Triumph Repair: Zenith Stromberg carb rough idle - rebuild, zenith stromberg, vacuum hoses

Question
I've got a 1979 spitfire with an auto-choke zenith carb. Idle was ok, at some point idle became very rough, unstable, idle speed up and down, stalling. Not sure what precipitated this. I had wanted to do a rebuild so went ahead and did that. No improvement. A few things. There's a throttle bypass valve, in it is a diaphragm that looks like it should be replaced but wasn't in the rebuild kit. Anyone know what the bypass does and how to adjust it? Also, I know the auto choke gives people lots of trouble, i ended up setting the bi-metal spring so that it always remains completely off for now. Thoughts on what might be going on?

Answer
Hi Jonathan,

The diaphragm is usually dried up and hard and if you suspect it, you can run the screw adjuster so as to close off the bypass (counting the number of turns so if you want to set it back you can do so).

The trouble is that with a symptom of rough idle and stalling etc, that can be caused by a hundred things so it is not good to get one thing in your head that you think it might be.

If you brought that car to me in a dealership and told me all the symptoms, I would nod my head as you told me and when you left I would have to just drop every thing you told me and go test the engine and fix it.

First you have to establish that the combustion chambers are good. A compression test is necessary to do that and if it fails that test it is no use proceeding until that is corrected.

Fire is second. We had a scope to test that in just a minute but most people don't have access to a scope so there are other ways to check this. First check the ignition timing with a timing light and note if the timing is changing much at your rough idle. If it is, remove any vacuum hoses on the vacuum advance or vacuum retard and plug the hose and note if it changes anything.

Carburetor is always last. While it is idling rough, very slowly start restricting the air going into the intake. If at any time while progressively closing off the air supply, the engine smooths out or speeds up, then you for sure have an intake air leak some where.

Without CO/HC test equipment you can look at the plug color to see if the plugs are showing any signs of a flat black powder or a very clean white center electrode. Flat black powder is a sigh of too rich mixture (for some reason) and very clean white center of a plug is an indication of a lean mixture. Also for a test remove the vent hose from the carburetor float chamber to note any changes.

Somewhere is all this you should find your problem.

Howard