Autolite 4100 Secondaries Not Opening

HammerG26 · Jan 31, 2006

I have a 66 A Code with an Autolite 4100. The carb had a tag on it when I got it saying it had been "remanufactured by Holley" - though I have no idea when that was done. I cannot tell if the secondaries are opening or not. A fellow here in Atlanta said they are not, and offered to rebuild it for $160. Questions are as follows:
1. How does one tell if the secondaries are opening? I can open them manually via the linkage with the car off, but if the car is running, the linkage will not budge.
2. If they are not opening, could it be a vacuum leak?
3. There is a plastic piece in the fast idle bracket (appears to be held in place by the fast idle adjustment screw), that is limiting the motion of the secondary linkage, is this supposed to be there?
4. What circumstances should a carb be rebuilt?
5. Is the $160 a good price?
Thanks in advance.
Hammer

krash kendall · Jan 31, 2006

First, they are locked out untill the car is up to operating temperature. If they are vacuum secondaries they may not open when you are bursting the throttle while parked as they will only open after the initial drop in vacuum is overcome. You would probably go over redline trying to open them for a relatively short duration while parked.

That's about all I can help you with on this one.

LMan · Jan 31, 2006

You cannot test this at rest by zinging the throttle by hand. Vac secondaries work off engine load as well, so they won't respond in your garage. You have to put the motor (yeah, sue me) under a load to get enough vac to operate them.

Its kinda like testing inertia reel seatbelts. You can sometimes yank on them all you want with your hand and they unreel....but in an accident the additional intensity/force makes them lock. Crappy analogy, Im sure.

180 Out · Jan 31, 2006

The 4100 is a vacuum secondaries carb. To check for actuation, put a piece of masking tape (lightly) across the secondary linkage, go for a drive (floor it), pull over and check if the masking tape came loose.

krash kendall · Jan 31, 2006

You may risk sucking the tape into the intake.

180 Out · Jan 31, 2006

krash kendall said:
You may risk sucking the tape into the intake.

I'm no expert on 4100's, but I didn't mean to put the tape inside the throttle bore, I meant to put it on the linkage somewhere. Here's a pic of a 4100:

View attachment 489167

Looking at the pic, it looks like there's a vacuum motor aka dash pot on the back of the carb. There's a lever attached to it. That lever is going to move when the vacuum motor is actuated. So you rig a piece of tape somewhere on that lever, such that it will be pulled loose when the lever moves.

bnickel · Jan 31, 2006

tie a piece of string to the secondary linkage and somewhere else on the carb then go drive it hard enough that the secondarie should open, when you park it again you should be able to tell if the secondaries are opening by looking at the piece of string, if it's still attached they aren't opening if not then they are.

krash kendall · Jan 31, 2006

Sorry 180out, I missinterpreted you. Makes sense now.

Autolite 4100 Secondaries Not Opening

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