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318 stumble

2255 Views 6 Replies 6 Participants Last post by  68RT
I've got a newly rebuilt '82 Chrysler 318 in a '34 Plymouth project car. I have a couple thousand miles on engine and started noticing a little roughness at idle. At part throttle the stumble is more noticeable, but not steady. I noticed that all header pipes are a little discolored, except #4 & #5 cylinders. With a digital temp. gauge at idle all cylinders read 300 to 400 degrees, except 4 & 5, which read around 185 to 200. I swapped spark plugs with other cylinders and nothing changes. I swapped plug wires, still nothing. Everything is brand new, aftermarket electronic distributer, accell 8mm wires, plugs etc. I did a compression test. All cyl. are good. Leak down test, all cyl. good. Pulled valve covers thinking maybe I had lost a coulpe cam lobes, but all rockers arms are moving properly & no bent pushrods. I have a dual plane Weiand manifold with 600 cfm Edelbrock carb. I am really stumped at this point. The only thing I haven't been able to swap is the distributer. I have an in-line spark tester and it shows every plug is firing. HELP !!!
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Definitely odd given the two cylinders do not share a runner with each other, they are on the two opposite two H portions. The only thing similar the two cylinders share is the fact it would be the two cylinders which feed the EGR ports from the exhaust ports. Did you block the two ports in the head that feed the intake manifold with a piece of tin to slow the amount of gasses going from the heads to the intake manifold? I usually do this because the aluminum intakes don't require as much heat in this fashion to warm up, and not a big old engine EGR person myself. This is the only item I can think which could cool the two exhaust pipes given it shows both plugs, wires and the distributor are functioning properly.
Are you sure you got the correct intake gasket? Might be as simple as that.
Thanx, everyone. I've checked for vac. leaks and found nothing. This intake is a non-EGR manifold. I'm pretty meticulous on engine assembly and think I would have noticed if the intake gaskets weren't aligning properly.
Well, I'm puzzled. The cooler exhaust indicates that normal combustion is NOT taking place in those cylinders, or the exhaust is escaping elsewhere.

To review:

Spark is good in all 8 cylinders.
Compression is good in all 8 cylinders.
Leakdown is good in all 8 cylinders.
Valve actuation appears good in all 8 cylinders.
There is no reason to think that fuel distribution is blocked or uneven.

I'm stumped at the moment. Did you put a vacuum gauge on it? What was the reading at idle? Was it rock-steady, or was the needle flickering regularly, or floating? What happens to the reading when you rev up to 2000 RPM, and when you hold engine speed there?
The one thing that you cannot check under normal test would be failure of the intake gasket at the bottom edge. With a warm (so you would have visible vapors) engine, can you slightly pressurize the valley through the oil fill area and with the carbs in the open position see if you have a leak between the valley and the intake area by seeing vapors come out the venturis.
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