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MG MGB Technical - Emissions
Had problems with my emissions (not mine- the cars) for some time.
MOT'd 2 years ago and could not get the CO below 11% without the car running badly.
Rebuilt the carbs with a service kit and last year the CO was 4.6% ( 4.5% limit).
Took it for MOT today and the CO was 1.1% having done absolutely nothing since last year.
It is running fine at the moment but takes a few miles to warm up.
Should I leave the mixture as it is or is it running too lean? Is the ideal CO setting the one the car runs best at?
Also, the MOT tester said their was some play in the OS rear wheel when rocking from the 12 o'clock to 6 o'clock position and suggested the wheel bearing was on the way out. When I had a look later there was some play but the wheel spun smoothly and there is no noise or whine from the axle.
Do I leave it alone?
|I'd guess you have an air leak. 1.1% is too lean but rememeber the emmission test is done at idle -most of your driving isn't. Check the plug colour for clues. I'd expect about 4.5%- or have to tweak it to that! Did he do a lambda reading reading? (not part of the test but can be done as "live" readings. lambda = air/fuel ratio so big clue as to mixture!|
|It shouldn't be taking a few miles to warm up i.e. push the choke fully back at this time of year (even with our present 'summer'), so it may be too lean. It depends how carefully the carbs were setup and balanced, but you could adjust both carbs by turning the screws clockwise by say a quarter turn and see what that does. But if its running well apart from that I'd tend to leave things alone, SU carbs get a bad reputation from people who fiddle and poke rather than doing it right then leaving them alone.|
The best CO reading is one that gets you through the MOT! That's not necessarily the one that gives the best running.
|The car is running really well at the mo. The choke can be pushed in when moving - I have to use it a little to maintain tick over if I have to stop, say within half a mile from home but once warmed up the tickover is a very steady 800 - 850. Before I rebuilt the carbs I had difficulty in getting any reliable tick over - I assume there probably was air leaks around the butterfly spindles. I remember some of my cars from the mid 80's. They all had to have a CO figure of around 1 - 1.5%. The limit for mine for MOT is 4.5% CO and 1200 HC - I don't think this means that it has to be set to 4.5%.|
I was just surprised that the CO was as low as 1.1% ! In fact the figure at 2500 rpm was also low.
My main concern was causing damage to the engine by running too weak. I don't find looking at the plugs very reliable unless the mixture is very rich or very very weak.
Once warmed up though the car is running very well. It accelerates well, no hesitant spots, no running on, very good pick up.
|If it goes well -leave well alone but in which case I very much doubt your CO is 1.1%!|
|"I don't think this means that it has to be set to 4.5%."|
Certainly not, it can be that or anything below it, as long as the HC is within limits as well.
Like Michael I very much doubt your CO is as you say. MGBs have notoriously 'dirty' engines, the GT with its extra weight was pulled from America because it couldn't stay below their steadily reducing limits, and to get the roadster below was a struggle - 4.5% in 71, 3.5% in 72, 2.5% in 73. To reach those they needed the single Stromberg carb, electronic ignition, transmission controlled spark advance, and air pump and all the associated emissions reduction kit. And even then the cars had to be individually tested, with some having the points plate in the distributor pinned to prevent any vacuum advance, if that was the only way they could pass.
Can't agree with what you say here.
"MGBs have notoriously 'dirty' engines,..."
>>No worse than other things of the time.
"the GT with its extra weight was pulled from America because it couldn't stay below their steadily reducing limits,..."
>>I think the GT was pulled more because it was not a great seller here (closed "sports cars" are horribly under appreciated this side), and it did require a whole extra set of expensive certifications, many of which were "safety", not emissions related. And of course, the BL MG/Tr schizo market problem of the experimental GT V8 competing directly with the pet TR8 project.
"and to get the roadster below was a struggle..."
>>you need to meet some of the US cars of the time!
" - 4.5% in 71, 3.5% in 72, 2.5% in 73. To reach those they needed the single Stromberg carb, electronic ignition, transmission controlled spark advance, and air pump and all the associated emissions reduction kit. And even then the cars had to be individually tested, with some having the points plate in the distributor pinned to prevent any vacuum advance, if that was the only way they could pass"
>>No. Those were problems with rapidly changing standards that everybody faced; BL's problem was that the company was falling apart and they did not have the resources (or the management!)to catch up. But, they did meet the standards, often better than other cars of the time. Most of the things you cite are after the period you mention. ZS, electronic ign, pinned advance all go with catalytic convertors, which came in pretty universally in 75/76, again with further changes in emissions limits. Air pumps were on most cars from 68, and lowered compression (71)/crappy cam timing(75) was to meet NOx standards, the tightening of which is what ultimately forced cats.
|I,m only taking the figure from the emissions print out from the MOT station!|
I was surprised to see the figure that low - perhaps I ought to get another reading from a different gas analyser!
|Yes, it could be a suspect reading...or maybe he was taking it from another car ;o)|
|"No. Those were problems with rapidly changing standards that everybody faced;"|
Everybody in America maybe, but not the rest of the world, although everyone outside the UK eventually got the North American spec because it was cheaper that way, even Japan got LHD when an RHD country.
This thread was discussed between 14/06/2012 and 18/06/2012
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