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Discussion Starter · #1 ·
I’m trying to find out if my intake gasket is leaking or not. I was low on carb cleaner, so I only half-a$$ed that test until the store opens. Vacuum gauge is reading a steady 10-11” at idle. If you blip the throttle around you can get it to peak at around 15”. I’m not too technical, so I’m wondering if anyone can tell what vacuum should be at idle with the cam spec in picture.
Comp Cams 08-422-8
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1965 El Camino,78 Caballero sold
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I have seen WD 40 used in place of carb cleaner to test for a leak. If you do a quick internet search a ton of information comes up.
"using a vacuum gauge"
 

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87 Caballero Amarillo, stock 305/200R4/QJ
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Vacuum should be closer to 15-18 for these cars in a stock configuration, but that ran many things, like heater controls, smog crap, cruise control, tranny kick down etc. That said, it really depends on what you want to end up with and what else you have. It's not uncommon to dissociate the vac system from the carb entirely and run the vac system from an electric vac pump.
 

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Vacuum actually falls as the throttle is opened up because the throttle plate opens up to let air in - vacuum level falls. Steady numbers are good but where you attach the gauge is very important. Some carburetor vacuum ports are only partial vacuum, not full.

Where did you attach the gauge?

Rick
 

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Discussion Starter · #6 ·
Vacuum actually falls as the throttle is opened up because the throttle plate opens up to let air in - vacuum level falls. Steady numbers are good but where you attach the gauge is very important. Some carburetor vacuum ports are only partial vacuum, not full.

Where did you attach the gauge?

Rick
It’s connected to the ported side of the carb. I should go connect it to the manifold port and see how much different it is.
 

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87 Caballero Amarillo, stock 305/200R4/QJ
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Best place to pull vac is at the brake booster. It's a large manifold port off the back of the carb, doesn't interfere with any potential smog or ported or pcv lines and you'll definitely know if it's getting a good seal.
 

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Discussion Starter · #8 ·
Best place to pull vac is at the brake booster. It's a large manifold port off the back of the carb, doesn't interfere with any potential smog or ported or pcv lines and you'll definitely know if it's getting a good seal.
So testing on the ported side of the carb = bad results I just found. On the last carb it gave me same readings on ported and manifold side so I didn’t bother checking on the new carb. But the manifold port on the new carb is way better vacuum. It’s pulling around 17-20 depending on the timing. With the vacuum advance off, initial timing is around 25* and it’s running better than it ever has. You can go from idle to WOT with just a tiny hesitation, no carb popping or engine stalling. Now I’ll just have to adjust my vacuum advance can and I should be close.
 

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Do you know how much your distributor's mechanical (centrifugal) advance is and have an idea (or graph) of when it comes in?

The distributor I have brings in 22 degrees centrifugal advance usually starting at 1 degree about 1000 RPM and then 22 at 2500 RPM.

If you have an initial timing of 22 degrees at idle, then added another 22 from centrifugal advance, you would be at 44. That might be high for a SBC.

Then you would have your vacuum advance added to that once you reconnect it.

Are you running with a "locked out" centrifugal advance to keep that out of your variable timing equation?
"Locked out" is a phrase for causing the centrifugal advance to not add timing as your RPM increases. Basically, you neuter it.

If you run a high initial, you might get kick back from your starter when trying to start.
This can damage starter components.
That is more likely to happen with a higher compression ratio engine.

Although I don't recommend running a "locked out" centrifugal advance, I know some people who run them, even higher than 22 degrees.
Fuel mileage is likely to suffer.
 

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Discussion Starter · #10 ·
they say Vortec 350s run a little less (32-34). But yeah, I’m sure I’m going to have to recurve it. But you are correct. If I plug in the vacuum advance, the starter isn’t liking it. The mechanical advance is full open at 2500 rpm. The only other springs I have make it open at 3500.
 

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I’m trying to find out if my intake gasket is leaking or not. I was low on carb cleaner, so I only half-a$$ed that test until the store opens. Vacuum gauge is reading a steady 10-11” at idle. If you blip the throttle around you can get it to peak at around 15”. I’m not too technical, so I’m wondering if anyone can tell what vacuum should be at idle with the cam spec in picture.
Comp Cams 08-422-8
View attachment 141825 View attachment 141826
To be honest, it has been years since I owned or worked on a carburated motor, so I can't be sure if your cams duration is excessive. You could perhaps google Comp Cams web page and study up. I can tell you that it is very common for the Exhaust valve to close after TDC. They are using the outgoing air charge to get theintake charge moving when the intake valve opens. Likewise, I believe they always open the Intake valve befor BDC. There are several degrees leading up to 180 degrees (and after) where the crankshaft journal isn't actually moving up or down, but more of a sideways action, per se. The cam grinders feel they need these extra moments of valve opening when the crank is rotating near 1000 rpm's (500 rpm's for the cam) and then again at 5000 or more. Be sure to spray around the throttle shaft bushings. That is where I find my intake leaks, and you mentioned the vacuum reading changes when the throttle angle changes..
 
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