and begin to get hesitation during cold does that mean that would be more optimum for my carb/engine?

 

[Dizzy]

I have a 1.37 mm main primary, and a 2.23 mm main secondary on my 49 states February '75 40. Is this for sea level, or 7,000 feet? Installed is a 1.09 mm power valve, a 1.32 mm primary slow, and 1.50 mm secondary slow. I'm measuring with a set of numbered drills, not pin gauges, so I might be reporting undersized.

 

[FJ40Jim]

I have a 1.37 mm main primary, and a 2.23 mm main secondary on my 49 states February '75 40. Is this for sea level, or 7,000 feet? Installed is a 1.09 mm power valve, a 1.32 mm primary slow, and 1.50 mm secondary slow.

Sea level '75 would be 1.44/2.30 main, 1.20PV. don't remember the slows, but no bigger than 80/110. Your numbers seem pretty far off.

 

[Spike Strip]

I'll just add installing a wide-band A/F meter makes a world of difference with carb tuning... For me in Kalif, it was the only way I could reliably and consistently pass smog.