I had this problem on my 1985 1.8 sierra and it had a Pierburg carburettor. The choke pull down unit (not sure of the exact description) was faulty. It has a rubber diaphragm inside which can split. This means the vacuum from the engine can no longer operate the unit and allow it to open the valve in the venturi and so it runs rich when cold.
I think it was held in place by a pin. I used to test it by sucking on the pipe and see if the unit acts on the flap and also if it holds the vacuum by placing your tongue over the end of the pipe. You need to do this on a cold engine and check that the valve is closed. Please bear in mind the unit may have petrol fumes in it!
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Yep soung like the Choke pull down unit has failed. Regards Peter
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maybe its overchoking? does it chug and smell of petrol ? any black smoke?
Yes to all the things above, when cold but no when warm/hot.
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The Carb could be dirty, take off the air filter and with engine running warm, spray carb cleaner down until it's clean.
Check if the air filter if needs renewal. If the problem is still there, then the choke/carb is at fault or too much mixture setting or sticking flap. Other things to look out for is block fuel filter, gummed fuel line, fuel treatment will do the trick.
See how it goes.
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