dumb question on twin carbs

[Willowbilly3]

OK, my home made twin carbs are working great so far but here's the head scratcher. I have only 6-7 inches of vacuum at best. Here's where I'm pulling it from. The carb spacers have the big vacuum port. I plumbed those together and came off the center of that connection with a hose to the pcv valve. The pcv valve had an extra port on the top so I just took off that with the gauge.

 

[Paul_J]

What cam are you running? That could make a big difference in how much vacuum you make at idle. I don't see a problem with the location for your reading but closer the the intake plenum wouldn't hurt.

 

[rainman]

The more air you let in through the carbs the less vacuum you'll pull! More carbs more air, less vacuum!!
Timing also has a direct effect on it!

 

[Willowbilly3]

Bone stock 1963 260 Ford, other than the carbs and pipes.
I might be wrong but to maintain a certain idle rpm, even with multiple carbs, the volume of air is the same so each carb is actually letting in less, hence the idle speed screws are adjusted to close each carb to half the airflow of a single carb. Also the idle mixture screws would also be run in more than a single carb. Unless there is a hole in that theory, the manifold vacuum under the throttle plates at idle shouldn't change considerably.
BTW, carbs are the original 2 bbl. and another identical to it. The only changes other than adjustments is the accelerator pump rods are move to holes closer to the throttle shaft to slow them down.

 

[Sam_Fear]

6-7 does seem low. I'd expect a drop of 3-5? with 2 idle passages.

 

[Bonehead]

Mine dropped a bunch when I went to twin carbs. I am putting a vaccume can in line to help on my brakes.

 

[oldskool]

When I tune multi carbs on bikes the vaccum is lower than a single...alot has to do with the cam overlap. make sure that the a/f mixture screws are equally tuned and then adjust the throttle sync.