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MG MGA - SU Carb pussle

My new toy has got me stumped. Twin SUs like my 1500 A but on a 2.5 litre 1950 Riley. Spits back very badly when cold and badly when hot. I've checked everything, timing, float levels, correct needles and positions etc, etc,. Idle is ok, lift the pins and mixture is ok. Open the throttle and it seems very very lean, spitting back on both carbs, but the plugs are black. Lift the front carb piston all the way up and the engine stops, lift the rear carb piston and the engine continues to run.
This I don't understand !
The carbs were overhauled in recent times , but the car has been laid up for a while. Before I strip them, and the inlet manifold I would welcome any suggestions. It has a cross flow head with a heat transfer tube from the exhaust side which I suspect is not doing its' job. Another problem
Thanks Sean
S Sherry

Could it be an inlet valve sticking open on one of the back two cylinders? A compression check will tell you. Lindsay.
Lindsay Sampford

Sean, Did you know there is a branch of the RM club in Australia? Area secretary is Cliff Goodman, 323 Mortimer Road, Casuarina, Western Australia 6167
Tel: 08 94192692
Cheers mike
m.j. moore

Are the dampers correctly filled with oil to the correct level? They act to richen the mixture when you open the throttle. I would not suspect the heat transfer tube, it is not necessary on the SU's where the caburetters and exhaust are on opposite sides on the mga twin cam nor the MGA cross flow HRG cylinder head.

Mick
M F Anderson

Sean, I think I'd start by adjusting all the valves and doing a compression check as Lindsay recommends. It could also be a vacuum leak in the inlet manifold. Check the core plugs at both ends, they have been known to fail.

Andy
Andy Preston

I've now removed the carbs and inlet manifold and the heat transfer ports have been blocked off. I'm going to reinstate them ,as even when the engine is warm the manifold is still stone cold. That can't help vaporisation very much. May be I've found the problem in one of the jets, the carbs were assembled with copous quantities of black silicone sealent and a piece appeared to be in one of the jets. I don't know if this was picked up when withdrawing the jet. I'll fit new jets and needles and keep my fingers crossed.
The carbs don't have dampers , just heavy brass pistons. Compression is ok on all cylinders.
Thanks Sean
S Sherry

This thread was discussed between 16/06/2009 and 17/06/2009

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