MGB not running right

[ate1980]

Hi,

I have a '79 mgb gt. After being off the road for around 8 years, I replaced all the ignition parts, cleaned the carbs and replaced the pump. Evenually got it running well and it passed an MOT in May with flying colours. It ran and started well. I did the Brighton run and all went well, other than the homeward bound journery were it lacked power and was lumpy.

Since then its done a handfull of short journeys and the perfomance it getting progressively worse. It starts very very well, runs well while my foot is gently on the pedal, but as soon as i press it for acceleration it goes very lumpy, lacks power and makes a phurt phurt phurt noise until I ease back and then its fine. All this is fine if I want to drive at 25 mph or slowly build up to a speed but of course this makes most journeys impossible.

Any thoughts of where to start ?

Thanks
Andrew

[JPB]

Compression testing would be my first port of call, if that shows no obvious lack of compression - for example as a result of valve seat recession - then check fuel delivery (has the tank been run low at some point?) and ignition system.
Although you've checked the ignition system already, it's quite common to find that modern replacement condensers are leaky since most are of shockingly poor quality now and a coil that's breaking down could also display similar symptoms to those you're experiencing.

Bs are uncomplicated things so by being methodical you will get to the bottom of this pretty soon, IMHO.

[suffolkpete]

Sounds a bit like a weak mixture to me, so I would also check the inlet manifold gasket and brake servo hose, and also make sure the carb pistions are moving freely. If you didn't centre the jets when you cleaned the carbs, this could be the cause of your problem.

[rich.]

i was running a 77 gt & i couldnt get it to run right, i spent a fortune on carbs ignition etc. finally the rac diagnosed a faulty resistor in the wiring loom for the coil. if it still has the origiinal loom it might pay to get it checked.

[ate1980]

Thanks for the comments so far.

Spent some time checking this evening,
- plug gaps correct. Two were dry black (1 and 3) and two were wet black (2 and 4).
- tappets were good, one sligtly tight. Now adjusted.

I then started it and found the idle uneven. It races for a split second then slows for 2 seconds or so. As I press the accelerator it hesitates and stutters.

Only other thing I noticed was a blue capacitor looking thing bolted to the bodywork right next to the coil has a broken black lead coming off it ?

thanks all
Andy

[JPB]

Unless someone fitted an external replacement for the ignition condenser, that'll be a suppression cap for radio interference reducing purposes.

[Talking Hoarse]

How about petrol? If the car has sat for so long have you replaced the old petrol and flushed the crud out of the tank?

[rich.]

Unless someone fitted an external replacement for the ignition condenser, that'll be a suppression cap for radio interference reducing purposes.

sounds like what was fitted to my b to get it running right..

[JPB]

Unless someone fitted an external replacement for the ignition condenser, that'll be a suppression cap for radio interference reducing purposes.

sounds like what was fitted to my b to get it running right..

Rich, you big old cowboy!

I've done that with many old cars, reason usually being that dedicated ignition parts are more expensive and less likely to be from fresh stock than a suitable cap bought from a radio spares specialist in boxes of 100 or so at a fraction of the cost per unit.

I'm known for being ridiculously generous and the above shouldn't be taken as an admission to having a "frugal" nature. Not even a little.

[ate1980]

Success.

This was a lesson in not overlooking the obvious. Although the points and condenser had only done 200 miles a closer inspection spotted heavily detoriorated points. Replaced both and the car runs well!

Thanks all.