MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - Rough Running/Misfire

Further to my previous post (1977/1500) still not sorted the problem. Plugs, points condenser, leads are OK. checked fuel in float chambers, floats OK, fuel in there on inspection. Cleaned suction chambers & pistons. Fired the car up, let it warm up for 20 mins or so, took it out, went like a rocket, then 1.5 - 2 miles problem occured, loss of power, feels like running on some of the cylinders, then picks up, everything fine then 1.5 - 2 miles same again, then picks up again.
I have a theory - I renewed the fuel pump in August, also renewed the flexible hose sections. It had been running perfectly OK until last week.
I calculated the the volume of fuel in the float chambers will run the car (up to 40 - 45 mph) for about 1.5 - 2 miles. It therefore seems like it could be the pump at fault - not pumping sufficient fuel. Why would it start doing this. I have read elsewhere in the archives of dodgy replacement pumps. Anyone else had this problem. The replacement pump came from Moss
Nigel Axtell

Probably not your problem, but some time ago my 1500 would start misfiring under load, such as accelerating up a hill, after a couple of miles, but then run sweet as a nut until the next hill.
It was due to fuel contamination, I had put twice as much Millers octane booster and ethanol protector stuff in the tank as is supposed to go in and it must have being fouling the plugs or something.
All is well now after correcting the dose.

Mechanical fuel pumps either work properly or leak fuel into the sump due a damaged diaphram,so I doubt if they would cause an intermittent fault like yours, but I could be wrong, as ever.

Although you think the dreaded ignition components are OK, it might be worth trying another set of points, condenser and rotor arm, one at a time to see if a faulty item shows up.
JB Anderson

Had a similar issue once when I changed the pump for a different type - the fuel supply pipe then became slightly kinked. Was OK to start with then started to struggle as not enough fuel coming through.
John Collinson

Nigel,

I have seen flexible hose colapse when used on the suction side if the hose is not stiff enough.

Charley
C R Huff

What sort of pump? SU?
Lawrence Slater

I had similar problems with my 1500 once. It would drive well enough at steady speeds or gentle acceleration, but with sustained pedal down driving it soon started playing up. It stopped on the long hill going up out of Moffat. It turned out to be a split in the diaphragm inside the mechanical fuel pump. Fixed it at the roadside with a piece from a plastic carrier bag which worked fine for the remaining 350 mile journey to Edinburgh and home again.

I know yours is supposed to be a new pump but undo the top and check the diaphragm. It might just have a split in it.
Guy W

Lawrence
It's the Moss equivalent of the AC Delco pump
Nigel Axtell

Sorry, I should have told you to ignore my question.

I noticed later that you have a 1500, and therefore not an SU under the rear arch.
Lawrence Slater

This thread was discussed between 26/11/2012 and 27/11/2012

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.