Since I rebuilt the engine 18 months ago the car has been going well. It is completely standard with SU carbs and Delco distributor still with points. Tonight it started to misfire, but only when cruising on a steady throttle at about 60 mph. On give and take roads where you are accelerating and slowing with frequent gear changes it performs nicely. But once on the dual carriageway it runs along sweetly for a mile or so then starts to stutter and won't accelerate. Get back onto normal roads and all is well again.
It's going to be fuel or sparks, but which? Could the mechanical pump be unable to keep up with demand, or is the horrible Delco dissy more likely to be the culprit? It's a weird one. Why doesn't it misbehave when not cruising? Any ideas where I should start?
If it is OK on "give and take" roads and under short bursts of acceleration then I would rule out ignition and go for fuel starvation. Either knackered pump or partially blocked fuel line. A good test is to find a stretch of road with a long incline and put you foot down, if it gradually dies I would say fuel starvation. Ignition usually shows up with a) warm/hot engine and b) under acceleration where the mixture has richened up causing a rise in HT to get a spark, causing any weaknesses in the HT side to show up. A temporary fix for a blocked fuel line is to disconnect the fuel line at the pump and use a compressed airline to blow it back to the tank. Let it settle for a while before starting otherwise you will just suck the crud through again.. Also check any inline fuel filters, they could be clogged. The crud comes usually comes from rust in the tank. It could be the condenser on the way out on the ignition side, they nearly always give odd symptoms before they fail completely.
You could just disconnect the fuel pipe from the carbs and crank the engine and make sure you get a good fuel delivery out of the open pipe into a suitable collection "jug", that usually gives a good indication of potential fuel starvation issues.