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Triumph Repair/TR6 carb idle


Hi Howard,

I recently had my carbs rebuilt. With the carbs reinstalled, with the choke in, the cars runs at about 3000 rpms - pulling the choke out gets it down to 1500 or so. Wondering what I've done on reinstall. I would have thought rpms would have been higher with the choke out. I'm also getting some uneven idle, ie: even at 1500, it cycles up and down.


Hi Cam,

There is a basic fact and that is no engine can idle at 3000 RPM without an air supply and a fuel supply. If the throttles are closed restricting the air you can add all the fuel you want and the engine can not go to 3000 RPM. If you lean out the mixture it can not go to 3000 RPM.

So that proves that you are taking in an excessive amount of air somewhere. Either your throttle plates are not closing or you have a air leak somewhere like a gasket or a vacuum port or if you have the throttle plates that have the spring loaded valves in them and the springs are weak.

When you have too much air entering the intake and you add fuel it by pulling the chokes out that would normally raise the RPM unless you are already too rich in mixture and in that case the RPM would lower and get rougher and start blowing black smoke out the exhaust.

First thing to do is to run it and let it idle at the 3000 RPM and start blocking off all vacuum lines including the brake booster to see if any change the RPM. If that don't change the 3000 RPM start spraying either propane or butane around the intake and around the carb gasket area to see if that changes RPM. (intake leak) next slowly start restricting the air flow to each carburetor by hand to see if it speeds up the RPM or slows it down.

If that makes the engine only slow down then remove the carbs and look at the throttle plates (butterflys) to see if they have the spring loaded valves in the throttle plates. If they do have them you need to either replace the throttle plates with plain ones or solder the valves closed. Also, on the side of the carbs are a large brass screw the must be screwed all the way in and as a test the long compensator valve under the plastic cover needs to be temperarily disabled by placing a piece of anything soft under the plastic cover so as to hold the valve closed on both carbs. Also remove the bypass valve and diaphragm on the side of the carb and adjust the screw so as to force the valve closed all the way.


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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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