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My 1984 E21 (316 (1800cc) with Zenith 2B4 Carb) is emitting a continuous strong plume of extremely smelly black smoke. This is not a barely detectable smoke trail - even at tickover the tailpipe looks more like that of a truck diesel engine pulling 40 tons uphill than a small car. And the smell of carbon monoxide is so bad that I can really run the engine only for a few minutes, even though the car is outdoors. There are no nasty mechanical noises at low or high revs. The smoke and fumes do not improve at normal running temperature (not something I can try often though due to huge toxic black smoke cloud surrounding a guilty looking E21 on my driveway). This seems to point to an excessively rich fuel mixture, maybe due to a float problem or blocked airway in the carb, although adjusting the air and mixture adjustment screws fully in either direction makes no difference at all to the level of smoke / smell, although tickover speed does change.
The car had been stored for 10 years before I bought it a few weeks ago. It was running only on two cylinders when I got it, and the spark plugs were covered in black soot. With a new set of plugs it ran on four cylinders, although these are already coated in soot. A compression test shows there is very good compression in all 4 cylinders. The choke is not stuck in the on position. I have removed and cleaned the 4 visible carb jets. There are no vacuum leaks that I can see. With the air breather removed, the carburettor does make a much louder air sucking noise than I have seen on other engines, although this might be normal for this engine. I have removed the carburettor and re-seated it with a small amount of blue silicone. I resurfaced the rubber / metal base plate which seems good as new. I have not stripped and rebuilt the carb as I cant find a kit that includes a new float. I don't have an alternative carburettor to swap over and cant find even a secondhand one to try. My question really is should I carry on concentrating on the carburettor or is anybody aware of any other reason for these symptoms - could it be for example a weak spark due to a faulty distributor / coil / control unit etc causing partial combustion? Does anybody have any experience of this problem please?

Welcome here, and congrats on the buy! I can't help you on this one. How is the oil consumption?

Just wait for Peter V. to respond, or sent him a pm.

Welcome,

First indeed back to the basics.

You had checked the sparkplugs already and the car runs but is the timing OK.
Your freeplay of the valves?
How is your oillevel at cold engine? overfilling gives a lott of smoke also.

Extremely smoking (black) is indeed a rich mixture. Have you checked the rest of the parts of the carb. TN-starter etc (warming-up items). It looks that there are some items that are stuck in the wrong position.

The 2B4 has two floating devices and there are almopst no rebuild kits that provides them. You probably have no other choice then go to the dealer but pay the max price or get a normal rebuildset from a good brand. The floating devices are in plastic and you can test them. A used carb is probably good to find.

Thanks both for your very prompt replies. Yes new spark plugs were fitted last week, although will need to be cleaned now as they are fouled with black soot! I intend to check the valve timing and ignition timing this weekend as suggested. Good point about the oil level, I know only that its very black (and have been putting off changing it until I get to the bottom of the current problem) but will check the level this evening. If that doesn't turn out to be the problem I think I will take the carb off again this weekend and give it a good check over, to make sure nothing is blocked or stuck. If that doesn't work I will keep searching for a used carb, but at least I now know that there is no other obvious cause that i am missing.
thanks again for your help and I will let you know how I get on.

Sorry I also meant to say that I don't know how much oil its using as I cant run it for more than a few minutes due to the amount of smoke being produced! There is very good compression in all 4 cylinders and smoke is definitely just black, there is no hint of blue at all so I am hoping its not burning much oil.

Why? If a car is stored for some years, start changing all the fluids. Oil, with filter, coolant, fuel with filter. Basic.

Hey, and remember fuel is aging pretty fast these days, fresh fuel might solve your smoking M10 problem.

And....welcome here

Welcome! Wise words spoken here!

OK good point, I didn’t change the oil as I didn’t want to get the new oil polluted with fuel if there is a mixture problem but fair enough 10 year old oil won’t be helping. I have drained some oil off as it was indeed overfilled but still running just as smokey and stinking of carbon monoxide. Will change oil and filter before stripping the carb. Fuel and filter is new. As is coolant. Thanks for all the advice, I will report back.

Well, I stripped the carb to pieces at the weekend and during reassembly after a good cleanup noticed that something was indeed stuck in the wrong position as suggested by Peter V - the choke piston in the carb housing was stuck solidly in the "choke on" position, this is the little piston activated by the TM valve. Lots of WD 40 soaking later I concluded that this was going to be impossible to free up as there was nothing to grip onto, as I could not pull it out I had no option but to tap it fully in to the choke off position, at least for now. It reluctantly and slowly moved fully home, but is extremely well jammed in this position. The choke air butterfly still works ok so I am hoping this will be enough to allow the car to start in cold weather, but I think in any case I can probably live without a choke, at least until such time as I can find a replacement carburettor. After adjusting the valve clearances and changing the oil and filter (and cleaning the spark plugs yet again) the car now runs much better, there is still a hint of black smoke but am hoping most of that will clear when I adjust the carb correctly, but first need to replace the back silencer box as its blowing badly and making the engine mote hard to judge. Either way the extremely large plume of black "diesel" smoke has disappeared so thank you all for your help.

Nice to hear that the main problem is fixed. The rest comes later. First try to drive some miles and you will see that everything wil react much better.

Thanks again, I need to strip and rebuild the brakes, get a little bit of welding done, plus lots of other jobs before I can drive it unfortunately, as the car has not been used for at least 10 years. So the whole idea was to check out the engine first before starting on all the other work, and as that now is running very well I don't mind doing the other jobs over the winter, and hopefully get it back on the road by spring 2019.

Good progress, and nice to aim for next spring! Feel free to throw some pics at us as well mate!

You'll get there, this progress is the first step to full recovery!