MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - CO adjustment

The manual for my '79 B with 890AE block notes a CO of 5.5% CO max (plus or minus 1%).
This is for a car with an original ZS175 and normal filter and emission pump.
Mine has a single SU HIF44 with K&N and the emission pump removed.
Any suggestions of what influence this might have.
I'm used to setting the mixture "by ear" but would like to come to some kind of "scientific" approach with an exhaust gas tester.

Carl
C. J. Gaasbeek

Yes, you have effectively gone some way towards the UK spec of the engine, the UK engines of that year have a max CO reading of 3%.

In my experience these engines actually run better with a slightly richer mixture than that and I would suggest you aim for 5% throughout the rev range - that may mean changing your carburettor needle to achieve that - but that needs a rolling road to be able to measure the mixture under load at all revs.
Chris Betson

Chris,

The rolling road test won't be a problem as I will have the opportunity to use a Dynaflow bench within a couple of weeks. When I set the mixture by ear I end up around 6% but that seemed a bit much. Also this manual set-up is tricky because I need to comply with the local legal limits of 4.5% once a year to pass the MOT. Do I need do do something specific to achieve 5% throughout the rev. change or just take an average of the readings and go with that?

Carl
C. J. Gaasbeek

Carl,

Just set the mixture at idle to 5% CO and then when it is on the rolling road, note the CO readings up the rev range and note the carb piston height at each reading.

Where it varys from 5% then the needle needs to be richer or leaner at that point on the needle if all the points are needing to be richer then file a small flat on the needle where needed and retest.

If some of the points are leaner then you need to find a leaner needle and start all over again.
Chris Betson

Chris,

looks I will need the Dynaflow for a day or two :-)

Thanks for the info, I will give it a go during the upcoming holiday, but the family will not be pleased.

Carl
C. J. Gaasbeek

This thread was discussed on 04/07/2003

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.