Vacuum?

[PussyPusher]

I have an AMC 360 in an older jet boat. My question is I have a Holley 4Barrel carb with Vacuum Secondaries and a vacuum advance on my distributor. When I get on it, it seems that the second the secondaries open I lose my ignition curve and it goes out of time. What can I do???

[Nubbs]

The vacuum in your manifold will decrease when your secondaries open. With decreased vacuum, your vacuum advance distributor will not be as advanced as it was before the secondaries opened. When you step on the throttle, two butterflies open. The manifold pressure (vacuum) will be at a certain level. The distributor will be at a certain advance because of the vacuum. When the secondaries open, the pressure drop across the carburator will be less. As a result, vacuum will decrease. With less vacuum, your distributor will have less advance.
You can advance the base timing of the distributor or you can adjust the internals of the distributor to alter the advance curve. I don't know what will work best for you.
Good luck!!

[PussyPusher]

Im not quiet sure whats going to best either. I understand the physics of it all. I just was wondering if I should try to go to a mechanical advance or somehting

[Nubbs]

You could try blocking off the vacuum port on the manifold or carb and setting your timing to work with just the mechanical advance of your distributor. It wouldn't cost you any money to experiment. You could also try opening your secondaries at a lower vacuum.

[PussyPusher]

How do I adjust the secondaries?
Will the distributor advance without the vacuum hooked up?

[Havasu Hangin']

PussyPusher:
How do I adjust the secondaries?Different secondary vacum spring.

[PussyPusher]

OK I am a little uninformed when it comes ot this so... How do I change the spring on the secondaries? I really appreciate your guys help on this.

[Havasu Hangin']

PussyPusher:
OK I am a little uninformed when it comes ot this so... How do I change the spring on the secondaries? I really appreciate your guys help on this. There's a vacum plunger on the side of the carb, which is connected to the secondary linkage. I think it's held by 4 screws on my Holley...
..or have a carb expert do it for you- it's not expensive.
[ January 08, 2003, 01:11 PM: Message edited by: Havasu Hangin' ]

[Craig]

The vacuum advance on a distributor is strickly there to enhance fuel economy. The moment you barely crack open the throttle from a steady state, the vacuum going to the distributor drops off. Plug that puppy off if you want.
You can change the carb secondary spring easily in 15 minutes, max. Unbolt the cover, pull the spring out and put a light one in. Remember if your carb is set up right you won't feel the secondaries "come in" like you do on a mechanical carb. [img]http://free.***boat.net/ubb/Avatars/Gearhead_Garage/Other_Equipment/carb2.JPG[/img]

[PussyPusher]

Craig,
Thank you for the diagram. I was also told to try using the ford vacuum inline diaphrams that are supposed to hold a little more vacuum for just a few seconds longer. So let me get this straight, what you are saying is that when I jump on the gas and secondaries open I lose vacuum to the distributor and that is what is supposed to happen? I thought that the vacuum help with the ignition curve under hard throttle???