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MG MGB Technical - Engine rough idle, putting, lacks power

I know someone with a '72 MGB GT, he asked me to take a look at it as a second opinion. Both him and I are stumped.

Everything on the MGB is stock for '72, except he's put a WEBER DGV, air pump removed. Engine is approx. ~26k on rebuild he says. Problem, it has a high/rough idle and pops from time to time. It lacks power and will not climb a hill.

I checked the ignition system, good powerful spark on all four plugs, advancing or retarding ignition made no difference. I replaced his Bosch coil with my new Lucas Sports Coil... no difference.

Moving onto carbuerator... primary and secondary throttle linkages moving freely and choke operating fine/not sticking. Engine accerates, with slight hessitation and popping. Accelerator pump squirts fuel into throats.

Theory of mine... possible timing chain issue? Either loosing tension or stretching. He says its been getting worse over the last year or so. I also think he could benefit from a vent tube from rocker cover to air cleaner on DGV. His crankcase vent tube is open air.

Any ideas welcomed, Thanks!!!
~ Ron.
midgets...etc.


Ron Koenig

Check for air leaks at the base of the carburetor and where the intake manifold bolts to the cylinder head. A bad intake/exhaust gasket will cause the symptoms that you have described. RAY
rjm RAY

Ron. I agree with Ray that this would be the first place to start. One clue is the DGV conversion. The intake manifolds for these conversions do not match the exhaust manifold flange thickness (neither the exhaust manifold for the HS equipped engines, nor the manifold for the HIF carbs which is thinner). Thus, unless the owner has made up custom spacers to make both manifold's flanges the same thickness the possibility of air leaks is the first thing which must be considered. Use either carb cleaner, sprayed around the base of the carb and around the manifold flanges, or move an unlit propane torch around these joint areas. An increase in speed and smoothing of the idle are an indication that there is an air leak.

Yes, the Weber air cleaner should have a 90 degree plastic elbow in one corner to hook up to the tappet chest cover. Using the "road draft tube" system is neither environmentally friendly, nor is it a very effective system.

What do the spark plugs look like? What do the wires look like, in the dark with the engine running? Do you see sparks around them? Deteriorating ignition components can also cause these symptoms. But, considering the DGV conversion, an air leak has to be ruled out before any other trouble shooting can begin.

Les
Les Bengtson

Air leaks!
J.M. Doust

Basics first
Compression test and check tappet clearances
Willy
William Revit

This thread was discussed between 19/09/2011 and 20/09/2011

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