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MG MGB Technical - Loss of power

My 77B has been losing power the last two times out. Starts fine and runs extremely well at 60 MPH plus but getting up to 60 the car seems to struggle. Today I had the pedal all the way to the floor trying to get up to speed. Also, it seemed to take a great deal of effort to brake the car today. The brakes seemed to be far less responsive than usual, coincidence? Any ideas?
JCH Hibbard

If it still has the ZS carb,a dying diaphragm will explain the lazy accel. But it should also show signs of running rich; it'll get worse if that's the case, and it will contaminate the oil with unburned fuel. However, the brake thing indicates a large vacuum leak, causing the servo to be useless. Usual cause is the manifolds falling off the side of the engine, with a resultant bad manifold gasket. This might also explain the lazy attitude. Also check that the screws holding the auto choke on the side of the carb are tight, as loose choke can cause odd running and fatal fuel leaks onto the exhaust.

FRM.
FR Millmore

Thanks FRM...I replaced the needle in the ZS Carb a few months back and at the same time replaced the diaphram with a new one. The manifold gasket is less than 1 year old as well and I checked the tightness of the manual choke and all is well there.
JCH Hibbard

Is a brake be binding? It should be getting really hot if so.
SteveP

If the two *are* connected it could be the servo diaphragm ruptured, or the hose, causing a vacuum leak and affecting running as well as braking. You could remove the hose from the manifold and seal the port, and if that corrects the running (braking may be worse so take care) then it is hose or servo.
Paul Hunt

JC-
Did you check the manifold bolts? A year is about the time it generally takes a new gasket to settle in to where the bolts are loose.
I had thought of mentioning servo diaphragm, but in all my years I've never met a bad servo diaphragm. The hose is certainly a candidate though.

FRM
FR Millmore

Funny you should ask but just recently checked bolt tightness and everything was ok. I'll check the hose later today, not sure how to check the diaphram just yet.
JCH Hibbard

As Paul said, disconnect the hose and plug the manifold port. Or hook a vacuum pump and gauge to the servo vac input.

FRM
FR Millmore

Checked servo hose today and that seems to be ok as well. A little bit of a mystery here until I get it figured out.
JCH Hibbard

Found a bad rubber grommet holding the non return valve in place at the brake servo. Replaced that and double checked the ZS diaphram, all is well. Took the car out for another drive today and had the accelerator all the way to the floor in fourth gear trying to get up to speed. Timing is right on and I rechecked the spark plug gap and fuel filter. The mystery continues.
JCH Hibbard

This thread was discussed between 02/11/2009 and 08/11/2009

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