On one of my engines (1967 427, port side), I have noticed what looks to be fuel out the exhaust at very low (idle) speed. I can't tell for sure, but it looks more like fuel than oil to me.  Just a slight boost in RPM and it stops doing it.  Could it be a mixture adjustment on the carburetor providing excess fuel at the low rpms? If it isn't fuel, I guess it could be oil, but we use very little oil over the course of a season in either engine, although occasionally there is some smoke, again noticeable at low rpm. The engine seems to run very well otherwise.  This is the engine that had the plug wires installed to several of the wrong plugs before I got it.  Any ideas would be appreciated. 

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Hi Tim....good to know. I am in no way a mechanic so I will have that looked at. This is the first I have heard of this so I am glad you chimed in. Dick, I am not sure if you are responding to the original post or mine but this is when I fire up and idle. I did not really get a chance to run her last year to check if it does it warmed up as I had other issues. So I can't answer as to does it do it while running all day. Tim, is that something you have to take apart the engine? As I said, I am not a mechanic. Thanks all.

I'm replying to the original post. There are 2 things to consider here. First, on a running, idling, warmed up engine with the choke valve wide open, remove the flame arrestor and shine a flashlight down into the running, idling carb. If you see any liquid fuel anywhere, you have a problem and definitely are rich at idle. The carb needs to see a doctor. The problem usually is a leaking needle & seat or leaking secondary diffuser gaskets. If all is well here, the problem may be a plugged exhaust crossover passage in the intake manifold. There is a passage in the intake manifold that runs from one bank to the other and exhaust flowing through this passage heats the manifold floor right directly under the carb. The purpose of this is to warm the incoming fuel air charge and aid in atomizing it. At higher engine speeds, the velocity of the air fuel mixture flowing through the manifold does a pretty good job of atomizing incoming liquid fuel droplets, along with the heat of a fully warmed up engine. But at a cold idle if this passage is clogged with carbon ( a very common occurrence) you're going to see a sheen from the exhaust water. On our previous 38 both engines did this when cold, but after a run on the lake when I came back to dock the sheen was no longer present as the heat in the engines were atomizing the fuel. Both of my crossover passages were plugged. When I did a valve job on the port engine, a plumber friend with a small power snake was able to break through the blockage AFTER I had soaked the passage with carburetor cleaner (nasty stuff) for 2 days. Upon engine reassembly, you could actually feel the passage portion of the intake manifold heating up and the sheen form that side was gone.

There could also be other reasons for a sheen, but these 2 are really common.

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