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MG MGB Technical - Jittery with low throttle

My '79 mgb roadster has recently been doing a funny thing where, at low throttle (regardless of RPM), it will start jerking, as though it were misfiring but far more subtle. It happens sometimes at idle as well. This is not a constant behavior; probably 60-75% of the time, it's as smooth as can be expected from a 26-year-old Britmobile. It never happens (at least, not that I've noticed) when I press the gas more than a whisker or two.

I suspect fuel. (Full disclosure: I know little to nothing about the fuel pump, so what follows is rank speculation.) Is it possible that the points in my fuel pump have simply become slightly misadjusted? The car just under 10K miles past a full rebuild. Or is it more likely something more mundane, like a piece of gunk in the fuel filter?

Or, heaven forbid, could it be a problem with the Weber downdraft carb? My impression has always been that the downdrafts tend to be *very* solid carbs, and that it's more likely something squirrelly like a point set. Thoughts?
Chris St. Pierre

Sounds like a vacuum leak which affects engines more at low speeds than higher speeds. Check the plugs for mixture which could also cause the same problems.
Mike MaGee

I had this kind of problem today, I was low on fuel and it occured as I pulled up the incline into work, so I thought I was about to run out of petrol. Problem was not improved by filling up, topping up the dashpots, checking coil & condenser contacts, points and plug leads. I coaxed the car home as rain was expected, finding it would run ok at idle & above 3000, but was likely to stall as the throttle was opened. On final look around before light became too bad I noticed the vacuum connection had come off at the manifold.
Replacing this cured the misfire instantly.

Good luck
Vic
V Todman

Punctured distributor vacuum diaphragm?

Test it with a Mity-Vac tool.

Just a thought.
Daniel Wong

That can also be points closing up...check the gap or better yet the dwell with a meter...mine runs crap if the dwell drops below 55 degrees....setting is 60.
P J KELLY

I had this effect on a Triumph Spitfire and traced it to a worn bearing surface in the carb body allowing on of the SU butterflys to move. The effect was only a problem at small throttle openings.
S Best

Another one: Check for worn distributor bushings.

Set the points gap - then wriggle the distributor
cam/spindle. Look for any noticeable wobble,
or movement in points gap.
Daniel Wong

May be a long shot but both soundness and smooth operation of the throttle cable could be checked from both ends. A simple solution if it is the cause.
Roger T

This thread was discussed between 31/08/2005 and 03/09/2005

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