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Triumph Repair/1976 TR6 Idle Speed


QUESTION: Hello Harold:

My 1976 TR6 idles at about 1000 rpm at start up and increases to about 1400 rpm when warm.  Specs indicate that idle speed should be about 800-850 rpm.  I've don't have much adjustment left in the screws.  The emission control systems have been removed and I thought I might have a vacuum leak so replaced all connectors and the cap on the bottom of the forward carb.  No change after installing the new connectors.

Right now the rear carb is connected to the distributor and the port on the bottom of the forward carb is capped.  Can you suggest how I can bring the idle speed back near the range indicated in the specs.?



ANSWER: Hi John,

First thing you need to understand is that no engine can run at 1400 RPM without a lot of air. So you need to spray around the intake manifold to check for leaks and check the choke fast idle screw to see that it is not holding the throttle plates open and check the decel diaphragms and valve to see that they are closed all the way. Some had a threaded rod sticking out with a speed nut on them that you can turn the threaded rod in to force the valve to stay closed all the time.
Some early British cars also had decel valves in the throttle butterflies that would leak and thus cause a high idle so look at the throttle butterflies to see if you have them.

Keep in mind that no engine can idle at 1400 RPM without a lot of air. If you don't have a stethoscope to listen for vacuum leaks then use a piece of vacuum hose in one ear and use a spray can to spray around the manifold to see if you get an RPM change.


---------- FOLLOW-UP ----------

QUESTION: Hi Harold:

I disconnected the vacuum hose between the vacuum retard unit on the distributor and the rear carb, and capped both openings.  I started the engine and was able to adjust the idle speed down to 800 - 850 so I'm guessing air was entering the system between the vacuum unit and the rear carb.  Since the car runs fine without this connection is it ok to leave the plugs in place and drive without it?  I don't really understand what the vacuum retard unit does.



It is most likely a bad diaphragm in the distributor vacuum unit. You need to find some shop or someone with a Mity-Vac to test the retard vacuum unit. The vacuum unit when it is a retard unit has manifold vacuum on it at all times holding the timing in a retarded timing and when the throttle is opened you loose a lot of manifold vacuum thus advancing the ignition timing.

Some emission control cars put switches and sometimes delay unites in the vacuum line for emission control. If you have any, you need to test them for leakage also.


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Howard M. Fitzcharles III


Triumph TR-4 up & Spitfire, and Engine theory


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

Associate member SAE EAA member

Import Car magazine

ASE Master Auto with L-1 certification up to 2000

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