Low vacuum on stock 383

[Yellow Submarine]

Now that the over temp issue on my stock 383 issue has been put to rest I need to figure out why the vacuum is only 12". The engine was rebuilt several years ago but the car was never really driven as it was garaged midway through a restoration. The machine shop indicates a complete stock rebuild including having the cam re-ground to unknown specs. The idle indicates a docile cam, not a thumper.
I have the baseline timing at 12deg. The idle mixture screws at 2 turns. I have tried blocking any and all vac lines to rule out leakage from them. I have not checked for intake manifold leaks yet. I also plan on doing a compression check.
One of my thoughts is the cam may have been ground incorrectly or the cam timing  may be retarded. Anything I am missing?


John

[Strawdawg]

Without knowing what cam is in the car, it is really all guessing.

What is the idle rpm?  And what is the cranking compression (when you check it)?

[Chryco Psycho]

EVen if it is a good cam if the cam was installed without degreeing it that could be the whole problem as I have seen cams 11* off just lining up the dots

[cudabob496]

12"?  Is this at idle?

what are cam specs, may be an ok number.

Increase idle?

[Yellow Submarine]

Finally checked cold cranking compression today. This is on an engine completely rebuilt in the mid 90's but not driven. It idles a little rough, has low vac at idle. About 1,000-900 rpm, no functioning tach to verify it has about 12" of vac from front base port on factory carb. The cam was reground to unknown specs during overhaul. Piston cr is unknown. Heads are 906's. Plugs are light brown. Plugs #6 and #7 have some darker color and are a little wet with gas or oil, not sure which. Distributor has Pertronix module in it. Base timing is 12deg.
Compression is as follows
1-118, 3-120, 5-120, 7-100
2-125, 4-130, 6-110, 8-120
Any ideas?
John

[Chryco Psycho]

To me it sounds basically like a lame cam in there now , i would be looking at something like the 702 Voodoo with short overlap & duration to build a lot more cylinder pressure this should idle well bring up the vacuum & compression & make the engine a lot more fun to drive . Bad news is also the engine doesn't seem overly healthy as you have a 30% variance from high to low , 15% is generally considered max , although a cam swap may help the compression #s even out & the engine may not have completely broken in & seated the rings etc if it was never driven so putting on miles will help too .

[Yellow Submarine]

Thanks cp. I want to drive it a little even though most of the interior is not in place. When I take it apart is when I would be ok opening it up or swapping it out.
Other things I'm thinking about is are the ignition wires ok as well as the carb. Also someone, maybe you, mentioned the cam maybe indexed wrong. Given the compression does that still seem likely?

John

[734406pk]

To me it sounds basically like a lame cam in there now , i would be looking at something like the 702 Voodoo with short overlap & duration to build a lot more cylinder pressure this should idle well bring up the vacuum & compression & make the engine a lot more fun to drive . Bad news is also the engine doesn't seem overly healthy as you have a 30% variance from high to low , 15% is generally considered max , although a cam swap may help the compression #s even out & the engine may not have completely broken in & seated the rings etc if it was never driven so putting on miles will help too .

  looks like you may have a compression leakage there. Sorry!

[Chryco Psycho]

Yes the cam could still be installed off a few degrees but given the variable of thousands of cams out there I would rather put a good known cam in there than guess , yes you have compression leakage but that could quickly change as the engine breaks in & seals up .

[cudabob496]

vacuum gauge accurate

vacuum leak

carb in tune

[Yellow Submarine]

Bob

The carb is another ? It is a stock carter that was probably rebuilt along with the engine in the 90's. I think I will "borrow" the Holley 670 off my Cobra to see if it runs the same.
About how many miles or hours of running do you think it takes for the rings to be considered well seated?

John

[cudabob496]

switch from dino oil to synthetic after 1500 miles

[73_Cuda_4_Me]

How about warmed up compression test, with WOT... fully charged (and charging) battery with all plugs out...

Cold compression can vary 10-15 percent from operating temp engine...

[Strawdawg]

Oops

[Chryco Psycho]

I would put 500 miles varying rpm as much as possible to seat it in , if it has chrome Moly rings 5000 miles won't seat them in .