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MG MGB Technical - Choke on = BLUE smoke?

I have a '74 w/HIFs. If I put the choke on at startup, it smokes blue smoke. It runs great otherwise, but why the oil smoke when the choke is on? I've got an idea, but what do you guys think? The original engine is nearing the end of it's expected life (from my experience) of 80,000 miles and probably needs freshened with the usual rings, guides, and seats, but is doesn't smoke at all when the choke is off.

Any usual suspects that would cause this?

Blue? Or black? Blue indicates oil burning, usually after coasting down a hill on a trailing throttle or idling for a long time, and caused by worn valve stems/guides and/or rings/bores. There can be other causes like sucking up excessive oil via the crankcase breathing system but that is nothing to do with use of the choke - with SUs anyway ... unless you have overdone the upper cylinder lubricant. Black is over-rich. The choke control must be used as a continually variable device and not a switch i.e. off or on. Only ever use the minimum choke necessary for smooth running at small throttle openeings, and don't use large throttle openings until the engine is fully warmed and the choke can be pushed home altogether.
Paul Hunt 2

It's blue alright. The choke workings is old hat. I used to drive a midget all year round. I've never had one burn oil when the choke is on, though. To be more specific, it only does this at full choke.

If running rich when the choke is on the blue smoke can be caused by the excess fuel washing the oil off the cylinder walls resorting in the blue smoke. If it runs well and the oil use is not excessive, don't wory about it. 80k miles is not really that bad, we have several club members with well over 100k on their engines with no problems.
John H

I've never driven either of mine on full choke, that would be way too rich especially on the 4-cylinder. With that I crank it on full choke (HSs) then push it half way back as soon as it fires, then progressively reduced. With the V8 (HIFs) it does need full choke for the first five or 10 seconds only, then progressively reduced. Driving on full choke sounds horrendous to me, old hat or not.
Paul Hunt 2

I guess to be even more specific... I thought maybe the situation was clear when I mentioned "at startup", but I guess not. If the car is stone cold, it requires full choke to start. It starts and the choke goes to half and I back out of the garage. It warms up while idling and the choke is reduced even more. When it has a steady idle and will take the gas with the choke off, I drive happily away. If at any time I pull the choke out all the way, cold or hot, the blue smoke is deployed. I'm going with the fuel washing suggestion, which is probably helped along with worn guides and/or rings.

This thread was discussed between 12/05/2006 and 17/05/2006

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