Question on air fuel mixture

[ColoradoCarlisle]

Hello, I have a carburetor on our 71FJ 55 and trying to adjust the air fuel mixture. However, the air fuel mixture screw which to my knowledge is the screw above the circled brass fitting does not seem to have any effect on the idle speed. Starting at around 1 1/2 two turns out with the engine warm un screwing half a turn, then full turn then another half a turn and so on the engine speed does not increase. I’m curious what this circled brass knob does? Is it possible one would need to unscrew this to allow more air into the circuit?

Any ideas thank you very much in advance

 

[charliemeyer007]

You should look for pinhead's vid on rebuilding and adjusting carbs.
Or you can look for the carb in the manuals.
Free down loads Land Cruiser Factory Service Manuals (FSMs) - https://www.cruisercult.com/factory-service-manuals

 

[wimberosa]

That is indeed the idle mixture screw and when you screw it out ... your RPMs should increase. Fundamentally you follow the "lean drop" method documented in the FSM. There is a very good youtube tutorial as well



As to why yours isn't working...i cannot guess.

 

[cbmontgo]

@ColoradoCarlisle if the rig runs rough and the idle mixture screw adjustment doesn't do anything, it's possible that your idle circuit is blocked/ineffective and it's having to pull idle fuel through the main barrel via running at a higher RPM to overcome. I've run into this a couple of times over the years.

Does it have a steady idle? How's vacuum look?

 

[numby]

Another possibility is that your idle speed screw has your throttle plate opened too far for the idle mixture screw to do anything without the circuit actually being blocked. It's knd of the same as what @cbmontgo said above but doesn't require a rebuild to correct.

 

[Pighead]

I’m curious what this circled brass knob does? Is it possible one would need to unscrew this to allow more air into the circuit?

I'm thinking that that plug is left over from drilling a hole between the slanty threaded hole that the screw goes in and the straight up&down hole that the air comes down in...I would leave it alone.
As I understand it, and I do not understand everything about this carb so don't do what I do, the idle air screw regulates a little tiny amount of air that flows from up high in the carb to down low.
I figured this out once when I put a gasket in backwards and did not notice, then my idle mix screw didn't work well at all.

 

[wimberosa]

That gasket flip possibility is just evil.