AC idle up screw driving RPMs all the time

[Seth_O]

I have a Mark rebuilt carb that runs like a champ. I overhauled my previously non-working AC and converted to R134a.

When I first charged the system my truck would surge. No problem, I adjusted the AC idle speed up to the point where it doesnt surge (~1500rpms.) However, it doesnt drop RPM's back down when I turn the AC off.

What am I doing wrong? Its 105* today and I would really like to have AC.

 

[3_puppies]

maybe the vacumn switch isn't working properly, either has power with the switch activated or not releasing/venting the vacumn when switched off?

seems like 1500rpm is high for the a/c idle up

 

[Seth_O]

I read in the FSM that the idle up speed is adjustable, so I'll dial it back down to 700-800rpms where it is supposed to be. I can check the VSV. I think maybe I just dont understand how the circuit works.

I think the issue is that I was trying to adjust it with the screw on the carb, and I should have been using the adjuster in the AC amp instead.

 

[BlackCat]

You sure you didn’t adjust your base idle speed by accident instead of the idle up?

 

[3_puppies]

the vacumn switch gets manifold vacumn, when the a/c button ispushed the valve gets power which then opens allowing vacumn to move the actuator on the carb. when a/c button is pushed again the valve doesn't get power and the valve closes which cuts the vacumn from the actuator. fairly simple circuit

 

[Seth_O]

I'm not clear what role the adjustable screw on the AC idle up plays. That's what has me confused. I cant find anything about it in the FSM, and when I tighten it down it adjusts the idle even when the AC isnt engaged.

 

[Seth_O]

You sure you didn’t adjust your base idle speed by accident instead of the idle up?

Yep, I adjusted the screw on the AC idle up circuit, on the side of the carb closest to the radiator.

 

[3_puppies]

I'm not clear what role the adjustable screw on the AC idle up plays. That's what has me confused. I cant find anything about it in the FSM, and when I tighten it down it adjusts the idle even when the AC isnt engaged.

the screw should be adjusted when the a/c is engaged, you want the idle raised slightly while the a/c is running
if you adjust the screw with out the a/c engaged, you're taking the vacumn switch and actuator out of the loop.

 

[3_puppies]

here is a link to a recent discusion on the vacumn routing

87 fj60 Brake Booster intake port question

Where does this hose route to on the little elbow on the finned brake booster plug located on the Intake manifold between carb and firewall.? Circled in red. My Engine is an 87 2F. As of now mine is currently hooked up to nothing.... Thanks

forum.ih8mud.com

 

[Seth_O]

So the way I understand it: the AC amplifier controls when the idle circuit turns on/off by allowing vacuum through the VSV to the idle up diaphragm on the carb. But that just pulls the lever until it contacts the screw. Which should be set to 700-800 rpms.

When the AC is off it should release the diaphragm, allowing the idle to return to its "normal" setting.

 

[John Staton]

Here is a great thread on the AC amplifier. I am desmogged with a weber 38 and when at a stop sign my compressor will stop. I really dont need it to as my truck runs strong, so I am considering snipping the resistor shown to have the compressor running at any idle speed. I have an extra in case that doesn't work out for me. Anyway, I have not seen a better thread with pics explaining the AC Amplifier.

The A/C Amp Thread

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