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Old 07-02-2012, 10:00 AM
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Hard to idle with the A/C on

Guys,

I'm having trouble getting my 69 mustang to idle when I put the A/C on. I have a 302 with a moderate sized solid roller cam and 4 spd. i live in Texas so a few yrs ago I put a vintage air system in. It idles fine at around 1,000-1,100 until I put the A/C on and then the idle drops way down to 400-500 and I need to tap the gas to keep it running. In order to get it to idle ok with the A/C on I have to turn the idle up to 1,500 with the A/C off. It is especially a pain when I stop for red lights/stop signs as the car wants to stall. Other than that the car runs fine. Any thoughts?

Mike

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Old 07-02-2012, 11:21 AM
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Not enough initial timing at idle.

Turning n the AC compressor puts a load on the idling motor.
Increasing the idle timing ( especially with a big cam in the motor)
increasees the engine torque output at idle. Allowing the motor to spin the compresssor under load while maintaining its idle speed.
Recurve the distributor so the timing does not drop off if when a load ( compresssor) tries to pull the engine idle rpm down.

I suggest 24 to 26deg base timing at idle that does not drop off at a low idle speed.

check you idle timing with no compressor and when the compressor kicks in.
I bet you will find the timing retards when the AC kicks in.

More base initial timing less reliance on manifold vacuum for idle timing.
use ported vacuum instead of manifold vacuum.

limit the mechanical advance curve from 20-22deg (stock) to around 10deg

to allow 24-26deg at idle and 34-36deg at max advance.
Make sure the idle timing stays the same at 600rpm as it is at 1100rpm.
(24-26deg)

Some cars with AC also employ a idle speed solenoid on the carb linkage to increase the throttle position ( opens the throttle a bit more when ever the AC kicks in)

fix the timing first. I bet the timing is moving arould a lot between 600rpm and 1100rpm.
Its a matter of correct distributor mechanical advance curve for the cam and then proper intergration of vacuum advance.
Again use ported vacuum.

24-26 deg at idle base timing that does not varry between 600rpm and 1100rpm.
The lightest advance springs are too light tension and will allow the timing to jump around at idle.
use medium tension advance springs that are way more stable at idle.

short 10deg mechanical advance curve , starting to progress around 1200 rpm and finishing around 3000-3200 rpm

34-36deg at max advance

10-12deg of vacuum advance when manifold vacuum is highest ( hiway cruise steady driving light load part throttle)
ported vacuum.

Last edited by F-BIRD'88; 07-02-2012 at 11:31 AM.
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Old 07-02-2012, 12:55 PM
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Further:

After checking your previous posts ,,

This will also solve your stinky motor , stinky 750 carb , burning eyes issues.

Once you have corrected the ignition timing curve both these issues will suddenly and miraculously disappear.

And your cammed up 302 will idle and run a whole lot better overall.
You will suddenly find that 750 is not too big after all.

You need to get that distributor advance curve dialed in correctly.
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Old 07-03-2012, 04:08 PM
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You could simply put in a solonoid that kicks up the idle (pushes on the carb linkage) when you turn on the A/C. This was a typical arrangement for many stock engines and carb brackets and solonoids are common. The A/C unit may even have a "switch" that comes on with the compressor just for this purpose.

Actually, isn't this the beastie from a Ford Mustang of your year with your engine?


http://www.ebay.com/itm/New-67-68-69...sories&vxp=mtr

Last edited by toddalin; 07-03-2012 at 04:14 PM.
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Old 07-03-2012, 04:33 PM
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Thanks for the feedback. Actually, I have a Mallory Unilite distributor with mechanical advance, and the distributor is already set up exactly as you suggest: 26 initial with 36 total, all in at around 3000ish. I rebuilt the 750 carb and was able to eliminate all the problems I posted about previously. Now the car runs well, except when the air is on. I think I will look into putting in the A/C solenoid you suggest. I was not aware that even factory cars came with those. It makes sense and I think it will take care of the problem.

Mike
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Old 07-03-2012, 10:47 PM
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Is the spark timing changing when the AC kicks on and loads the motor?
It must stay stable 26deg. at idle and not change when the compressor
kicks in.

there are two difference types of these idle speed solenoids.
One is for A/C and has the power to move the throttle linkage by itself, when it energizes.
It is wired to the AC compressor power circuit relay. The compressor control relay controls the compressor drive clutch and the carb idle speed solenoid.

The other type solenoid is a anti deiseling, anti run on solenoid that has less power and requires that you tap the gas petal with your foot to move the carb linkage and set the idle.
(it closes the throttle when deactivated by the key switch to prevent run on)
It energizes when ever the ignition key is on.

They both look pretty much the same but serve two different purposes.

Last edited by F-BIRD'88; 07-03-2012 at 10:57 PM.
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Old 07-04-2012, 12:06 PM
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I'm not sure, but I understood that with the way my dist is set up the timing can never be less than 26 total timing. this dist has an adjustable advance timing limit. Currently it is set to give only 10 degrees advance. then I set the wieghts and springs so that the timing mark moves through that ten degrees when I rev the enginge, with full advance at around 3000. Then I power timed the car to 36 degrees. so AFAIK the most the timing can retard to from the peak is to 26 degrees, even if the idle drops down to zero the timing can never be less than 26 degrees, correct?

Thanks for the info on the solenoids. I will be sure to pick up the right one. I think that will fix the whole problem.

Mike
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Old 07-04-2012, 07:03 PM
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yup as long as its not dropping off when the idle rpm goes down, your good to go.
Try the AC solenoid and tell use how that works for you.
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