Soot and water in exhaust
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500, 472, 425, 368 Discussion, Soot and water in exhaust in Cadillac Engine Technical Discussion; I noticed a bit of soot under the place where my tailpipe aimed down, so I figured I was running ...
  1. #1
    davesdeville's Avatar
    davesdeville is offline Banned
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    Soot and water in exhaust

    I noticed a bit of soot under the place where my tailpipe aimed down, so I figured I was running rich. I turned what I think's the idle mixture screw all the way down, it's the screw with a spring on it on the drivers side of the front of carb. Anyway I started the car and revved it a bit. A lot of soot and water were deposited on the gutter under where the tailpipe was. So I turned it the other way until it was pretty far out, and started it and revved it a bit. The same thing happens.

    What gives, what am I sposed to do about this?

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    elwesso's Avatar
    elwesso is offline Cadillac Owners Master
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    Re: Soot and water in exhaust

    75 was when they started all that smog crap, right???

    Whatever the case is, it sounds like theres just a lot of crap built up inside it...... Run it hard for a while and see if it clears up......

    Turning the adjustment all the way in will be full lean, and all the way out will be full rich...... On a 327 I worked on one time, I adjusted the mix by screwing it all the way in, and then I THINK 1 3/4 turn...... I wouldnt go much past 2.5....

    The steam could be from (if you have it) the cats.... A normal by product of the catalytic conversion is steam, so thats normal.... Thats why when you put your hand up to the pipe it feels moist..... I see water coming from the tail pipes of cars all the time (I mostly notice it in fords)

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    500-CID's Avatar
    500-CID is offline Cadillac Owners Member
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    Hey Dave, you didn't say what engine you have i'm guessing it's the 425?? with the q-jet cab.. On my 500 q-jet carb it has two mixture screws on the vary bottom one on the left and one on the right and one for idle up higher and on the drivers side of the carb. the one you say you turned sounds like the idle to me and would do nothing but make the engine idle higher. the two bottom ones are kind of hard to get to.. Anyway hope this helps

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    brougham is offline Cadillac Owners Master
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    The biggest thing that causes carbon is when there's something wrong with the carburetor or it's not setup right or the timing is off. Some other things too but that's the main ones. But it's also common on bigger engines that just maily get used for short city drives, which is the kind of driving the car I have that does this does.

    Usually 2 and 4bbl carburetors have 2 screws and you adjust them with the car running. I've always been told that when you turn the screws all the way in the engine should pretty much want to stall or will, and you start with them turned out a couple turns each and start adjusting from there. But according to the Cadillac Haynes manual I have (which I pretty much regret buying) you start with both screws out 6 turns and once it's warmed up you turn one screw colckwise untill the engine slows down 10rpm and then do the same to the other. You probably already read this somewhere, or whatever the actually setting is if that's not it.

    Condensation is normal. It's from the engine, not the converter. It should be less noticable when it's warmed up.

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    davesdeville's Avatar
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    Yeah it's the 425 with the Q-Jet.

    My Haynes is no help, the exploded view carb diagram doesn't even show this screw on there. Anyway it's on the very bottom of the carb on the drivers side. I was trying to get it to idle at a higher speed just after I'd rebuilt the carb about a month ago, and I turned it then and it didn't seem to do anything. Then last week, there was the smaller amount of soot on the ground, and I got gas yesterday, then I adjusted that screw today and got more soot.

    The secondary metering rods power tips are both broken off, and I've locked out the secondaries until I get my replacements, but that shouldn't have anything to do with this.

    Anyone want to buy a 78 Fleetwood? lol..

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    Katshot's Avatar
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    Re: Soot and water in exhaust

    The screws you adjusted are called "idle mixture screws" and do nothing beyond Idle. Proper adjustment is to turn them in until the idle falls, and then back out until you obtain maximum idle. Do each one seperately. Moisture in the exhaust stream is very common on ALL cars and is usually due to condensation. Once you warm the exhaust system long enough to burn off the moisture, you should notice no water. Wes IS right that the Cats do create water but it's generally not enough to create noticable water in the exhaust stream, although long idle times CAN produce water accumulations in the exhaust. The soot you're talking about is generally caused by a rich cylinder or and overall rich condition which could be caused by a number of different things.

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    Re: Soot and water in exhaust

    You might just pull the plugs to see if theres one that fouled or one cylinder thats overly rich.....

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