low engine vacuum

[Frank V.]

Re: low engine vacuum

I have not been able to test vacuum in drive because the engine won't idle in gear. At 650 rpm in "park" with all vacuum ports closed the engine is only producing 13-14" of vacuum. At 950 the idle becomes smooth and the engine pulls 20". I will get the exact cam specs. But if it cant pull vacuum under no load at idle, it's not gonna under load? And if there is insufficient vacuum will this not force the quadrajet into its power metering thus causing an over-rich mixture? I did all my testing with ALL ports plugged both on the manifold and the carburetor.

[Duke W.]

Re: low engine vacuum

The no load (neutral/park) speed/vacuum you report is odd to say the least. If it makes 13-14" at 650 I would expect something less than 20" at 950. Unfortunately I can't put my finger on a specific problem that would cause this strange behavior.

The power valve likely has nothing to do with it. At these these speeds with no load all the fuel is going through the idle/off-idle system, and none is being delivered through the main and power circuits because there's not enough signal at the booster venturi to cause fuel flow.

Rather than "cam specs" what you need to provide to indict or eliminate the camshaft as a suspect is the manufacturer and part number.

Another suspect is the distributor - assembly (dimple pointing the right direction?), installation, and wire indexing on the cap.

Correct wire indexing and distributor installation orientation is in the CSM. The "dimple" issue is a matter of system knowledge and a lot of guys, including professional mechanics, don't understand it and screw it up.

Duke

[Frank V.]

Re: low engine vacuum

I'am not sure I follow the part about the dimple. The distributor is installed with the tach drive running perpendicular to the engine centerline as shown in the assembly manual. The basic timing, no vacuum advance, is set at 8 degrees btdc as per spec. The wires are installed in the correct firing order and orientation according to the manual. My thing about the carb is this. The power piston allows more fuel to flow through the primarys as vacuum drops (high load condition). If there is a low vacuum condition to start out with, at no load, would this not force the carburetor to sense the need for additional fuel?

[Duke W.]

Re: low engine vacuum

I see you are a new user. Welcome aboard! This site has some true experts, and we have been at it for nearly ten years. The archives contain a wealth of information, and the "dimple" issue has been well hashed out. It should be easy to find educational threads. I don't think the word "dimple" shows up in too many unless there was one about Shirley Temple that escaped my scrutiny.

Spend a little time in the archives and get a real education. The C3 SB distributor installation orientation and (I think) the plug wire indexing is different than a C2 SB.

I know Terry McManmon posted a photo of correct C3 distributor installation a year or two ago. The dimple deal is something some of us learned in the school of hard knocks, which was circa 1966 for me since there was no NCRS TDB back then.

From what you describe about your distributor, it might be okay. I'm not sure without looking at it myself and checking against the various references. If you can post a photo with the #1 plug wire marked, Terry and others should be able to confirm whether or not the distributor orientation is okay. If the dimple is indexed incorrectly the distributor orientation will be way off normal.

As far as your carburetor theory is concerned, all I can offer is see my previous post.

Duke

[Frank V.]

Re: low engine vacuum

I spoke to the machinist about the head/intake manifold matching. He told me that the surfaces were checked and found to be parallel and with the proper gaps. He felt very confident that the intake was sealed properly.
However he admitted anything is possible and is willing to pull the manifold again to reinspect it for cracks and misalignment.

[Joe L.]

Re: low engine vacuum

I have not been able to test vacuum in drive because the engine won't idle in gear. At 650 rpm in "park" with all vacuum ports closed the engine is only producing 13-14" of vacuum. At 950 the idle becomes smooth and the engine pulls 20". I will get the exact cam specs. But if it cant pull vacuum under no load at idle, it's not gonna under load? And if there is insufficient vacuum will this not force the quadrajet into its power metering thus causing an over-rich mixture? I did all my testing with ALL ports plugged both on the manifold and the carburetor.

Frank-----


One other thing I thought of: are you sure about the engine RPM readings you're getting? You're not just going by the in-car tachometer, are you? Have you cross-checked that with another tachometer?

[Jim T.]

Re: low engine vacuum

Frank hook up the vacuum to the distributor,direct from full time vacuum source to the vacuum advance cannister and see what your vacuum guage reads.
My 68 and my 70 run full time vacuum all the time. If the vacuum hose is separated from the vacuum advance canister, idle speed decreases.
My 70 did not leave the factory with full time vacuum, first modification I made 38 years ago.

[Timothy B.]

Re: low engine vacuum

Frank,

I know you are getting tons of suggestions and I will offer one more. Your description of the engine at idle sounds like the 350HP cam to me. Better check that camshaft # as my 72 showed the same idle problems in drive after I installed the 350HP cam. Car also had TH400