Technical Multiple carbs = lower vacuum?

I have a 53 Merc flathead with three Stromberg 97s in a 40 Ford. Carbs are synced and adjusted properly.....idles and runs great on all three carbs.....progressive linkage. Timing is set to 7 degrees at idle with 25 degrees all in at 2200rpm. Manifold vacuum at idle is a steady 16. I can get the vacuum reading up to 18 inches if I advance the idle timing to 12 degrees or so, but that's obviously too high of an advance at idle.
My question is; does having multiple carbs typically cause lower manifold vacuum at idle?
I've played with 3-2 setups in the past, but they were bigger carbs on bigger engines and I don't remember being concerned with lower vacuum gauge readings on them.
Thanks
Norris

 

No, assuming the same advance, the idle speed is set by the amount of air let through the carb (or carbs). So, same advance, same idle speed = same airflow and same vacuum.

 

I’d say if outer carbs are not sealed at idle, vacuum would be lower. But 16 inches seems ok. But how does it run at 12 BTDC?

All I recall from my stock flatheads was they didn’t like a lot of total advance, seemed to run the best around 22-24

 

My outer carbs are not sealed. They are set exactly like the center carb. Same airflow and mixture at idle. That’s the only way to setup 3-2s with a progressive linkage. When the outers kick in it’s as smooth as silk.
I guess it’s not a big deal but, it’s one of those things I wonder about; not worry about.
It runs ok at 12 initial at idle, but better at 7-8.
Most every car I’ve had has shown 18 inches or better at idle on a vacuum gauge with everything setup correctly.
Oh, and yes, I’ve spent quite a bit of time looking for vacuum leaks at the manifold and carbs.....none to be found.
I just thought low vacuum might be inherent with multiple carbs.
Thanks!

 

Can you add or move the vacuum line to the distributor to full time vacuum? This will give a advanced timing at idle, which will bring the vacuum up, yet still give decent performance with initial timing set where it runs best. Vacuum should be the same with one or three carbs, if the idle speed is the same, and sealed end carburetors are only good if the idle circuits are also blocked.

 

I’m running a mechanical only Chevy distributor.
The lower vacuum at idle does not bother me.....just asking the question

 

What Curve is in the Dist.?????
That usually indicates the Timing
if you have 12 degree's than you doubly it & put the Rest on the Damper!
like mine is 12 in the Dist & I double it & witch comes to 24 & I put 14
on the Damper because my SBC likes 38 Total.

Just my 3.5 cents

Live Learn & Die a Fool

 

The curve is 20 degrees. All mechanical advance. I'm sitting at 27 total now at 2200rpm. Runs great.
Idles at 675rpm.

 

Just curious, why is that too much advance? Thanks.

 

Because it starts "pinging" under load at higher RPMs.

 

If your end carbs are set up exactly like your center carb, you will have a bit lower vacuum at idle. Instead of 1 vacuum leak, you have 3. If it runs good, idles good and accelerates without stumbles, I wouldn't worry about it.

 

I don't know if flatheads are different. But factory stock OHV engines should pull a steady 19" to 21" at sea level, at factory ignition timing specification, or it indicates a defect of some kind. There is a correction factor applied for altitude, San Angelo looks to be about 1,800 feet.

Manifold vacuum also depends on the camshaft, a performance camshaft will tend to be lower, sometimes a lot lower. So it can be a little tricky to determine what is "normal". Generally speaking the higher the better, or put another way if the engine does not pass a vacuum test, if it should be 18" and it's only 16" find out why, because it will not run right.

Valve timing/camshaft phasing will also affect manifold vacuum in a big way. This might be something to look at further, because advancing the base timing compensates for this or brings the manifold vacuum up.

 

I’m really not too concerned. The engine does have an Isky Max1 cam which is by no means a radical cam. It runs beautifully. I was just curious. The needles on two different vacuum gauges are extremely stable right between 16 and 17 inches.
Jaracer answered my original question.
Now....I’m gonna install one of my Winterburn CDIs.
I cannot leave well enough alone!!!!
Thanks guys