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MG MGA - I'm baffled - car won't run except at high rpm

Ran fine all through Watkins Glen two weeks ago, including lots of spirited driving on the old and new tracks. Ran home at highway speeds (3500 - 3800 rpm) for 300 miles, then stop and go through Queens, then back out fast on Long Island. Put the car away, then several days later very hard to start. Once started and fully warmed up, ran fine. Thinking it might be fuel related, filled up (6.5 gallons). Next day exact same problems. Now I've replaced the fuel filter, blown out the needle valves, replaced the condenser, cleaned and gapped the plugs, cleaned and gapped the points, replaced the distributor cap.

Nothing has improved, if anything, it is getting worse, running rough at low rpm when warmed up and some backfiring when revving it


- Ken
Ken Doris

Rotor is new- just replaced before Glen trip.
KR Doris

Just because the rotor arm is new don't assume it is OK - there are a lot of iffy ones out there - I changed everything a few weeks ago including the coil and it turned out to be the newish rotor arm in the end although backfiring sounds more like timing.
Cam Cunningham

Thanks Cam - I'll try a swap-out with the prior rotor. As for the backfire, it happens when, as the engine is dying at idle, I flutter the accelerator and it coughs and sputters before revving up. Some of those coughs are backfires out the exhaust.

- Ken
Ken Doris

Your centrifugal advance hasn't jammed, has it? ...or advance springs broken?
Dave O'Neill 2

You could try advancing and retarding you distributor to see if it makes any difference (after marking the current location first).
Mike Ellsmore

Manifold leak?
Art Pearse

Ken. Art is correct. The most common problem associated with these symptoms is a manifold leak. And, it can be easily checked without adjusting timing or making some other change to the existing system. Start up the engine and spray some carb cleaner, WD-40, or similar product along the base of the carbs and at the cylinder head where the intake manifold joins it. If the problem goes away, it is an air leak somewhere in the system. Tighten nuts and replace gaskets as necessary.

If it is necessary to check out the timing, why not just use a timing light and see what your reading is at idle? Always worked for me.

Les Bengtson

Art and Les - I thought you were on to something, as the 4 bolts holding the carbs each took a half turn or so, but after spraying WD-40 as well, a try from cold start showed the same results - barely caught, and ran on only a couple of cylinders.

I'm pretty sure the problem is fuel/air related though, as the timing hasn't changed since I set it before Watkins Glen, and the car improves as it warms up.

- Ken
Ken Doris

Ken, If you don't have electronic ignition try changing you capacitor. I had a similar issue years ago.
J Butler

Jack - I have plain old points. Changed the capacitor yesterday and checked the points. All on the ignition side seems to be fine. Getting a solid spark to all 4.

- Ken
Ken Doris

Another data point - I did a compression check just to rule out head gasket - all 4 between 145 and 150 lbs.
Ken Doris

Are the carb pistons rising and falling ok? Perhaps a bit of 'stiction' when cold.

Steve Gyles

Steve - yes, both pistons rise/fall OK. I pulled the air cleaners and checked them both with and without the dampers in place.

- Ken
Ken Doris

Have you checked the carb spindles? both clamped ok on the connect & both on idle to start.

Mog - good question - yes, just checked the spindles are tight and the idle screw is set on the front, with the back one up a bit, not touching. Seem to be the same as I set them over a year ago before setting off to the Savannah Regional. Since then (April 2013) we've done almost 7,000 miles (Savannah, Asheville, Ottawa, Watkins Glen) with no problems up to now. That is why this is so baffling/frustrating.

- Ken
Ken Doris

Check the floats in the carbs. might have developed a leaky one and is sending too much gas through the float chamber. Just an idea.
C.R. Tyrell

C. R. - yep, I had the same thought, but the floats are at the top of the level in each bowl.

- Ken
KR Doris

Try some dry gas....If the car was running fine and started running like crap its often a little water mixed with the fuel. When did you get gas last? Was it running well before your last fill? At least its a $2.00 cheap fix and if its not it you can rule out fuel on your check list and continue.

Steven Devine

Have you had a chance to check ignition timing with a timing light or turning distributor as per Barneys method for a Casual tune up?
Mike Ellsmore

sticky chokes underneath the carbs? pull choke on, then off, then reach under carbs and see if the jets are all the way home.

AJ Mail

OK guys - all good suggestions and I've checked out a bunch of them, thus far to no avail. At this point I'm closing this thread down and taking a step back - going to basics of setting timing and then setting up the carbs. I may post some specific questions along the way in separate threads, and I'll let you all know when I have it solved.

Thanks again!

- Ken
KR Doris

not something blocking the exhaust system ?
G C Pugh

This thread was discussed between 20/09/2014 and 22/09/2014

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