Here's another hole in your head! 
AMC V8's are just as notorious as the Gen 1 Vortec's at intake manifold leaks.
I could be wrong as I am not all that familiar with AMC's, but your vacuum is actually a bit high I think from my experience. I've tested a few injected AMC 360's and they were right around 12-13 inches at idle. I'll have to check Alldata. You are a 360 right?
Most other V8's are typically in the 16-20 range at idle. You might have a small vaccum leak, but that is not what concerns me. The most important part of performing a manifiold vacuum test is not the actual pressure reading, but the way the needle behaves on the gauge.
If the needle oscillates quickly between a 3-4 in/HG variation, this normally means that you have worn intake valve guides. If the needle still bounces around in the same fashion as above, it points to tired valve springs.
Valve timing does have an effect on vacuum in the manifold. If your timing chain has slipped one tooth and/or the timing chain is excessively stretched and effectively retarding your valve timing, your vacuum will be lower but it won't oscillate
Too little ignition timing and bad rings can also show low vacuum readings, but they will not make the needle bounce around near as much.
The higher the vaccum the lower the MAP output voltage; just the opposite for low vaccum. This is another reason I think you may have too much vacuum at idle.
Do you have access to a scanner that can show you the barometric pressure reference the MAP obtains at key on? Is the vaccum port on the manifold larger than the gauge hose? If so, you need to use a reducer coupling, or else the vacuum readings can be inaccurate.
If it ends up being that you do have too little vacuum, a good place to start would be the CTO valves and/or the CCP solenoid. The CTO valves are a common vaccum leak problem from what I'm told.
AMC V8's are just as notorious as the Gen 1 Vortec's at intake manifold leaks.
I could be wrong as I am not all that familiar with AMC's, but your vacuum is actually a bit high I think from my experience. I've tested a few injected AMC 360's and they were right around 12-13 inches at idle. I'll have to check Alldata. You are a 360 right?
Most other V8's are typically in the 16-20 range at idle. You might have a small vaccum leak, but that is not what concerns me. The most important part of performing a manifiold vacuum test is not the actual pressure reading, but the way the needle behaves on the gauge.
If the needle oscillates quickly between a 3-4 in/HG variation, this normally means that you have worn intake valve guides. If the needle still bounces around in the same fashion as above, it points to tired valve springs.
Valve timing does have an effect on vacuum in the manifold. If your timing chain has slipped one tooth and/or the timing chain is excessively stretched and effectively retarding your valve timing, your vacuum will be lower but it won't oscillate
Too little ignition timing and bad rings can also show low vacuum readings, but they will not make the needle bounce around near as much.
The higher the vaccum the lower the MAP output voltage; just the opposite for low vaccum. This is another reason I think you may have too much vacuum at idle.
Do you have access to a scanner that can show you the barometric pressure reference the MAP obtains at key on? Is the vaccum port on the manifold larger than the gauge hose? If so, you need to use a reducer coupling, or else the vacuum readings can be inaccurate.
If it ends up being that you do have too little vacuum, a good place to start would be the CTO valves and/or the CCP solenoid. The CTO valves are a common vaccum leak problem from what I'm told.
. Yeah, its a 360, fully balanced and rebuilt not but 2 yrs. ago. I've put maybe 1000+ mi. on it. I can also check my Chilton's manual when I get home and see what it says the vacuum is supposed to be at. Not sure if it will make a difference now that its EFI-ed and not carb-ed.
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