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MG MGB Technical - Checking Carb Balance

Recently read on another site that the balance on the SU carbs can be checked with 2 bits of wire: take out the pistons from the top of the carbs and fit carefully a piece of bent wire into the slide tubes so it fits tightly and bend them at the top so they are at the same height. When running the engine if one carb is pulling more air then the wire will be at a higher level.
Is this true and has anyone done this or is it rubbish?
AdrianS

Adrian. It is true and I have done it. I used two lengths of aluminium tubing that was a "nice" fit into the tubes with pieces of wire held in place with a bit of silicone sealant. Worked like a charm and although I have done the job with a rubber hose and listened to the hiss I think this is a more accurate method.

I believe that a similar set up was supplied by SU as part of the tool kit in some cars.

Cheers
Tony
Tony Oliver

Will work ok as long as you have an accurate means of measuring!! I bought, and still use, a device I bought in the 70's, which works on the same principle. There are two adjustable stops, which fit in the dash pots and a DTI, of sorts, which measures the lift. The same "Dti" can be used to set the jet heights as well. Never seen one since though.
Allan Reeling

Allan. No accurate measurement is required. The wire is bent so that the two ends are very close to each other and at the same level when the engine is stopped. It is then simply a case of adjusting the throttles until they are the same level with the engine running. It is the relative position of the ends not the absolute position that gives the accuracy.

Tony

Tony Oliver

I knew I'd seen a picture of it somewhere. Shows the tool and how it sits in the carbs.

http://www.mgaroadster.co.uk/su_carburettor_tool.htm

Tony
Tony Oliver

I like it!!! Always appreciate simplicity.
Allan Reeling

It is neat isn't it. It also allows you to check the balance at higher revs. I read somewhere that it is more important to check the balance at about 1500 to 2000 RPM rather than tick over. Don't know why, I just read it somewhere.

Cheers
Tony
Tony Oliver

Pretty slick - are these kits still available?

Pat
Patrick Callan

It's better to check the balance at higher engine speeds due to the fact that, if you have any wear on your throttle shafts, it will not not make as big a difference in balance at high speeds as it will at idle. RAY
rjm RAY

Thanks Ray. Now I know. Is the RPM range I quoted about right?

Cheers
Tony
Tony Oliver

What to do if they are not in balance at higher rpm?
Art Pearse

Good question Art. From what Ray said I assumed it would be better to balance them at the higher RPM. I suppose it is reasonable to assume that what happens at idle has little relevance to how the car is normally driven, that is at part throttle.

I wonder if this would apply to mixture setting as well.

Tony
Tony Oliver

Patrick. A supplier over here sells these kits. I wouldn't mind betting that they are supplied by Moss.

Tony
Tony Oliver

I did a google search and found the kit for sale at Minimania for about $35.

Decided to make my own with what I've got in the garage:

Found that an 8mm long frame fitting - the ones used to fit window and door frames ( take the screw out) fits well into the dashpot tube. Bent and twisted some plastic coated garden wire so it was a snug fit in the plastic frame fitting and left about 8 inches sticking out.

Put these into the dashpots until seated and bend the wires at the top to the same height - job done!

You do lose the dashpot oil so this needs to be topped up after removal of the very expensive tools!
AdrianS

I balance the carbs at 2,500 rpms. This seems to give me a good balance between low and high speed performance. As the ads say: "Your results may vary". RAY
rjm RAY

I made a manometer thing with some plastic hose and a metal plate I bolt to the front of the carbs. Two metal tubes are soldered to that pointing into the carb mouths arranged to be in the same position on each carb. The manometer is just a U shaped tube filled with water and some food colouring. The plastic tubes are the same length and each goes to one of the little metal pipes.

Probably total overkill but it seemed to work OK.

There is a picture here: http://www.asciimation.co.nz/pics/page10.html

Simon

Simon Jansen

Hi Simon,

I like you setup very much but wonder if there's a reason why you didn't use the tubes on the HIF to which the engineventing hoses are fitted?

I have used those tubes in the past with good results from the tube containing coloured fluid.

I also used and liked the bent wire variety and used wooden sticks in the damper for fear of damaging the bore.

(and my language teacher always told me not to start a sentence with "I"....)
Willem vd Veer

I never thought of that! Aren't those tubes on the downside of the pistons though? Would that affect things? I am guessing not.
Simon Jansen

AAAAH the ingenuity of classic car owners. I used aluminium tubes, Adrian used parts of a window frame and Willem used wooden sticks! Simon made a more complex test kit but used bits available to all and sundry. It goes to show what can be done to produce "special tools" when you put your mind to it.

Tony
Tony Oliver

rjm Ray - to repeat my qury - what do you do if the airflows are different at 2500 rpm? Is it just a check that the front and rear cylinders are pulling equally? And how do you know how much flow is equalized in the balance tube in the manifold?
Art Pearse

Art,

Differences in the air flow at speed is normally due to the linkage between the carbs not being correctly set up.
If one side moves earlier than the other you will get a difference in suction - remember they are not rigidly fixed! Pull the throttle cable by the carbs and watch the linkage.

Also as the mixture can mix in the manifold it does even up any inaccuracies in balancing the carbs. I have seen some really badly adjusted carbs and the engine still appears to run sweetly! It is a very forgiving engine.

My main problem is that the engine runs better with a richer mixture than that allowed for our annual MOT test which for my car is 4.5% CO in the exhaust gas.
AdrianS

If the dashpots have different rate springs in them then that might cause different opening rates too imagine? Shouldn't be a problem with normal cars but it wouldn't surprise me on a car which is a bitser like mine.
Simon Jansen

My (vintage) Edelbrock Uni-Syn A has always made carb
balancing a snap.

I've read that the new ones are not made as well as the older
models, but that they can be brought up to snuff if they are taken
apart and the rough flaws removed before reassembling.
Daniel Wong

Art, Sorry to get back to you so late. It's been a busy week. If the carbs balance out at idle, but not at 2,500 rpms, I would adjust the linkage to have the carbs balanced at 2,500 rpms. You spend a lot more time driving the car than you do at idle. Of course, having the carb throttle shafts replaced, or re-bushed, is the proper way to have the carbs balanced across the rpm range. RAY
rjm RAY

Of course if you really want to be clever, once having balanced the carbs the true objective to seeing how high the dashpots ride with the aluminium tube or wire is that this corresponds to that point on the needle and the jet, or a stage.for a given point og the revs range. Now let us say you had a flat spot at this point, most flat spots are caused by a fuel weakness, so here one could 'tune', the needle by finely reducing its diameter. All in the SU tuning manual, it is tricky and time consuming but one can really get a set of carbs to 'sing'!
J.M. Doust

Just to mention that these SU tuning kits are available in the UK from Moss and also from MGOC at around 14 plus p & p. Have just ordered mine !!
Geoff Watts

This thread was discussed between 22/04/2011 and 01/05/2011

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