I have a severe problem with rich fuel mixture. The plugs, valves, even rings are fouling with heavy carbon and the car smokes black at idle. I lost compression in two cylinders and had to inject oil into the spark plug openings and let it loosen up the rings to get it back. The car runs very strong and the secondaries kick in as expected although they make the engine a little rough. It is very smooth under normal acceleration and idle. The idle mixture screws will not clear the smoke without producing a rough idle. The carb has been rebuilt and the engine has about 250 miles on it since complete overhaul. The smoke started after the overhaul. I did not replace the jets during the overhaul. Also, the float bowl will empty if I let the car sit for a couple of days and I have to crank it for a while until fuel is available. Did this before the overhaul. Suggestions??? I am considering replacing jets next.
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Re: Black Smoke
Would help to know what type engine/carb you have, but from your description, there is a problem with fuel leaking out of the float bowl. If it does it during a static condition, it stands to reason that it will likely leak while running. I would change the carb, or not run the engine until I found the problem, or you are going to be rebuilding your engine again. -
Re: Black Smoke
Jim-----
You did not mention what kind of carburetor that you have. If it's a QuadraJet, I'd recommend replacing the float. This is a common problem with Q-Jets; the nitrophyl float's coating breaks down or gets scratched, then the float becomes "heavy" as it absorbs gasoline. Rich mixture and stalling/stumbling at idle are usually generated by this problem with the float.
The fuel leak down problem nay be caused by leakage of the well plugs in the bottom of the float bowl. There are four of them----two large ones that are vertical and two small ones that angle in. Seal these with JB Weld or similar epoxy and your problem will be reduced. It won't be completely eliminated, though, since fuel in the float bowl will tend to evaporate to some degree on long sitting.In Appreciation of John HinckleyComment
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Re: Black Smoke
Carburetor is the standard AFB for the 340 hp engine. The float is not leaking but he seals in the bottom and side of the bowl probably are. I guess they could be leaking enough to cause the smoke. Will repair. Why no comments on jets? Is this not normally a problem?Comment
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Re: Black Smoke
Jim, yes it's a problem. the excessive rich mixture can wash down the cylinder walls on your new engine and score the walls. I haven't been inside an AFB in 25 years, but IF they have a piston similar to the one on a WCFB, it could be stuck in the open position allowing fuel to pass throught the main jets all the time. This is something that could be a sudden event that would cause a problem all of a sudden. Jet wear would be gradual. Could be from your carb sitting while engine was rebuilt??? Just a guess with no real information.Comment
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Re: Black Smoke
Jim---
As I recall, the float in an AFB is brass. These should give no trouble. The Q-Jet float is a nitrophyl plastic material which is the basis for the problem with them.
As far as jets go, I don't see how yours could be the source of the problem. Generally, these do not wear and are not otherwise compromised. In the old days of leaded gasoline, the scavengers used with the leaded fuel caused all sorts of carburetor problems, including deterioration of the metal parts of the carbs. Presumably, such damage could have included jets, although I never experienced that problem. In any event, the unleaded gas which has been just about the only thing available for the past 10 years, or more, does not cause deterioration of the metal parts of the carb. That includes unleaded gasoline of which MBTE is a component.In Appreciation of John HinckleyComment
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Re: Black Smoke
Jim, Ed raises a real good question. Basically is there enough vacuum to keep the AFB power piston in the down positon at idle and off idle throttle? First, check your engine vacuum at idle aginst the engine specs. Also, did you change cams? Regards,Comment
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Re: Black Smoke
Not only is there enough vacuum, but is the power piston moving freely in the bore? If the answer to either question is "No", then you are on the track of the problem.Comment
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Re: Black Smoke
I would look down the throat of the carburetor and see if any fuel is dripping into the engine. If it is some possible causes: -Float level too high -Throttle is open too far at idle, allowing air flow through the boost venturis -Brass float that has a hole, filled with fuel -Bad needle and seat, or dirt on the seat -Romote possibility that the aluminum is porus. As far as the jets, they do not come into play until the throttle is opened enough to allow air flow through the venturis to cause a low pressure, which will pull fuel from the fuel bowls and into the engine.Comment
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Re: Black Smoke
Ed, The power piston moving freely and not binding is a good start. Sounds like the original cam is still in the engine. I can't relate to the statement, having enough vacuum. The spring under the power piston is "matched" to the vacuum that the cam pulls when the (healthy) engine is idling and off idle. As an example only, if the standard engine spec is 17" vacuum at idle and a vacuum gauge shows 13", that would not be enough vacuum (as far as the AFB's power piston system is concerned). Checking vacuum is a simple & quick place to check when one is experiencing a rich idle. If vacuum is OK then go to the next checkpoint. Regards,Comment
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Re: Black Smoke
I changed the cam installing a Crane A5780-260. It was from Ecklers and included roller lifters and pushrods. Vacuum is very good at idle. Distributor is dual point mechanical advance. I have not looked down the carburetor throat while the engine is running. How does fuel get to the engine at idle if not through the main jets? Is there an idle circuit I should be looking at? I will check the freeness of the power valve.
Thanks for the ideas so far. I have been trying to figure this out for a few months and it's a tough one for me.Comment
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I Like Black Smoke
because I can deal with it. If a Carter, I check the choke and if OK then I rejet with bigger rods or smaller jets. If smoking at idle I examine the idle jets and repair if too big. (soldier up the hole & redrill). If a Holley, I remove and discard then later install a Carter. Same goes for Rochester.
Dale.Comment
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Re: Black Smoke
The idle circuit is used mainly when engine RPM is low, and the throttle plates are in the nearly closed position. There is not sufficient air flow through the venturis to allow fuel to be drawn into the engine. The idle jet is a crimp at the end of a brass tube aprox. .035 to .045". There is a drilled hole below the throttle plate on the two primary barrels, which is exposed to manifold vacuum. Fuel is drawn up the tube, air from an air bleed is mixed with the fuel to create a fuel air mixture, then down a passage and regulated by the mixture screws. I doubt this is your problem. You cannot see this circuit, except for the idle mixture screws, without disassembling the carb. As I stated in my post, I would look for fuel dripping at idle.Comment
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Re: Black Smoke
I think Ed and Jim are correct. My 65 Lincoln has an AFB and it sat all summer out in the sun. It belched black smoke and barely ran. Upon tear down, the power valve was stuck due to varnish.Comment
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Re: Black Smoke
Thanks guys. I have enough information to proceed. I will let you know what fixes it or I will be back to aks more questions.Comment
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