Black smoke at idle

I rebuilt my holley 4160 and fired the bronco up today. It's been sitting for a long time and I honestly can't remember what I did last to it that would cause this. I can see puffs black smoke coming out of the passenger side header just installed those too) and some of the primary tubes are cold. I am going to change the oil since i rebuilt the carb due to a flooding problem. My firing order is 1,5,4,2,6,3,7,8.

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Bitch'nBronco said:

I rebuilt my holley 4160 and fired the bronco up today. It's been sitting for a long time and I honestly can't remember what I did last to it that would cause this. I can see puffs black smoke coming out of the passenger side header just installed those too) and some of the primary tubes are cold. I am going to change the oil since i rebuilt the carb due to a flooding problem. My firing order is 1,5,4,2,6,3,7,8.

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Here is a picture of the bronco

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Fouled plugs.
Stuck or maladjusted choke.

I set the choke to stock i guess I've got some homework to do. I also popped the cap off the distributor. I'm not real familiar with the HEI but it kind of looks worn

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Black smoke is over fuel. Not sure how it happens only on one side as I'm not totally familiar with a lot of fuel systems. Blue smoke is oil and white is a actually water vapor from an intake leak.

idle circuit? maybe screws on side of metering block out of adjustment...

since i rebuilt the carb due to a flooding problem.

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Full Disclosure: I don't know Broncos anywhere near as well as these guys....

But here's what I do know...black smoke = rich gas mixture. Since you just rebuilt the carb my advice would be to look there first.

You may have set the fuel level too high in the float chamber?

If you're confident with the carb check your air filters, they could be musty since you let it sit for too long...

Fuel is a hydrocarbon. That is it has the elements of hydrogen and carbon. Hydrogen loves to burn and is the first to be consumed in the fire of a combustion chamber. So long as there is enough oxygen (air) the carbon will burn off as well. Ideal combustion results in the hydrogen burning with oxygen and making H2O, water. And the Carbon burns off into CO2 (carbon dioxide). When there isn't enough air the carbon is starved for enough oxygen to bond to. Normally it is just a high CO (carbon Monoxide), the carbon can only grab a single oxygen instead of two that it really wants.

In this case there is so much excess fuel being burned that there isn't even enough oxygen to burn the carbon into CO, you are just getting raw carbon out the tail pipe. Black soot carbon.

It's a fueling problem.

First, good lookin Bronco!

You could probably use a new cap and rotor but don't think this is causing your black exhaust smoke... it's getting too much fuel.

What all did the rebuild consist of? Just thorough cleaning and new gaskets or were there hardware changes, like needle & seat, power valve, accelerator pump, etc.

Things I would check and try

**check and adjust your float levels if they need it - you'll want the fuel level just at the bottom of the sight hole with a slight trickle when you rock the Bronco back and forth.

**air mixture screws at the metering block - if they are too far out, you could be getting too much fuel. You can screw them all the way in so they slightly seat (do not over tighten these, you can damage the seat), then back out 1.5 to 2 full turns and start from scratch. You'll want to adjust these the same on both sides and you are trying to achieve maximum vacuum at idle. If you do not have a vacuum gauge, you can turn the screws CW until the motor stumbles, then back out 1/2 turn

** did you make any jet changes during the rebuild? If yes, what was the old / new sizes?

If none of these remedy the black smoke, you may have to pull it off and go back inside and re-evaluate the rebuild and check for internal problems.

If the air/fuel mixture screws do not cause any changes, the power valve is probably blown. When you turn the motor off, do you notice any vapor coming up and out of the carburetor? This is usually a sign of a blown power valve.


DJs74

The floats on my 4160 are not adjustable. When I "rebuilt" the carb all I did was take it apart, clean it, made sure the floats moved freely, replaced the accelerator pump diaphragm, the vacuum secondary diaphragm, and replaced the gaskets. I did not change any adjustments to the screws in the metering block. I did remove the electric choke and reinstalled it at the factory position.

I looked at it again this morning and it looks like the squirter in the primaries isn't really squirting, but just dumping fuel. I guess I'll pull it apart and replace the gaskets for that. I guess my choke could need some adjustment from stock, I'll reattack that after I resolve the squirter issue.

Also, I did not replace the power valve and I have 65 jets in it. I did not change them, they're the same ones I have been running in it for years without issues. Thanks for your help

Broncobowsher said:

Fuel is a hydrocarbon. That is it has the elements of hydrogen and carbon. Hydrogen loves to burn and is the first to be consumed in the fire of a combustion chamber. So long as there is enough oxygen (air) the carbon will burn off as well. Ideal combustion results in the hydrogen burning with oxygen and making H2O, water. And the Carbon burns off into CO2 (carbon dioxide). When there isn't enough air the carbon is starved for enough oxygen to bond to. Normally it is just a high CO (carbon Monoxide), the carbon can only grab a single oxygen instead of two that it really wants.

In this case there is so much excess fuel being burned that there isn't even enough oxygen to burn the carbon into CO, you are just getting raw carbon out the tail pipe. Black soot carbon.

It's a fueling problem.

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Great explanation!

Thanks for the help. I replaced the power valve and while I was in there I noticed my metering block gasket was missing a hole so i swapped the gasket and it runs great again. Just need a new timing light and I'll get it tuned/ready for the road

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