Triumph Repair: Zenith Stromberg carburetor, zenith stromberg, nova socia

Question
I am getting after running on my 1978 Spitfire 1500 when I shut my engine off even at the idle speed at about 900 rpm. Even on short drives. The operating temperature is normal.  I use regular gas with the 10% alcohol in the gas. The timing is set at 10 degrees B.T.D.C. The spark plugs are gaped as .025.  Is there an adjustment on the carburetor I can make to correct this?  I drive the car a couple of times a week.  I have been on long drives also.  I drove about 7000 miles last October when I drove to Nova Socia from Florida.  Thank you for any guidence you can give me.

Answer
Hi Don,

"Running On" is "Pre-ignition" which can be caused by several things. Your RPM is low enough that it should not do it and your engine temp is normal so overheating is not the cause. Excess carbon build-up in the combustion chamber can also be a cause.

The 68 and later cars were emission control cars (for the US) and the factory tried to clean up emissions by raising combustion chamber temp. Which caused Pre-ignition. Several of the British cars imported into the US had anti-dieseling valves that applied vacuum to the float chamber of the carburetor to prevent any fuel leaving the jet when the key was turned off, in an attempt to stop the pre-ignition.

Backing the timing off a little sometimes will stop it but then your performance will suffer. I would first try switching to high test gas first. You may then stop the pre-ignition and pick up a little more performance in power and fuel mileage. If it stops it, check closely the fuel mileage and you will probably see a better fuel mileage which will offset the extra cost.

The alcohol in the gas helps you so don't listen to those who claim it makes an engine run poorly. They don't know what they are talking about.

Let me know,
Howard