Rough running at lower-medium throttle

[Paul Kelley]

Before I do anything drastic like buy replacement carbs, any suggested causes - and fixes - for a consistent rough running on lightish throttle openings? Engine pulls well under acceleration and at higher revs and starts well and idles ok, cruises fine at 70. Doesn't seem affected by being cold or hot particularly. I don't think it is the fuel pump or dirty fuel, or on the spark side, reasonably sure it is inside the carbs. I have had them balanced and serviced not that long ago and this has been something of a recurring issue but I think is getting worse and results in a lumpy driving experience on light throttle. Any ideas welcome! Thanks.
Paul

[new to this]

Have you checked theres oil in the dash pots

Dave

[jakesmig]

Have you checked for an air leak in the manifold?

[richardthestag]

get it running and warmed up, spray a mist of wd40 around the inlet and carb area and any rise in revs would indicate an air leak.

difficult to say without looking at it but does sound like fuel mixture may be out.

That said 95% of fuelling issues turn out to be ignition related and vice versa

[Paul Kelley]

Thanks guys. Yes, oil in dash pots and yes have done a spray test around manifold a while ago and no apparent leaks but will re-check that.

[flying farmer]

Sounds like a weak mixture might be the problem.

If you have a stromberg adjusting tool I would suggest doing something fairly drastic like giving both carbs half a turn richer and see if it makes any difference.

If you can feel a difference it at least narrows it down to a carb problem.

When you said you had the carbs serviced, did whoever worked on them actually know what they were doing?

I ask this as when I bought my first Stag it had had the carbs adjusted by a garage for the previous owner to pass the MOT. One carb was hopelessly rich on idle, the other hopelessly lean but they balanced each other at idle due to the hole between the two halves of the manifold. As the throttle was opened four cylinders developed a weak mixture stumble. This disappeared at higher throttle openings due to the needles and jets being badly worn and the mixture going horribly rich.

Fitting new needles and jets fixed the problems.

[Kithmo]

Is there any sign of water in the fuel filter ?

[Paul Kelley]

Thanks Neil and Kithmo. The fuel filter looks completely normal - no misting - so I think it's ok. I used the same mechanic for the carbs as I do for most things, and he's generally very good and knowledgeable if not a Stag specialist as such (but does lots of classics and builds his own cars and trucks). I don't know the history of my engine that well and have found a few bodges here and there. I think new needles and jets is a good starting point, thanks Neil.