Ok, so I've been away for a bit. Resolved some fuel delivery issues. Corrected some electrical issues (lights, not ignition). and now I'm getting down to tuning my 1975 F-350 super cab...and I'm flummoxed!

The engine has edelbrock valve covers, edelbrock performer 460 intake, edelbrock 1406 carb, and the engine compartment has been painted so both of the stickers identifying idle, timing, etc are gone, but the PO printed out some little stickers, laminated them, and stuck them to the front of the radiator and one tire well inside the engine compartment. He listed the timing (14*BTDC) and idle (850 No A/C, 1250 A/C). Now here is where I am lost.

What does the No A/C and A/C actually mean? The truck does have AC from the factory, however there is a toggle switch mounted to the dash that doesn't look factory to me. It toggles the A/C compressor clutch. I know that newer vehicles have an idle increase switch/solenoid/toggle/whatever that increases the idle when the A/C is turned on. Would a '75 F-350 with A/C have this? If so, where/what is it? I don't see anything referencing it in the Haynes book. Now if there ISN'T some kind of idle-up, AND this toggle switch for the clutch is not standard from the factory, does that mean the A/C in these models was always engaged, and therefore the idle was to be set at the higher value?

Basically, where should the idle be set for this beast? 1250 seems way to high to be the standard idle, but if it's 850, and I engage the clutch is it going to die from the additional load at that low RPM?

Thanks.

 

That toggle switch isn't stock

In the 1970s, the typical A/C unit had the compressor running all the time, when A/C was desired. Things like STV (Suction Throttling Valve) were used in the A/C system itself to vary the amount of cooling so the evaporator wouldn't freeze up.

Later years brought the FFOT (Ford Fixed Orifice Tube) system, where the A/C system itself ran full-bore when compressor was engaged, but the compressor was clutched in/out depending on system pressure (therefore, the temperature).

I would have expected the original carb on your truck to have some sort of A/C Idle-up solenoid on the throttle linkage at the carb. With A/C OFF, no power to the solenoid, the plunger would not be extended. With A/C ON, 12 volts would be applied to the solenoid, extending the plunger, which would prevent the throttle from closing all the way.
So base idle would be set in Drive if Automatic, with A/C OFF and engine fully warmed up (off of choke fast idle cam).
The increased A/C idle would be set with same conditions, just add A/C on and blip the throttle so plunger can extend to hold up idle.
Sometimes the increased-idle fine-tuning was done by turning a threaded screw on the end of the plunger, or on a dashpot on the end of the plunger, or by loosening and moving a bracket that held the solenoid itself.

1250 RPM sounds awful high! The early high-compression 460s (1968 1/2 - 1971), is where my 460 experience is from. An A/C compressor wasn't much of a load for them

I think you'll just have to try a lower RPM, and see what it will do. If the A/C compressor is too much of a load without increasing the idle, maybe there are aftermarket idle-up or throttle-stop solenoids that you could press into service, and have it turn on with the A/C clutch to lift up the throttle.

 

Thanks. That's pretty much what I suspected. So now this leaves me with all kinds of other wonders. I'm wondering if this electric clutch A/C unit is a replacement for the original (the factory climate controls do include the AC, so that tells me the truck originally had A/C). If so, I wonder if it was built with R12 or 134a. I wonder how I would find out.

 

You didn't mention what cam you are running, but a big lumpy cam can require higher than normal idle settings to stay running at stop lights, etc.


On my truck, when hot and idling in Park with the AC off, its set to idle around 1250-1350. When put into drive with the AC off, idle speed drops to 850-950, when the ac is turned on, the solenoid engages and maintains idle speed in drive at the same level. Previous posters described the solenoid adjustment process pretty well. If you post a picture of your carb with the air filter off, we can probably provide better instructions for adjusting ac solenoid, dashpot.

 

I wouldn't know if the cam was changed, or what was installed, without opening up the engine and pulling it out. Or would I? Is there some way to ID the cam?

Here is a picture of my carb If there is an A/C solenoid on my Edelbrock 1406, it's somewhere other than the carb itself.

 

The Ford A/C compressor of the 1970s was the York or Tecumseh 2 cylinder reciprocating compressor. They all would have had a electrical clutch on the front of them. IIRC, the only way to tell the difference between a York and a Tecumseh is a small difference in the cylinder head shape.
Here is a pic:


For more pics, go to Rockauto.com and look up your truck. I couldn't link their pics, because the movable detail window there screws up the link.

Looking at the compressor pic, mentally rotate it 90 degrees for the way it is usually mounted to the engine. They were all R-12 back then, the first year for R-134a is 1994, though someone may have converted it to R-134a sometime past. If a conversion to 134a was done, it should have the high and low port conversion connectors on the system, allowing 134a-style gauge manifold set hose couplers to fit.

The idle speed in Drive on my early 460s was something like 550 RPM!

A big help to you would be a picture of the driver's side of the stock carb setup mounted on a 1975 460, so you could see the solenoid and other stuff. Maybe post a request for that up in the 1970's F-series forum on this website.

I thought I had an underhood pic of a 1975 Lincoln, but I haven't found it yet. The Lincoln engine from 1968 1/2 through 1978 was the 460. It had the Autolite 4300 carb on it through 1974, in 1975 it switched to the Autolite/Motorcraft 4350. The 4350 had real small primaries, where the earlier 4300 had big primaries, almost as big in diameter as the secondaries.