The generator on my 1956 Ford Fairlane (292/Fordomatic) was only putting out 11 volts at idle or when cruising with the lights on. When cruising with no lights on, it would put out about 12.2 volts.

I recently had a shop install an alternator in place of the original generator. At idle I was getting a 13.5 volts reading on the aftermarket voltmeter. On the way home from the shop there were two occasions where I had to stop for a traffic light. Both times with my foot on the brakes, the car started shaking from the motor acting like it was not idling fast enough. I thought the engine was going to stall but didn't. It also felt like I had lost the "boost" on my brakes. The aftermarket voltmeter began to pulse each time - going back and forth from 5 to 14 volts. When driving, everything seems normal.

Is it possible that the installation of the new alternator is putting a little more "load" on the engine and to compensate for this, the carb idle needs to be adjusted (increased)? The carb is a Holley 4160/465 cfm - 4 barrel. If it does, what is the best way to make the needed aljustment? I am not experienced in carb adjustments and the shop that installed the alternator does not work on carburators.

JMO  The shop that installed the alternator is remiss because they should have tested it in operation BEFORE they turned you loose.  It sounds as though the curb idle speed is set a bit low but other issues may be present (example, vacuum leaks).  This is a minor adjustment that you or a technician can do.  An accurate tachometer, manifold vacuum gauge and a few hand tools are all that is needed.  There is a stock idle speed specification which will get you close.  Variables will include engine condition, equipment such as A/C, temperature, etc.  If you are not knowledgeable about all of this, consult a tech.

It is likely the alt.has a little more "drag" than the generator.You dont have to be an expert on carbs to set the idle speed a little higher.If you look at the carb throttle at the base of the carb you will see a screw whicvh contacts the lever when the carb is closed.That controls idle speed.A little turn clockwise will raise idle speed slightly.They really should  have checked that at the shop.If you go back they should be able to adjust it for you if you dont feel confident doing it your self.the 465 is a really simple carb to adjust.

  

Another possibility is that while performing the work on your car, they inadvertantly knocked a vacuum hose loose, causing a vacuum leak and poor idle.  Do you hear a whistling sound that you never noticed before?

John - "The Hoosier Hurricane"

Could be the belt tension is too much or the new alternator is causing too much drag and highlighting some previous undiscovered issues like a small vac leak.

Mike Rizzo

1963 F100 "Rudy"

Daniel Island, SC

No whistling noise. I failed to mention that the vibration/shaking only occurs when the car is in the drive position and the brake is applied. Vibration/shaking is not present when the car is in neutral.

Danny,

Turn the idle screw in about 1/4 a turn to see if this helps. It will increase the idle speed by a little bit. The screw can easily be turned back.   

cheers

Warren

Grizzly (Aussie Mainline)

You say the shaking occurs when the brake is applied.  Does the car have power brakes?  If so, the brake booster can have a bad diaphragm which causes a vacuum leak when the brakes are applied.  Does the idle speed change when the brake is applied in park or neutral?

John - "The Hoosier Hurricane"

The idle speed does not change when the brake is applied in park or neutral.

I gradually increased (by one quarter each time) the idle speed one whole turn. The vibration decreased somewhat but still there and the car feels that it won't stall when I stop at intersections.

The only drawback about this change is when I shift from Park to Reverse, the car shifts very abruptly. When I shift from Neutral to Reverse, the car shifts normally. For the time being, I plan to start the car in Neutral then shift in Reverse.

I am beginning to believe that the vibration problem may be with the connection between the engine and transmission or the transmission itself.

Sounds like a carb with one idle passage plugged.  Only idles on 4 cylinders.

John - "The Hoosier Hurricane"