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MG MGA - Spark plug colour - carburetter adjustment

I have just checked the valve rocker clearances on my car and noticed spark plugs 1 & 2 were awful black (not oily but sooty)- looks like the front carby is running too rich. (see photo attached; Plugs 1-4 , right to left)
I was planning to lean off the front carby a couple of flats and see what difference it made. Any comments appreciated.

By the way, does everyone set up their carbys religiously by the method in the manual Section D.10 or are there some short-cuts or better ways?

The manual is not clear on whether the air cleaners are removed or not when setting up - I assume you need to remove them to gauge the suck but then putting the filters on may change the mixture? When it talks about lifting the piston out of the way, do you do this with the lift pin, or your fingers with the filter removed? Do the lift pins lock in postion or do you have to jam something in the throat to keep open?

(Having a crank handle sure makes setting the valve clearances easy!)

Happy MotorinG All
Mike


Mike Ellsmore

Hi Mike,

The richness of the plugs looks so excessive that I would look at bowl flooding and fuel height in the bowl before I even tried setting the mixture. Do you get fuel coming out of the bowl overflow tube? With the bellhousing and piston removed, does the bridge area around the jet flood when the pump is turned on? (Remember to mark the bellhousing and carburreter body to refit in the same position).
You seem to confuse setting air velocity through the caburetter with setting the mixture. You take the air cleaners off only to measure air velocity. You can have the air cleaners on when you use the piston lifting to check the mixture. Of course this is only if you have piston lifting pins on your carburetters.
The lift pins do not lock up, you only lift them for a very short time to check the mixture.

Mick
Mick Anderson

Mike,

I have attached an image of the first procedure, adjusting throttle openings. The clamp which connects the throttle shafts of each carburetter must be loosened.
I have never bothered with the first procedure mentioned in the Workshop Manual (putting each carburetter out of action in turn).

Mick

Mick Anderson

Mike,

The second procedure is adjusting the mixture. I have attached an image.
The linkage between the the jet levers on each carburetter must be disconnected.

Mick

Mick Anderson

Mike,

I have attached an image of the final result. Plug colours should also be checked.

Mick

Mick Anderson

Hi I was told by a mg garage that races a couple of MGA's that with the fuel available these days that you could not rely on lifting the pins anymore to check the mixtures.


gordon
g c pugh

Gordon,

Are you sure you heard that correctly?
With Australian new lead free fuel you cannot get a good idea from the exhaust pipe colour, it always appear black. Lifting the pins should work, it changes the mixture irrespective of the fuel type.

Mick
Mick Anderson

Hi Mick I certainly heard it right but don't know if it is true or not , I was quering why my exhaust note, plugs tec were all correct, but when I lifted the pins both carbs were weak and when I richened them so the pins showed a correct mixture it was so rich it would not even idle, it was then they told me to forget about using the pins because of the make up of modern fuels and just to go with the best the engine would run at


gordon
g c pugh

Thanks Mick,
I will check float bowl levels. My first attempt to just lean off the mixture nut on the front carby resulted in a few back fires thru the carby - too lean? I will now go to first principles and do each step by the book.
I looked in Piet Olyslager's "Handbook for the MGA" and it mentions removing the air cleaners, set the idle, set the mixture then refitting air cleaners and check mixture and idle again - no mention of lifting the piston to take the opposite carby out of service (I also cannot see the point of this if you make sure the butterfly is fully closed and the shaft coupling is loosened off).
I have piston lifters on my carbies - it is a pretty fine touch to check mixture, 1/32" lift, or is it really any lift between 1/32" and 1/4" and there should be no speed loss or increase except for a momentary blip?

Regards
Mike




Mike Ellsmore

In the words of a Quality Assurance inspector in a third world country "near enough is good enough".
It is not critical, just raise the piston a very small amount.

Mick
Mick Anderson

University Motors has some videos on YouTube and one is tuning some SU carbs, I think on a B, but the concept is the same.
Steven B

I use the method from a period tuning manual I have. It is very similar to the MG manual method and as my book shows the Jaguar D-type being tuned this way I stick with it!!
I use a plastic strip to hold the carb fully lifted to ensure it is out of action, the lifting pin does not fully stop the carb just makes it inefficient.

Neil
Neil Purves

Neil,

Could you scan that picture of a Jaguar E-Type with SU carburetters? I have only seen them with Webers and Lucas fuel injection.
The use of the lifting pin by 1/32" is not supposed to put a carburetter out of action, but just to alter the mixture. The effect on engine speed then indicates whether the mixture setting is correct, weak or lean.

Mick
Mick Anderson

Neil,

Sorry, I meant a scan of a D-Type with SU's. All E-Types in Australia have triple SU's.

Mick
Mick Anderson

Like with consumer electrical gear "when all else fails read the instruction book" I should have read the MGA instruction book first - Barney's wed site - it answered my queries on the easy way to tune the carbies.
http://mgaguru.com/mgtech/care/cf120.htm
Mike Ellsmore

This thread was discussed between 05/11/2007 and 07/11/2007

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