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MG MGB Technical - Backfire Black Smoke

I just can't figure it out. I drive slowly out in the country (15 - 25 mph) and things go great for awhile. Then my 78 MGB starts cutting out, backfiring and billowing black smoke. If I can nurse it up to 60-70 mph for a bit, things clear up. I have a fairly new petronix ignition, plugs, wires and coil. Also a manual choke on a Zenith Stromburg carb. The carb is adjusted fairly rich but won't run well at all if adjusted to leaner. Any ideas, anyone?
L Gooch

Does sound like very rich. That might be fouling the plugs (25mph? What's the matter with you? They were never designed to go that slow ...) then faster running may well be cleaning them again, especially if the mixture is closer to correct at higher throttle openings. I'd be looking at the needle and jet, could be the wrong needle i.e. too rich in the lower portion, or maybe worn in that area. Possibly the diaphragm?
Paul Hunt 2010

Thanks Paul. The carb as a new grosjet. Would a tiny hole in the diaphram do this? If the spark advance was not functioning properly... would it cause that you think?
L Gooch

Get rid of the Groose jet. They have been nothing but trouble for the last 10 years when the company changed hands. It is flooding out your engine. RAY
rjm RAY

Advance problems won't cause black smoke, but will result in other symptoms of course.
Paul Hunt 2010

This thread was discussed between 17/05/2010 and 19/05/2010

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