PINTO CAR CLUB of AMERICA
Shiny is Good! => Your Project => Topic started by: M Jennings on February 08, 2014, 09:44:01 AM
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Hello all. I'm a long time lurker.This is my first post.
I'm rebuilding my 78 Cruising wagon that I tore apart 25 years ago.
It's back together now but I have a problem with low vacuum in the V6.
The engine struggles to idle below 1500 RPM. If I turn the idle screw
lower the engine stumbles and dies. At 1500 RPM I'm lucky to see 12
inches of vacuum. Any lower RPM the vacuum decreases.
The engine is a 2800 cc V6. It has 10 to 1 .040 over pistons. A Norris
mid-range performance cam. Offenhauser dual port manifold with a Holley
390 four barrel and Hooker headers.
Some of the things I have tried:
Adjusted the engine timing. Has minor affect on the vacuum.
Plugged all the vacuum lines to eliminate other components causing the issue.
Ran a compression test. All cylinders around 175 PSI.
Ran a leak down test 4 cylinders at 8%, 2 cylinders at 13%.
Sprayed starting fluid around carb and manifold while running to see if
I could detect a change in the idle. None found.
Tried injecting smoke into the intake to see if there is a vacuum leak
around the carb or intake. Didn't see smoke escaping.
Took the carb off and made a plate to block off the carb opening on top of
the intake. Backed of the rocker arms so that all the valves are closed and
then connected a hand vacuum pump to test for leaks. None found.
The cam has no adjustment since it is gear driven and the alignment dots are OK.
I have rebuilt the carb.
Any help or ideas are appreciated. I would actually like to drive the car again.
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a leaking power brake booster gave me some grief on my pinto ...
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Assuming that the Carburetor isn't bleeding vacuum below the Butterflies when running, (Warped base or mounting surface) then if the Cam profile is the cause of low vacuum, you may need a vacuum accumulator to re-establish a 15-17 inch reading. This is common cause with Hi-Po cams if a external leak can't be discovered. Are the marks on the timing gears , accurate with the new cam specs? (Could need a thorough degree check to validate keyway relationships ,if any , to correct any flaws)
Porosity in any casting would have revealed itself with your other checks, unless the engine temp is an issue there.
Just some ideas , I'm sure others have more to add.
Pintosopher
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what is the total duration of your cam also what is your lobe separation?? To much duration: on a cam will cause you to lose vacuum. Thanks Chester
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Thanks for the replies so far. The brake booster has been eliminated as a problem since I have disconnected and plugged all vacuum lines to auxiliary equipment.
As far as the cam goes it was purchased 25 years ago and the documentation for it has been lost, but I believe it is a Norris cam 3039 grind. Specs might be .282 duration and .444 lift. I did not degree it since I didn't have the means or expertise. I can't find that Norris is still in business unless someone knows otherwise.
What is a vacuum accumulator?
Thanks again for the replies.
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Rebuilding carb means nothing if all ports and passages not guaranteed as fully open. If say a cross leg for idle was plugged where you could not see it then could cause trouble. Where are the throttle plates ending up at the idle speed it does best at? If too far open to get into transfer slots it never will idle right and vacuum will be low if too rich from that. If idle vacuum low enough the power valve could be open too, plug it and then reset idle.
You must guarantee cam timing as well, could damper (and TDC mark) on crank have slipped? Simply saying the timing marks are lined up could be an error. You need to know where the lobe center of cam is at. The 282 duration doesn't mean anything, you have to know at what timing that is, like at .050" lift. In that case it would be WAY too much cam.
Hydraulic or solid lifter?
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It's very common on factory dampners for the outer ring to walk, I always put a mark on both parts across from each other and always look before checking timing.