Black Smoke from Exhaust

[Whistler]

I have black smoke coming from my exhaust and the engine started running very very rough. Any suggestions on what the cause might be?

[Marian Claire]

Welcome to the forum. Black exhaust smoke could be a sign of a rich mixture. Do you have an adjustable main jet on the carb? If so turning the jet in will lean out the mix. Pull the plugs and check for black/sooty buildup caused by the rich mixture and make the engine run rough. How do you run you’re a-4? Short time at low RPM? What is your operating temp? Dan S/V Marian Claire

[hanleyclifford]

Black smoke usually suggests overly rich fuel mixture (whereas blue smoke suggests oil burning).

[Dave Neptune]

Whistler, welcome to the forum! Did the black smoke and rough running coinside with each other? It could be as simple as the choke being stuck partialy closed. Does it idle and run OK at low RPM & idle?

First check visually to see if the choke is actually opening and closing properly.

If all is well with the choke then it is probably in the carb. Usually carb trouble is related to running to lean as the fuel metering jets will get pluged up with some sort of debri. However when the carb goes rich which is indicative of the "black" smoke it is usually an air corrector plugging up from debri. It could also be the Needle and seat are stuck open but that will mostly cause the engine to flood out and not run~doubtful.

Check a couple of things and get back to us.

Dave Neptune

[Whistler]

Thanks for the suggestions. Today I replaced the points, condenser, distributor cap and cleaned the plugs (they're only one month old and they had a lot of black carbon on them). The engine is running smooth again.......but I have a feeling its a carb issue. I will look at the choke and see if it is functioning properly. Is there any simple way to clean the jets?

[hanleyclifford]

If you want to try and clean the carb without taking it apart, try a can of "Guaranteed to Pass" available at your local NAPA. Add it to a full tank of gasoline.

[Marian Claire]

Along the lines of Dave’s choke suggestion also make sure the flame arrester is clean. Do you have the adjustable main jet? Was the smoke a sudden change or a slow buildup? Dan S/V Marian Claire
Edit: I visited Whistlers public profile and found the answer to my second question. A sudden change.

[Dave Neptune]

Whistller, since the carb is rich I suggest that you spray around and through the air correctors and the discharge tubes with a good carb cleaner if you don't want to do the rebuild. As I stated the jets are probably fine as if they plug up you will go lean not rich.

First confirm the choke is working and releasing properly.

Then start on the carb, I suggest that if it isn't the choke just go ahead and rebuild the carb for piece of mind. The carbs are very simple and quite easy to rebuild.

A word of caution on additive type of "cleaners" added to the fuel~~they may and often do break stuff loose in the tank and lines causing a series of issues until the system is purged. This can be a real PIA in some cases especially old marine fuel tanks.

Dave Neptune

[hanleyclifford]

Dave is right about the "cleaners". If you do go that route, be prepared for what you might find and get ready to deal with it. On the other hand, if you do indeed have a dirty fuel system (including the tank), maybe it is time to get it all out at once rather than letting the ethanol do it piecemeal?

[Whistler]

Dave - what should I spray "around and through the air correctors and discharge tubes"?

[Dave Neptune]

Whistler, most of the "spray" carb cleaners are pretty good. I have been usig Gumout lakely because it was on sale. I would by the brand with the longest "spray rube" for ease of getting into the orfices. Since you will probably need to remove the carb to get to the tubes and correctors you may as well take it apart and do the redo.
I sugest you install a new hose to the carb from a polishing type of filter. Then the fuel being deliverd wil be clean downstream to the carb.

Dave Neptune

[StephenGwyn]

I'm going to renew an old thread.

My A4 has recently started to put out some black smoke, not a vast quanity, but more than before which was none. The engine runs as smoothly as ever. I know what an A4 with the choke slightly shut sounds like, and that's definitely not what I'm hearing. I recently fixed a chronic overheating problem, so my first guess was that some other problem had been masked the fact that an overhot engine burns better. However the smoke increases as the engine warms up.

Thoughts?

SG

[capnkenny]

Clean fresh fuel is the food of choice. I have 2 filters one polisher and other is a 10 micron in line before carb. take your carb out soak it blow it out make sure your float and needle is working. I run fresh fuel from a plastic outboard tank. Mine smokes but runs smooth.

[hanleyclifford]

Are you using the adjustable jet?

[StephenGwyn]

Nope. Late model, fixed main jet.

Is there a particularly good spray carb cleaner out there? Names like "Guaranteeed to pass" don't inpsire confidence. But then neither does "Marvel Mystery Oil" so what do I know?

SG

[hanleyclifford]

"Guaranteed to Pass" is a good product for adding to motor vehicle gas tanks to clean up a system to pass emissions. If you put it in your boat tank be ready for a very clean tank and you know where the gunk will end up. It sounds like you have a rich mixture. Have you tried leaning your idle? Try 1/4 turn counter clockwise increments and see what happens.

[StephenGwyn]

The tank is only 4 years old, and I have a 10 micron filter.

I've twiddled the idle mixture knob, but I don't get much of a change in either revs and no change in the smoke.

SG

[hanleyclifford]

Originally Posted by StephenGwyn

The tank is only 4 years old, and I have a 10 micron filter.

I've twiddled the idle mixture knob, but I don't get much of a change in either revs and no change in the smoke.

SG

Sounds like you have a blockage in the carb body, possibly in the idle jet. Suggest you clean the passages. Also note the two small holes on either side of the throttle plate which require a very fine wire to clean.

[sastanley]

SG, are you running fuel with ethanol in it?

I had lots of trouble with 10% ethanol. Once I found an ethanol free fuel source (albeit at over $4/gal) I've had ZERO fuel problems in the last two years. I also replaced my fuel tank in 2009 with a polyethylene tank from Moeller after a breech that leaked fuel into the bilge. But, I continued to have fuel problems even with the new tank until I removed ethanol from the equation.

The other issue was to make sure my fuel fill o-ring was new each year...I think I had some leakage there too. Even at the high $$ West Marine they are $5 for a pack of three!

[JOHN COOKSON]

Originally Posted by StephenGwyn

I've twiddled the idle mixture knob, but I don't get much of a change in either revs and no change in the smoke.
SG

This is happening because there is something else that is controlling the mixture. You need to get the carb apart and get it cleaned out.
Hanley picked up on this........

TRUE GRIT

[ndutton]

When you get the carb apart give the floats a shake and listen for the sound of liquid inside. Make sure the backfire flame arrestor is clean too. Is the choke opening fully?

[StephenGwyn]

Wouldn't a blockage on the fuel side cause the mix to be kean, not rich?

There is no ethanol in my fuel.

SG

[Marian Claire]

The idle mixture adjustment regulates air flow. So a blockage in that area will limit air and make the mixture rich. As noted by others the fact that you do not hear/see much change when you use the idle adjustment is very telling.
Dan S/V Marian Claire

[StephenGwyn]

Originally Posted by hanleyclifford

Sounds like you have a blockage in the carb body, possibly in the idle jet. Suggest you clean the passages. Also note the two small holes on either side of the throttle plate which require a very fine wire to clean.

Aren't those holes the where the fuel gets sucked into the airflow? So wouldn't blocking those holes cause a lean condition?

Also, why is the only symptom black smoke? Everything else I read talks about running rough, but the engine runs smooth at both idle and cruise. Is it the case of the fuel mix being *slightly* off, so I get some smoke, but not off enough to cause poor running?

Forgive me, I don't mean to dispute peoples' expertise, but I formed an idea of how the system works looking at the diagrams. Of course I may be wrong, so please enlighten me.

Turning aside to the larger picture, while I'd be happy if it was carb problem, since carbs are easy to fix (or replace, I have a spare carb), are there any other likely ways I could end up with unburnt fuel in the exhaust. Valves or rings? (he asked, hoping the answer is "no").

SG

[roadnsky]

Originally Posted by StephenGwyn

...Also, why is the only symptom black smoke? Everything else I read talks about running rough, but the engine runs smooth at both idle and cruise. Is it the case of the fuel mix being *slightly* off, so I get some smoke, but not off enough to cause poor running?
...so please enlighten me.

Exactly. You could be running just rich enough to have exhaust smoking but not necessarily so rich as to cause poor running.
In my experience, I've found these engines don't mind being on the rich side and will start easier (cold) and idle better.
The down side is the smoking AND likely fouled plugs eventually.
In this case, it's truly a case of YMMV...

The fact that turning the idle adjustment doesn't make a difference is cause for concern though.
How FAR have you tried turning?
AND, you're turning it OUT correct?