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MG MGB Technical - UK emissions

My 1980 LE roadster (1800cc) has gone back on the road after three years in storage. I tuned it before its MOT test yesterday using the lifting pin method with colourtune to confirm and a gunson's balancer. The test readings were CO 4.11% and HC 590 ppm. Do these readings sound reasonable for a UK car?
I came across an old article from a 1990s copy of "Enjoying MG" re dynamic timing in later cars. Quote "The revised setting is 14 deg BTDC for all MGB engines fitted with the new camshaft sprocket CAM5333 introduced from engine number 18V-847-F27799"
I haven't seen any other reference to this figure in the manuals, but it seems to be fine for my engine. Would a rpm of 1000 vacuum plugged be right for this application?
ph barton

Your CO level seems a bit high. Lean out the mixture, but not by too much or else your HC levels will rise accordingly. RAY
rjm RAY

Does it run well? If so leave well alone. 4.11 is about average, and it passed. UK test CO -4.5% HC 1200 ppm for 1980, even for post 1986 it's only 3.5% CO

14 BTDC- with todays fuels the only true test is, does it pink under load? Book figures from 25 years ago are pretty meaningless.
Michael Beswick

As well as the fuel, ignition timing will depend on which distributor is fitted, and how worn it is. Once the advance mechanism wears, the distributor no longer advances at the original rate so the timing will be correct where it was set and wrong everywhere else. The wrong can vary from a little to a lot, depending on the extent of wear and unless the distributor is serviced to restore the original advance rate then the best that can be done is a compromise of one sort or another.


BTW 14 at idle deg is the setting for earlier engines. The wording "The revised setting is 14 deg BTDC ... " suggests the context needs looking at. That something is revised suggests that something else has been changed. As changing to the earlier spec distributor spec gives a useful torque boost to rubber bumber cars, maybe it was that.
Paul Walbran

If you do lean it out, only ever alter both carbs by the same amount in the same direction, or you will unbalance them.

14 degrees, if that is at the original 1000rpm, is a lot more than it was previously, originally that figure would only have been reached at 4000 rpm. But as Michael and Paul say, the original figures are pretty meaningless with today's fuels and worn distributors, even if you have the original 45D4 41610 distributor. With high compression you can advance just short of pinking at any combination of throttle opening, revs and load, and that way you are getting the best you can with what you've got.
Paul Hunt

Correction, 14 degrees dynamic would occur at about 2000 rpm.
Paul Hunt

This thread was discussed between 16/04/2014 and 17/04/2014

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