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MG MGB Technical - MOT emissions

Hi, today I took my 76 roadster for it's annual MOT. I had to use a new garage as the recession closed the one I've used for the last 4 years. Any way it failed on emissions. CO was 5.17 against a pass of 4.5 and HC was 1562 against a pass max of 1200.
Now I've never had a problem with emissions. But for the last few months I've been using Shell V max and I topped up with this this morning before the test using a lead replacement additive.
I set the carbs up with a pair of gunson colortunes, and I pride my self on getting a tickover of 850rpm with a stage 2 engine.
Is it possible using Shell has raised the emissions?
c cummins

No! It has nothing to do with V power! Either use the MOT station's analyser on live readings and lean out the carbs to pass, or do the same at home as best you can and hope! You may have to raise the tickover a bit. Personally I find colourtunes more bother than they are worth. Average CO should be around 4.5. 5+ and high HC means rich. It should run better at 4.5 but if you think it won't ,note how many turns of adjustment you made to pass the test and.....work the rest out....Michael MOT tester
Michael Beswick

With that high an HC I'd be looking for ignition problems, or other problems of wear on the engine and carbs. Whilst a CO of 5.17 *is* higher than the MOT allows it is only a smidgen high and that in itself shouldn't cause an HC as high as that. My V8 runs poorly if I adjust it to pass the emissions, so I weaken it about a 1/4 turn for the MOT then put it back afterwards, which reduces it from 5.4% to 2.4% on my Gunson's CO meter, 2.14% in the MOT. FWIW the HC was 716 last time, bear in mind this is a V8, my roadster is emissions-exempt.

Like Michael I've never got on with Colortune, and unless I had two having to keep swapping it between cylinders while you adjust first one carb then the next would be a real pain. I've always used the lifting pin with good and repeatable results. However whilst the change in engine note with HS carbs is just detectable, the change with HIFs is very subtle, almost subliminal. But if you adjust it one way so it is just obviously rich, then count the flats while turning it the other way until it is just perceptibly weak, and position it between those two points (half the flats you counted) you will be pretty close. Repeated attempts will gradually tune your ear in, making it easier. Remember 4-cylinder carbs are interlinked via the balance tube and making a change to one affects the mixture on the other pair of cylinders, so you have to go through two or three iterations of adjusting first carb one then the other then back to the first again before they are both right.
Paul Hunt

High HC can indicate either weak or rich - either way it is poor combustion of the fuel.

Very small adjustments of the carbs mixture can have dramatic effects on the HC reading as does ignition timing.

I would weaken the mixture just 1/4 turn of the screws.

Check the timing - it may be slightly over advanced.
Chris at Octarine Services

This thread was discussed between 13/07/2009 and 17/07/2009

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