Periodic black smoke and slow idle



1986 Mazda B2000 2wd pickup. 150k miles, manual trans, SOHC, Carbureted.

Once the truck warms up, it will periodically idle very low (500 rpm or less), normally 900 RPM.

During this time, when I rev the engine, it shoots black smoke out the tailpipe. My gas mileage is not very good, so I am 80% sure it is running rich. It still drives fine like this, but becomes very hard to start if I shut it down and try to restart it like this. Normally it starts readily within 1 crank of the starter.

It also sometimes bucks or surges when cruising at very light throttle, as on the freeway and such. Maybe this is related to the problem, maybe not…

What could be causing this? black smoke means too much fuel, right? I did a compression check and all cylinders are within 10% of each other (175-190psi), i replaced spark plugs, distributor cap/rotor, plug wires, fuel filter, air filter, etc.

There is a glass window on the carb float bowl. I have been meaning to look at that when the truck is doing it’s black smoke thing, but usually I am in traffic and cannot pull over.

I am thinking either a stuck float in the carb, or a clogged catalytic converter, but I dont know how to check either one.

Any help would be appreciated I like wrenching on my own cars, so I am not afraid of doing any work on it myself.

thanks a lot.



I am thinking either a stuck float in the carb

I believe you are thinking right. However that stuck float valve may have now damaged the converter so it may also be bad. Start with the float valve.


It was a stuck/bad float. Extra fuel was leakign into the secondary throat at idle. After replacing the float valve and associated parts. it runs flawlessly. I should get much better gas mileage now!