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Discussion Starter · #1 ·
Just built a 350 with Performer RPM components. Got it running, but I have little to no vacuum. I've checked for a vacuum leak (checked hoses, sprayed edges on intake with carb cleaner, etc) but can't find one. At a little over 2000rpm, there is 10in vacuum, but at 1000rpm, there is nothing - the needle sits on zero (it's a fairly new guage). Any thoughts? I'm at my wits end and cant' figure out where the vacuum is going. Thanks
 

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Yeh, if its running at all its gotta have some vacume. You might have a bad guage, clogged line going to the guage or you are not connected to a good vacume source. If your connecting the guage to a port on the carb make sure your not connected to "timed" vacume. "Timed" vacume will be zero at Idle but will increase as you open the throttle plates a little. The vacume advance usually gets connected to that "timed" port.

Clark
 

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Discussion Starter · #5 ·
I'm sure that I have vacuum, just not very much. If I put a finger over a vacuum port, I feel a very little bit. The gauge is very new, but I will try using another guage. Someone else suggested the valves, but I did a compression test and all cylinders were between 160-170.
I have been connecting the guage to the carb. I will try connecting directly to a manifold vacuum source (but I tried this before and it read the same). Is there anything else I can try?
 

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Discussion Starter · #6 ·
Also Clark,
Any idea which is the timed and which is the untimed port on a Carter AFB - ?
 

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Discussion Starter · #8 ·
On the front of the Carter and Edelbrock, there are three ports on the carb base. The large center is for the PCV. Are either of the outside smaller ports "timed." I think when I put the guage on these, they both read little to no vacuum, but I'll try a different guage.
 

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John,

Is this a new Edelbrock carb? The reason I ask is that I have a new one that I have to return. Edelbrock's tech line told me there are some carbs on the market which have a machining error on the ports on the front of the base.

Specifically with the full vacuum port. When looking at the carb, the small port on the drivers's side is the full vacuum and the small one on the passenger's side is the timed. The error turns out to be that the full vacuum port is machined above the throttle plates when it should be below them, therefore no full vacuum at idle. The timed port is good as well as the PCV port. You can verify whether your carb is bad by looking down the primary venturies. Back off the idle screw to allow the throttle blades to close completely. You should be able to see a horizontal machined slot in the timed (pass.) venturi only. If you can see one in the full venturi, your carb is bad. Also, you can tell which is timed or full by looking at the front of the carb. The timed port is physically higher than the full port.

Note: There is also a vertical slot in each venturi, these are the idle fuel slots, don't confuse them with the vacuum ports.

I two am experiencing low vacuum at idle (10"), but get good readings of 14" at higher rpms. This engine had good vacuum before this carb. But, I also changed the intake gaskets so I need to verify these are not the source of the low vacuum. I have capped all vacuum ports to verify that my accessories are not causing the leak. My readings have been taken off of the intake to ensure I am measuring a full vacuum port.

Your symptoms are the same as mine, maybe the carb is the problem. Let me know what you find out from this inspection.


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Brian TC#1092, ACES#4374
My Garage
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1964 Malibu SS Convertible (Restored Garage Queen)
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Clark this has been beaten to death.The vacuum advance gets connected to manifold vacuum.This is why you disconnect it to set timing,it works opposite to what most think.
Vacuum its in the advance position, low vacuum its retarding timing to stop detonation...FRED

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Fred

I agree with you. BUT many folks on this site have thier distributors hooked up to timed vacume and seem to be happy with it. I have even seen new distributors come with instructions to use timed vacume. I followed many of the very heated debates on this site concerning which vacume to use for the distributor. I tried to remain neutral on the subject. I should have done a better job remaining neutral on this post.


I personally have had better results by using full manifold vacume as you state. (I have tried both) Many others have had better results using ported vacume. I dont know why one method works for some but not for others. It must be one the worlds great mysteries. I guess the only right answer is the answer that works, whatever that might be for your car.

Clark
 
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