Vacuum Level Question

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67Redfish

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Hey guys,

I am trying to get my 273 running right and am still having problems. I was running a stock 340 intake but recently converted to an Edelbrock LD4B. With the 340 intake my idle vacuum was approximately 15 give or take (Comp XE262H cam). With the LD4B my idle vacuum is approximately 7. I have not identified a leak yet so my question is whether the LD4B would account for the reduced vacuum. Should I keep hunting for a vacuum leak somewhere? Thanks.
 
I would say you have a massive vacuum leak somewhere. The intake would not change the vacuum. You have a 273 with 340 intake.. how does the intake mate with the heads? I know the bolt angle is different on the 273 but how well the intake mates with the heads is important.
 
Take a look at the obvious stuff like carb base gaskets first, as it sounds for sure like you have a leak somewhere.

I assume you are running a PCV valve. So,with the vacuum gauge connected, and the engine running at idle, cover the crankcase breather hole with your hand. You should feel vacuum build. If the vacuum gauge goes up and/or idle quality changes your vacuum leak is internal. Meaning you are leaking at the ports or your intake guides are seriously loose.
 
The LD4B is actually a 273/318 port intake, LD340 is the large runner style.

If all you did was change the intake, I'd suspect the following:
A cracked intake.
A possible slipped intake gasket on install.

It could possibly even be an intake that has been machined for heavily milled heads and no longer seals to stock heads correctly.

Which intake gaskets did you use?

Mark.
 
I did not notice any cracks in the intake when I looked it over after purchase, but I did not have it inspected for that problem. The only obvious modification was some grinding along the edges of the ports. It did not look substantial, but then again I had no frame of reference.

The gasket is metal so its probably thinner than a Fel-Pro or Mr. Gasket. I am going to try a thicker gasket and see if that helps any.

Thanks for the advice.
 
C130 Chief said:
Take a look at the obvious stuff like carb base gaskets first, as it sounds for sure like you have a leak somewhere.

I assume you are running a PCV valve. So,with the vacuum gauge connected, and the engine running at idle, cover the crankcase breather hole with your hand. You should feel vacuum build. If the vacuum gauge goes up and/or idle quality changes your vacuum leak is internal. Meaning you are leaking at the ports or your intake guides are seriously loose.
Click to expand...

Curious --- if you can feel vacuum build but the manifold vacuum reading stays the same, all is well, right?
 
Thanks Mark.....

So I must have a cam in this engine then.......or, a huge vac leak, but I cant find any....and one that big should be obvious....I would think.

I did find out that I am missing my secondary throttle linkage on the carb (its a Eddy 1407)......so up to now, I have only been running a 2v!

Hopefully I can find the linkage for it....
 
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