High Idle speed on 87 FJ60 with 2F and Aisin Carb (1 Viewer)

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I have an 87 FJ60 with a 2F and an Aisin Carb. The truck is desmogged. Ever since I bought the car in August it has had a high idle speed. The carb has allegedly been tuned by Jim C under previous ownership.

When the car is started cold, it takes a fair amount of pumps (5-10) to get it to fire. It initially idles at about 750-800.

The idle speed when warmed up is 1500 RPM. The High Idle when the choke is pulled in when warmed up is 2000 RPM. When I adjust the idle speed screw looser (counter clockwise) on the carb (air filter on and all vacuum lines hooked up as when I bought it), nothing happens. I can turn it 3 complete revolutions with no effect. When I tighten that screw (clockwise) from the position that I found it in maybe 1/2 to 1 turn, the RPM jumps to 2000 RPM.

Any thoughts on what the problem could be?
 
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Check the AC idle screw. I had this exact same problem only to find that a PO had tightened down the AC idle up. I backed it off a few turns and that got me sorted.
 

2mbb

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@Seth_O, Thanks for that. Another screw I need to find. BTW, I enjoy your Resurrection thread. We were all glad to hear you passed smog...

The ac idle up screw is on the front of the carburetor. It should only be "active" when the AC is on, but I suppose your AC idle up vacuum switching valve could be stuck open or your vacuum hoses could be routed wrong such that the AC idle up is always active.

Below is a schematic of that circuit. The actuator is located on the front of the carb. The vacuum source is from the rear of the intake manifold (where the brake booster vacuum connects). The connection of the vacuum lines on the vacuum switching valve is important. The lower port should be connected to the actuator as it is vented to atmosphere when the valve is closed.

1580747910784.png
 
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So I adjusted the AC Idle Up screw such that it wasn’t touching the lever at all and that took care of about 500 RPM.

However, before I did that there was no response from the engine when the AC was turned on, so I’m not sure that system is working to begin with.

I’m still idling at 900-1000 RPM when it is warmed up. With the AC on, it idles at about 700 ish.

I think the next step is to try the idle screw on the carb to see if there is any adjustment there.
 
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2mbb

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search for "lean drop" method for carburetor idle speed adjustment. But as someone said above, you should first check for vacuum leaks. For example, if your AC idle up is not working then maybe the tube from the vacuum port to the vacuum switching valve is not connected, creating an air leak into intake manifold. This will lean out your idle mixture and make the engine idle faster.
 
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I am pretty sure I have a few vacuum leaks. I am waiting my McMaster-Carr delivery of hose and I will replace all the hose. That should leave only components that are leaking. By that time the vacuum gauge I ordered should be here and I will "lean-drop" the carb and see where we are.

I drove it to work today and I get the vibe (pun intended) that I should not lower the idle further without reducing the number of vacuum leaks. Idles fine, throttle response is good, it just seems slightly rough. When cold, the choke is needed (it wasn't before). An improvement, but further work is needed.
 

cheezypoof

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Curious if you found the root cause. My '86 is doing almost exactly the same as your description. I was going to start with a lean drop but I get the feeling this is an AC idle up issue, or a vacuum leak that worsens as the motor warms up.
 
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@cheezypoof I adjusted the carb via the lean drop method. Then I learned that my VSV was not working at all, actually visibly broken. I replaced it with part number 88690-32280. Then I leaned drop adjusted again to make sure the "AC Idle up" RPM was correct. All seems to be well now. (well, with that part.... Its a 33 year old truck, so there are other things...)
 

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