The Stovebolt Site Forums The Shop Area Engine Shop Black soot from exhaust?

I seem to be getting some black residue from the tail pipe. Could it be I do not have dual carbs set right? Seems to run pretty good. Any ideas would be helpful.

Runnin too rich???

Could be something as simple as a sticky choke or dirty air cleaner.

John

Just a thought but you may have a float sticking on one of the carbs or the floats are out of adjustment. One is possibly leaking gas from the float bowl or one may have a gasket not tight enough and may be seeping down the throat into the manifold. One carb should be not working until you put your foot part way down on the gas. Too much fuel pressure could also cause it to flood and then soot. Just a few ideas that may help. Henry

Another quick thought, I have had it happen when the needle valve was not seating correctly or was dried out and would let more gas go by than needed and caused flooding and soot like you have. Also would drip into the manifold when sitting with the engine off. Rebuilt more carbs than I can remember. Just thought of this and I hope this will help you out. This is unlikely but a carb jet inside may be not tight and letting gas go by it or it could be cracked. Just trying to come up with some suggestions. H.

Most any carburated engine without catalydic converters will leave the black residue in the tailpipe. Does the oil smell like gas? Are you getting worse than normal gas mileage?
Just a thought,
Scott

When rubing your finger on the inside of the tailpipe this is what I would suspect.

A little dry grey residue on the inside of the exhaust is probably too lean.

Light dry black is probably pretty good.

Dry black is going to be too rich at the carb.

Wet black is oil.

I would "assume" that you are running rich with dual carbs and your discription of "black soot".

What do the plugs look like? I would use that as a more reliable indicator.

Hey guys I do not want to sound dumb, but I did think I may have been running too rich. I do I adjust? I am runnung carters.

Look at your tailpipe after running on the highway for several miles, it should be a bit grey to show a proper mixture. If it's black and sooty it is definitely too rich. Read the tailpipe same as you read spark plugs.

Adjusting usually just takes care of the idle mixture not running mixture. Make sure that the carb screws are tight that hold it together. A 1" or a 2" spacer plate can help lean it out. Worked for me. 48 Big trucks may be on to something. If the oil smells like gas, your fuel pump may have a bad diaphragm. On Chevies and some GMC trucks, it will dump gas into the oil and in a hurry. Fuel pump time then. Just another idea. Henry

At the bottom of the Carters on the side facing the valve cover, there is a little screw. To adjust my dual Carters, I turn the screw all the way in. Then I turned them out 1 1/2 turns each and started the truck. I turned in the screw until I reached rough idle then I turned it 1/4 back out. Remember, whatever you do to one carb, do to the other.

Do you have a uni-syn tool? If not, I HIGHLY suggest getting one off eBay for cheap money, your truck will thank you for it!

The only way to check high speed mixture correctly is to do a road test with a 4-gas exhaust analyzer connected. Swapping main jets and/or power valves, if your carb has them, is how high-speed mixture is adjusted. Nothing you do to the mixture screws will have any effect above idle speed. You can do a WAG by looking at spark plugs or the color of the tailpipe. A too-lean mixture will have a tendency to detonate ("spark knock") and be short on power climbing hills or accelerating. It will also run hotter than normal. Really lean, and the plugs will turn a white color, with glassy-looking deposits and/or powdery-looking shiny stuff on them as the pistons melt. Fuel is cheaper than internal engine parts, so don't get it too lean.

If anyone tells you he can change high speed mixture by adjusting the idle jet(s), one of two things is going on:

1. He's too dumb to be tinkering with your carburetor.

2. He's lying and knows it, and he's about to get into your pocket in a big way by charging you for a procedure he knows won't help.

In either case, RUN, do not walk away, and find someone who knows what he's talking about.
Jerry