Triumph Repair: rough idle, throttle shaft, wd 40

Question
1975 Spitfire purchased 10 days ago not ran for any time to speak of in last four years. Ran fine until we changed plugs and air filter. Old air filter was excessively dirty. I also sprayed carb cleaner on outside of carb to remove excess dirt. Did idle at 1000-1050, now idles at 650-750. Starts right up on choke but runs rough off choke. Is this running to lean due to so much more air to carb (only one carb).

Answer
John,

Couple of thoughts.  I agree that the dirty air filter may have been too restrictive, effectively enrichening the mixture.

Another possible problem is that the throttle shaft tends to wear in the carb body.  By cleaning the outside you might have removed some grease/dirt that was helping to seal it.   You can test by running the car and spraying carb cleaner (or WD-40) at the throttle shaft and seeing if the engine speed changes.

Last idea on the idle change involves the spark plugs.  It's possible that one cylinder isn't firing, either because of a bad plug lead connection or the plug lead itself is broken internally.
You can check for this using an insulated pair of pliers.  With the engine running pull one plug lead at a time and see if the engine speed changes.  If you find a cylinder with no change, there's the issue.


Cheers,

Jim