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Rough running MG

I have a 1972 MG midget and recently rebuilt the carbs, (needles&seats, floats, needles&jets, new fuel pump&filter) along with ignition,(points, condenser, rotor and plugs), checked timming and adjusted the valves. After tunning and balancing the carbs I take it for a test drive, it runs perfectly for about 5 - 7 miles then starts missing and running rough. I thought this might be heat related so I ran on a cold day with no change. I also put in my old coil with no change. After 25 years this is the first time I have been stumped. Help!! Thanks.

Wonder if the new fuel pump is having problems. Any way to to swap it out for the old one?

Also wondering about the fuel pump. My guess it is an electric pump and they use to be problematic in the old MG’s.

Your floats bay be a little too high causing some flooding.

Along with checking the fuel pump also make sure that the ignition is getting the proper voltage and isn’t low due to a high resistance in the line and is heating up after time causing a higher resistance.

I would look the choke. Once warmed up is it totally open? MG also had a big problem in that era with slipping time, double check the timing.

Any chance the fuel tank is not vented and after a short running time the fuel pump is pulling a vacuum on the tank?
You could try driving it with the cap removed and see what happens. The tank does have a vented cap I would hope.

I am not familiar with a 1972 MG Midget with dual carburetors. But have you checked the valve clearance?

Yes Rod, an '72 Midget shoud have dual SUs.

Have you rechecked your timing and dwell (AKA point gap)? The condensor is a component that is subject to failing under high heat.

When you rebuilt the carbs, did you put in throttle shaft bushings? If the holes that the shafts run in are worn out, the carbs will suck air around both ends of the shafts. The fix is to drill out the holes and put in bushings. The throttle shafts may be worn too. Check by attempting to move them up and down while the engine is running. ANY change, says the shafts and/or the holes they ride in are worn. I can’t think why that would be time sensative though…

You’re going to have to figure out if your problem is fuel or ignition related.

My first guess, since it ran for 6 or 8 miles after a tune-up would be that you forgot to put some grease on the points where they rub on the distributor cam and wore the points block away. This means that your points are no longer gapped correctly. It’s easy enough to check, just see if the points are gapped the same as when you installed them.

You mentioned everything in the ignition except wires. You did replace the wires, righi?

You didn’t by any chance run 12 volts directly to the run side of the coil did you? That would burn up the points.

I would put money on an exhaust valve being too tight on #2 and/or #3.

One odd fix I recall in a similar problem was a gas line resting too close to the exhaust. Don’t ask me why but the too hot gas caused problems. I doubt this is the problem but just a thought.