390 massive vacuum leak?
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Elder User
Joined: Sep 2003
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From: colorado
390 massive vacuum leak?
So I did a quick re-ring on this 390 I acquired for my 77.
Got it in and fired up, #5 cylinder is not firing. I have spark all the way to the plug. Checked with a vacuum gauge and it jumps wildly at idle, the needle is sweeping back and forth about half the gauge. At about 2200 or 2300 it smooths out but then I can't tell if it's still missing. Figured it was an internal leak around the manifold so I took it off and re sealed it. started up, exactly the same. I also lapped the valves when I had it apart and the heads were off and checked for a seal with soapy water and compressed air so I don't think it's a burned valve. I started it with the valve covers off and it looks like the lifters and push rods are working fine, I've got all new lifters, springs and hardware. When I re-sealed the manifold I checked the push rods and they're not bent.
FWIW the plug is black and sooty.
Any ideas?
Got it in and fired up, #5 cylinder is not firing. I have spark all the way to the plug. Checked with a vacuum gauge and it jumps wildly at idle, the needle is sweeping back and forth about half the gauge. At about 2200 or 2300 it smooths out but then I can't tell if it's still missing. Figured it was an internal leak around the manifold so I took it off and re sealed it. started up, exactly the same. I also lapped the valves when I had it apart and the heads were off and checked for a seal with soapy water and compressed air so I don't think it's a burned valve. I started it with the valve covers off and it looks like the lifters and push rods are working fine, I've got all new lifters, springs and hardware. When I re-sealed the manifold I checked the push rods and they're not bent.
FWIW the plug is black and sooty.
Any ideas?
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Joined: Jul 2002
Posts: 42,562
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From: Long Island USA
Well, that's a really good sign 
Swap the plug and plug wire with another cylinder (making sure you move the wire at the distributor cap too) and see what happens.
How did you determine that #5 wasn't firing?
Swap the plug and plug wire with another cylinder (making sure you move the wire at the distributor cap too) and see what happens.
How did you determine that #5 wasn't firing?
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From: colorado
I know because when you unplug that wire it makes no difference to how it's running. If I unplug the other wires it stubbles and misses. I know it's got spark all the way to the plug because I took the plug out and ran it with the plug grounded. It's got a nice fat spark at the electrode.
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