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MG MGA - Engine Stutters, then Runs Rough Under 3000 rpm

I had a problem the over the last month. It started out where the engine had just a hint of hesitation when coming off of a cost onto light acceleration. It was barely perceptible to start. I looked everyhing over the plugs looked just right. The fuel filter was filled with fuel.

Last week I went for a ride. Everything started off well. It was unseasonably warm in the upper 80s. We got caught in local town festival and the temps started to rise a bit. Not too warm, around 200. We got thru the traffic block OK. We were now about an hour into the ride. Then, just as the engine temps started to drop back down, the stumble started back up again. I decided to turn around and head back home. As we continued the stumble got worse and worse. I thought I was not going to make it home. Funny part about it was that if I got on the throttle and the engine got over 3000 RPM, everything smoothed out and the engine ran like a champ. Getting to that 3000 rpm was a chore. It was as though the engine wasn't firing on 1 or maybe two cylinders.

I did make it home and just parked it. Later I gave the car a look over. Nothing seemed to be blatantly out of whack. I checked the plugs again and they looked right. I checked for even movement on the SU dampers.OK there too. I waited until dark and started up the car to see if there was any errant sparking going on. The car seemed to idle fine. I did found some arcing though. My vac advance line to the disty has a loop of rubber right near the #4 plug. If I push it over to the plug, the HV would arc over to the rubber(?) and the engine would start to stunble. I really don't think this was the problem as it was a good inch or so away in its free state.

I also noted a light spark coming from down low near the front of the block. I turned a flashlight on and found that the rear generator mount was loose to the block was sparking across the loose. I tightened things down.

I really don't want to drive an hour or so more to see if this was the root cause of the problem I had. Anybody have similar experiences?
Chuck Schaefer

Could it be a sticking carb piston not closing off?

also, some of these types of problems can occasionally be traced to the rear filter box being fitted 180 degrees out or an incorrect gasket between the filter box and carburettor, blocking off the pressure balancing hole to the piston. This results in a very slow moving piston, most noticeable during acceleration and deceleration with a consequential erratic fuel/air supply from that carb. Once in cruise the piston has time to slowly balance itself out and the car runs okay.

See my pictures of what I mean: http://www.mgaroadster.co.uk/odds_and_sods.htm second picture down.

Steve
Steve Gyles

Steve. Thanks for the thoughts. Nothing on the car had been changed by me recently except for the SU bowl to cover gaskets and the addition of a new condensor which failed earlier this year. air filters have been in place for the last 2 seasons, so I doubt it is the balancing hole. I already checked for free movement of the SU damper piston. When it started to run right over 3000 RPM it is like a switch is thrown. Instantly it runs great.
Chuck Schaefer

I had a similar incident. Mine would start and then run rough at idle. Giving it gas only caused the engine to start to stall. After 3 - 5 minutes the RPM's would slowly start to climb and then it would run fine at 1500 plus RPM's. I thought for sure it was the carbs but the engine rebuilder looked at the plugs and went right to the rotor. 1000 miles and it was buying the farm already. He changed it out to a heavy duty rotor and checked the points. Bingo. Maybe this might help.
Cheers.
Lmazoway

Chuck,
Just a thought..you might check the advance to see if you've broken a spring. If the weights are stuck out it might explain why it runs good above a certain RPM and poorly below.
Cheers,
Gerry
G T Foster

Chuck,
Two thoughts from some experience - (a) could be fuel vaporisation causing rough running, or (b) another coil about to fail!
Peter.
P. Tilbury

This thread was discussed between 16/10/2010 and 19/10/2010

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