Running rough

[Bumpa]

A long shot this, but it sounds similar to what I experienced with my Dolomite, also on SU carbs. It idled fine but as it warmed up it lost more and more power until it wouldn't do more than 30 mph flat out, and as rough as rough.

It was shortage of fuel caused my trouble due to the waxstat jets on the carbs. As they warmed up they weakened the mixture much too much. Why I don't know. Could it be due to the formulation of modern fuel? Changing the jets to plain ones affected a complete cure.

One other possibility might be the fuel tank filler cap not venting. This prevents the pump from drawing fuel and starves the engine.

[MGB72]

Following this with interest as a similar problem here. Runs absolutely fine most of the time but then will have a really rough passage, tacho not flicking. I'm pretty sure it's not a spark issue - all replaced vai Distribution Doctor so I'm going to fully check for fuel starvation - when I get some time!

By coincidence mine isa 1972 Mallard Green (originally Autumn Leaf)!

[John 37]

Have you replaced the fuel hoses with modern ones to take E10 fuel?
Another possibility is sticking carb dashpots.

[Charles Farran]

Tom,
I have just be clearing out some old Safety Fast magazines. I found a Tech Tip referring to the condition of the Distributor Vacuum Advance Pipe. Following noticing that the spark plug in number 4 cylinder was almost white compared to pale grey of the others it mentioned that the pipe had perished & cracked,letting air in & thus weakening the fuel mixture. £2 replacement part & possible rough running & engine damage were avoided.
Is you advance pipe in good condition?
Cheers,
Charles