Triumph Repair: Spitfire loss of power on hills, competent mechanic, jet needle

Question
Hello. My 1975 Spitfire has a chronic issue: Although she runs fantastic on the flat (good acceleration, runs and idles evenly)and up short hills,when she encounters a decent longer uphill grade, the engine begins to lose power and, as more gas pedal is applied, starts to run unevenly, like it's flooding. It has the stock single carb setup and new electronic ignition and recent tune up. The engine has had a recent overhaul by a competent mechanic who is experienced in Spitfire issues but he can't seem to find the problem. I am considering getting a replacement/upgrade carb (Victoria British has a two-stage carb that apparently goes right in without modifications) but I want to get this problem fixed first. Any help or ideas appreciated. Thanks.

Answer
Hi Bob,
Any engine under high loads is strained but if it is all correct the only problem will be high engine temperature.
If your engine starts running "uneven" you need to do a test.
With the engine running good and up to operating temperature. Remove a few spark plugs and look closely and note the coloring. Then do your run up the long hill and get it to run uneven. If possible and you can do it safely, push the clutch in and switch off the engine. BE CAREFUL NOT TO LOCK THE STEERING. Pull off the road and remove the same plugs and look at them closely. If they are Flat Black and well covered as though they were spray painted flat black. Then it is too rich on the main jet.(needle too high) If the plugs are cleaner than normal and even have a clean white insulators, the mixture of the main jet is too lean. (needle adjusted too low) or you have a fuel pump problem as a steep grade makes the pump less effective because it can't pull fuel as well. This problem will usually be a hard "On-Off" "On-Off" running.
Unleaded fuel is not easy to take "Plug Readings" but when the mixture is so far off as to make an engine run bad it will show on the plugs.
You said "Recent Overhaul" How recent? In miles? and what was done inside the engine?
Let me know.
Howard