MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

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 MGB Technical - Spitting and sputtering at high RPM

Hi,
I have a 1971 MGB GT. Recently it has had a really hard time at high RPMs. Whenever I accelerate or go up a hill it the engine sputters and skips a lot. It idles fine. The plug wires are OK and it has fairly new plugs. I don't have a lot of experience with cars so I'm not really sure what I should look at. Any suggestions are appreciated. Thanks a lot!
-Lars
LM Lars

Because this is happening under engine load, it almost has to be the ignition system. Could be as simple as moisture in the distributor cap, corroded terminals, or worn and/or badly-adjusted points. It's the "load" portion that really stresses the ignition. Despite being faily new, you could have a weak spark plug, or even one plug wire that is breaking down.

This is what I would do: Never assume that anything is good. Start with the simple things - wipe out the distributor cap (give it a shot of WD40 and wipe - doesn't have to be completely dry of this). Look for cracks or evidence of carbon tracking inside the cap - are the posts carboned-up (some is OK)? Make sure the carbon button is undamaged and still in the cap. Examine the rotor for cracks and damage. Check the points for worn or pitted contacts and make sure the gap is within a thousandth or so of specs. Any broken insulation on the wire going through the distributor body to the points? Is the bare ground wire in good shape and could spark jump to it from the points or rotor? Is the screw on the condenser tight? Are the plug wires inserted fully into the distributor cap and are the boots pushed all the way down - same for the plug ends and both ends of the coil wire.

Pull the plugs - Gaps OK? Is one fouled more than the others?

There are other things, but get this done first.

Wayne
Wayne Pearson

Sounds like you are running out of gas. A plugged fuel filter inside the gas tank pick up line. All MGs , TDs, MGAs, MGB, MGCs came with an intergral fine mesh fuel filter inside the tank and they are impossible to get to without cutting a hole in the tank. Looking at the bottom of your tank you will see a depression about the size of a quarter. The filter is is located above the depression. I have removed two plugged ones so far. Remove the fuel line from the carb stick it in a quart container and see what kind of fuel flow you are getting. If you have good flow then it is fine. If the pump struggles to pump then the filter is plugged or the fuel line is plugged. You may get by with boiling the tank but it will need coating afterwards and the coating may plug the filter also. A 3/4" hole saw will do the trick also. A word about SAFETY. When working on fuel tanks FILL THEM WITH WATER 100%, CLEANED TANKS OR NASTY TANKS so they don't go BANG on you. It may be the LAST thing you ever do. After removal the hole has to be patched. Been there done that.

Ermit

r starnes

They've got it - fuel or spark. Keep trying things until you have it.

I'd start with new points and condensor just because that is cheap and easy.
Bill Spohn

The other thing to check easily if you have SU carbs is that the dashpot pistons have oil in them maybe?

Simon
Simon Jansen

"They've got it - fuel or spark."

Well, it couldn't really be much else ... :o)

Did this suddenly start? Was anything done to the car before hand?

Lack of dashpot oil would cause stumbling during *acceleration*, it wouldn't be rev dependant. Mind you, HT breaking down under load would do the same.

If, at high revs, you can cause the stumbling to stop and start by applying less or more throttle that could be fuel starvation as has been said, with a carb pipe directed into a container and the ignition on it should deliver *at least* one Imperial pint per minute, and in practice closer to two, in a steady stream of pulses with minimal bubbles.

It could also be a points phasing issue where vacuum advance is moving the rotor away from the cap contact when the HT fires, you could disconnect the vacuum and plug the source as a test.

If it happens regardless of throttle position it could also be HT like some issue with centrifugal advance, I've known of problems like this where the condenser screw was too long and the weights were hitting it.
Paul Hunt 2010

I have a similar problem with my GT, usually when pulling up-hill from cold. Try easing off the throttle as you could be flooding the engine, also do you stiil have choke lever out also increasing the fuel supply.

Try the same hill climb with the engine nicely warmed up and see if there is a difference! A simple exercise can sometimes saves you an unnessary headache.
si robathan

The new contact points finally came into my shop yesterday, I replaced the plugs and points and adjusted the timing. The problem is fixed! It was the points because I tried the plugs and there was no improvement. Now it runs and accelerates like a champ. Thanks for all the advice and help!
-Lars
LM Lars

"usually when pulling up-hill from cold. Try easing off the throttle as you could be flooding the engine"

More likely insufficient choke if it stumbles under load when cold. If over choked/too rich it would run badly all the time.
Paul Hunt 2010

This thread was discussed between 22/06/2010 and 28/06/2010

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now