[TR] 1972 TR6 Overzealous At Idle

[Ann and Tim Buja]

cole.jw at comcast.net wrote:
> My 1972 TR6 <snip> idle speed is very high.
>
> The idle remains very high even with the choke cable disconnected,
> the throttle linkage removed and the idle speed screws on both
> carbs backed out to where the venturi are closed. The carb linkage
> moves freely and is not binding.
>
> Disconnecting the vacuum advance causes the RPMs to decrease only
> very slightly.

If you've already ruled out leaky throttle seals, I would look at the bypass
valve diaphragms and the gaskets that attach the bypass valve to each
carburetor.  A leaky or misadjusted bypass valve will cause an abnormally
high idle, which may go as high as 2000 to 2500 rpm with the vacuum retard
capsule disconnected.

Many bypass valve gaskets have an hourglass shaped opening between the two
ports in the bypass valve.  This will cause a permanent throttle bypass that
will prevent you from lowering the idle below 1000 rpm.  TRF sells new
bypass valve diaphragms as
part number ZEBPV6 for $12.95.  I just bought one and it was sourced from
Joe Curto as "BP-014 Zenith Stromberg Bypass Assy Rebuild Kit".  This
package came with the rubber diaphragm with brass valve, a rubber ring to
seal the adjustment screw, and three gaskets.  One had the hourglass shaped
opening, and I believe the other two had two individual passages to the
diaphragm valve ports.  IIRC, one of these two gaskets has an additional
vacuum port while the other does not.  My 72 TR6 needed the third small
vacuum port to operate the valve.  Other models may have a separate vacuum
line to the valve body.

If your bypass valve adjustment screw is blanked off with a brass plug, you
can disassemble the valve and push the brass plug out from the inside of the
valve so you can adjust the valve while it's mounted on the carb.

Follow the ROM procedure 19.15.02C steps 23 through 26f (pages 223 and 224
of the Robert Bentley TR6 manual).  Remember that turning the adjustment
screw clockwise will reduce the force of the internal spring that holds the
valve closed, and turning it anti-clockwise will increase the force holding
it closed.

I replaced a "perished" bypass diaphragm and hourglass-shaped gasket with a
new diaphragm and gasket with two distinct bypass valve ports on one of the
carbs on my TR6 this spring and was able to drop the idle from 1100 down to
750.

Tim Buja - Rockford, IL - 80 TR8, 73 Stag, 72 TR6