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MGB High rpm problem

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Carlbanan56 said:
When I get home after school I will be taking the carburetors off and make sure the throttle valves are fully closed.
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Looks like the temperature is about 7 deg. in Sweden today. Put on a sweater and take it for a run first. 1500 rpm isn't way out of line and I am convinced that it is something simple. Butterflies don't go out of alignment ove the winter.
 
I think I found the problem. The throttle return spring was very worn. It did not close the throttle valve tight enough. I will be buying a new spring and se if this solves the issue. Thank you all guys for your time and effort. It is greatly appreciated.
 
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Carlbanan56 said:
I think I found the problem. The throttle return spring was very worn. It did not close the throttle valve tight enough. I will be buying a new spring and se if this solves the issue. Thank you all guys for your time and effort. It is greatly appreciated.
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Often the easiy thing! great news and well spotted
 
I had same problem turned out to be a crack in one of the bakerlite type insulator blocks that sit between carb and manifold. You could not see the crack but a smoke test revealed the problem.
 
the clue is that the idle was satisfactory before a long winter's respite. so it's unlikely to be any type of wear problem, or adjustment problem. but most likely a problem associated with inactivity. and things get stuck when they aren't used regularly, especially old things that only remain unbound by regular use and resulting lubrication. ergo, something is stuck...IMO.

remember if idling at 1300rpm, air and fuel is being drawn over the jet at sufficient speed to support that RPM. so looking for an air leak anywhere downstream of the jet will not likely solve the problem. in fact it would create a lean condition as only air can be introduced through a vacuum leak. the fact is that fuel and air is getting past the butterflies, unless the HIFs have a dedicated idle circuit, which i don't think they do.

my best guess (going for broke here) is that one or both of the spring loaded throttle valves (aka "overrun valves", aka "butterfly valves") associated with the butterflies is holding it/them open. again, the long winter's corrosion. or at least one of them may be. it doesn't take much of a gap in the butterfly/carb-throat to create 1300rpms.

good luck!
 
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