600cmf holly carburetor not getting enough vacuum

[Superbond]

I rebuilt my 72 351c 7-8 years ago. She runs great but it seems like more and more I’ve noticed the four barrels aren’t kicking in less and less.
I’m pretty sure it’s a low vacuum issue. It alway had low vacuum and I don’t know why. I upgraded the cam but a mild cam. Checked and replaced all the rubber hoses. I think once the motor produces enough vacuum the secondary work.
I put the paper clip on the secondary shaft linkage and sure enough. It doesn’t move most of the time. I put a weaker spring and it seemed to help. But at the end of the day. I’m thinking my issue is the low vacuum. I wish I could tell you the numbers but it been years since I tested it. I should test it again I know but I have been really busy with other things. I just thought I’d ask you guys what I should do. I did upgrade my brakes from drums to disc’s. So I don’t know if that is robbing enough vacuum to produce my issues.

 

[Steve McMahan]

Just for info, the vacuum secondaries work by the vacuum produced in the primary bores, as the air rushes by into the engine. Maybe that port is clogged or something, preventing the vacuum from reaching the diaphragm? Good luck.

 

[Barrysmach1]

I rebuilt my 72 351c 7-8 years ago. She runs great but it seems like more and more I’ve noticed the four barrels aren’t kicking in less and less.
I’m pretty sure it’s a low vacuum issue. It alway had low vacuum and I don’t know why. I upgraded the cam but a mild cam. Checked and replaced all the rubber hoses. I think once the motor produces enough vacuum the secondary work.
I put the paper clip on the secondary shaft linkage and sure enough. It doesn’t move most of the time. I put a weaker spring and it seemed to help. But at the end of the day. I’m thinking my issue is the low vacuum. I wish I could tell you the numbers but it been years since I tested it. I should test it again I know but I have been really busy with other things. I just thought I’d ask you guys what I should do. I did upgrade my brakes from drums to disc’s. So I don’t know if that is robbing enough vacuum to produce my issues.

Just to let you vacuum 4V works off venturi vacuum not engine vacuum, first check that vacuum pod diaphram is in good condition

 

[basstrix]

The paperclip test is a simple way to determine if the secondaries are opening, but many people don't realize that you have to run the engine hard (put the throttle to the floor like you're drag racing) to get them to open. If you're just revving the engine in neutral, you'll be very hard pressed to get the secondaries to budge.

With a vacuum secondary, if you can feel them open, they're opening too fast. It should be a smooth transition where they open as the demand for air increases due to RPM increasing.

How are you performing the paperclip test?

 

[boilermaster]

Superbond,
you could remove the complete vacuum pod as a unit
(3 screws and an Eclip and apply vacuum to the pod an see if the rod moves an holds.
a vacuum pump works well for this or simply push the rod in and place your thumb over the port and see if it holds.
depending on what version of pod you have, my guess is you already had a bad diaphragm when you changed springs, or lost the seal of the diaphragm when you changed springs.
P.S. the paper clip method sucks, use a micro hitch pin clip, it will stay in place much better.
Boilermaster

 

[Steve McMahan]

And personal experience, I’ve found the stock spring is a bit too stiff. If you find all is well on the diaphragm, you might try a softer spring.

 

[rcadd1ct]

Adding

The throttle shaft bushings can wear and lower the vacuum signal.

Try doing the spray test to look for leaks.

 

[Superbond]

Thanks guys. This is all really good information. I’m out of town but when I get back I will look into all of these ideas