Triumph Repair: 76 Spit runs rough when engine is warm, mixture problem, lean mixture

Question
I have owned a 1976 Spitfire since 1979.  It stays covered in the garage during the winter with limited usage during the cold weather.  The last couple of years, the Spit begins to run very rough after the engine is warm.  If I just take it to work in the morning or return from work (about 4 miles each way), everything seems OK however extended drives gradually get rough.  The fellow that has done some work on the car in the Knoxville, TN area has stated, "you car is cold natured"  Any thought?  Thanks

Answer
Hi Richard,
Running rough when hot narrows the problem to only one thousand possible causes so you need to do some testing. All tests must be done while in the rough running mode. Rough running when hot but not cold could be a fuel mixture problem among other possibles so while it is running rough note if there is any puffs of black smoke out the tail pipe when you rev the engine quickly. Black smoke is a sign of a over rich mixture. Also remove the spark plugs and look for signs of flat black powder on the outer edge. Another indication of a rich mixture.

The opposite is a lean mixture would be indicated by poor quick throttle response and very clean white colored plugs.

If either is noted you need to look for the cause first by removing the top cap out of the top of the carburetor and use a 3mm Allen wrench down into the oil to see if your carburetor has an adjustable needle. Clockwise to richen and counter clockwise to lean. Never turn more than half a turn at a time then check performance.

Some of the other things that can cause rough running are tight valves, incorrect ignition timing, Ignition failure when hot. Choke failure (common)

If you don't find anything in fuel mixture put a timing light on the coil wire and run the light out from under the hood and place it under the wiper arm so you can see the light when driving. Tape the trigger down so it flashes all the time and drive the car while it is running rough. Even when you only have a intermittent misfire, it will show on the flash of the timing light as a flicker in the normally steady flash. If you do the test in the bright sun light you may have to tape a piece of cardboard over the light so you can see it in the sun light.
Howard