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Carb. Intact flapper valve rattle fix - Eric Reitz

searlerb@...
 

Diagnosis
The “flapper” valve is a two and a half inch wide sheet metal plate on a bimetal arm inside a banjo shaped duct.  When the engine is cold, the plate covers the fresh-air intake and the engine pulls air from around the exhaust manifold shroud.  As the engine warms up, the bimetal arm warps and closes the hot air side and opens the fresh-air side.  The warping action is not very strong and the valve seating is rudimentary so vibrations picked up from the engine easily cause the rattle against the valve seat.
 
The Fix
The air valve is easily removable and leaves an unrestricted opening for fresh air to the air filter and carburetter. I considered just removing it and test drove the car this way and it ran fine.  But in keeping with my originality goal and the MGB being such a highly engineered machine, I was reluctant to remove even a seemingly useless component for fear of upsetting something else.
 
There was no rattle when the car was new (I’m the original owner).  And there is no evidence of original padding or other mechanisms to prevent it from rattling.  I assumed that over time the bimetal arm became weaker allowing the valve plate to rattle against its surroundings.  I could not find new parts on the Internet so I considered two repairs:  a disk of silicon rubber gasket on the valve face or attaching a spring to hold the valve plate more securely to its seat.  The gasket seemed simpler so that’s the way I went.
 
I bought 1/32 inch silicon rubber sheeting and cut out a disk 2 1/8” in diameter.  This material is rated from -60 to 600 degrees Fahrenheit.  I glued this to the hot air face of the valve plate.  I thought a disk of material covering the entire valve seat would be superior to the small square my mechanic had once installed because it distributes the impact across a much larger area and allows the valve to close evenly.  I used Permatex Black Silicone adhesive after testing several other rubber cements.  I did not use epoxy due to a concern that it would work too well and be difficult to remove if I had to do the job over.
 
I was surprised how well the fix worked.  Not a sound.  I’m not sure if the disk cushions the rattle or its extra mass changes the resonant frequency of the bimetal arm so the engine vibrations do not excite it.  Maybe both.
 
By the way, I noticed that Moss Motors charges $38.99 for the short flexible duct that connects the manifold shroud to the air temperature control valve.  The image in their drawing seems to be beefier than was original to my car which was just light aluminum flex tube.  Dorman MightyFlow flex tube 2 inch diameter is identical to my original part, is available from Pepboy’s and costs only a few dollars.

Kevin McLemore
 

Nice fix!  I seem to recall that the original sealing agent was a thin sheet of porous polyurethane foam rubber... it never lasted very long and rapidly turned to dust in the heat of the engine compartment.  That's probably why you never saw anything there.  I like the silicone idea.  I probably would have gone with a thin sheet of EDPM (Neoprene) foam, since it would have had a wee bit more 'give' to seal the flap better, but your idea will last longer.