09-11-2017, 08:30 PM
Hugh, I agree with others that it sounds like weekness at some point on the needle profile, remember that the wrong profile can be both week at one point and rich at another. As was suggested earlier have you experimented with the choke out a bit, this will lower the jets and effectively richen the mixture throughout the range. I wonder if the suggestion about the time of year may have something to do with it, your car I assume has no thermostat so may be running cooler at this time of year?
Therefore if the mixture was set during summer months with a completely hot engine, and was set up with the O2 ratio border line lean around low to mid piston stroke this could possibly give symptoms described when the engine is not getting up to full operating temperature. If having the choke out improves things try partially blocking the radiator and getting the engine fully warmed up, see it that alters anything. Otherwise I have to agree that a sudden air leak is likely, but that would show as different plug readings on the cylinders affected, you have twin carbs so not all cylinders can be affected by an air leak.
Therefore if the mixture was set during summer months with a completely hot engine, and was set up with the O2 ratio border line lean around low to mid piston stroke this could possibly give symptoms described when the engine is not getting up to full operating temperature. If having the choke out improves things try partially blocking the radiator and getting the engine fully warmed up, see it that alters anything. Otherwise I have to agree that a sudden air leak is likely, but that would show as different plug readings on the cylinders affected, you have twin carbs so not all cylinders can be affected by an air leak.
Black Art Enthusiast