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MG MGB Technical - Low power on starting and bad driving

I own a 1978 MGB GT who has very recently developed a strange problem when driving.

When I start her up she sounds very reluctant. The engine gives a low "mmmm" sound (good description!) and the revs are extremely low; directly giving her gas has no effect, I have to pump the pedal a bit to get the revs going.
Then when I drive her she feels like there is no power there and it's only if I give it a lot of gas do the revs suddenly rise and she becomes useable - very jerkily though.

When I accelerate, sometimes she works fine and other times she has a bit of a fit, occasionally with a bang from the exhaust before working fine.

At speed (i.e. motorways) she drives fine when the gas pedal is hard down, however if I release it a little the car either has a bit of a fit (to cure this I either release the pedal completely or push it hard down) or the rev counter drops to zero and the car sounds empty - if I give it a bit more gas then there is a little jerk and suddenly I have power.

When driving her it also feels like she is lacking a bit of power and acceleration.

Any ideas on this would be much appreciated!

Robin
Robin

See my post titled:

"Sputtering, popping, waking the neighbors - MGB"

Skip to the end if you like...
Rick Bastedo

The tachometer dropping to zero - if it does it suddenly while the engine is still spinning - or otherwise flicking around shows that the problem is in the ignition LT circuit. As Rick says skip to the end of his long tale and check the points and condenser first instead of last! However because this seems to come and go with pushing the throttle pedal it soulds like one of the wires inside the distributor is at fault. These are bent back and fore as the points plate twists under changing vacuum, which happens when you move the throttle. One is the wire coming from the coil, which looks like a standard wire but is actually more flexible than usual. Replacements for this come with new condensers. The other wire is the earth wire which is brown cloth covered and goes between the condenser screw and another screw on the body. I'm not aware of an exact replacement for these, Moss UK sell one but I don't know how flexible it is - one assumes (hopes!) sufficiently so.

If the ignition warning light is flicking on and off as well then the problem is lack of ignition voltage coming from the white on the ignition switch.

The back-firing in the exhaust also indicates it is an ignition problem, on its own without the tach jumping up and down it would indicate an HT problem.
Paul Hunt

Thanks very much for your help guys!

I gave the points contacts a cleanup, set the gap and she worked. Then adjusted the timing and gave the plugs a little clean and the problem is now completely sorted!!

Just to make sure I took her for a bit of a run - am very impressed.

Cheers!
Robin
Robin

Great - glad you didn't start out on the wrong side of the motor like I did!
Rick Bastedo

.... Still one more points problem.
Incredibly this little device may turn into a nightmare.
Rgds.
Renou

".... Still one more points problem. Incredibly this little device may turn into a nightmare."

I have to say I have never had points or condenser fail in over 40 years, nor be the cause of bad running unless it was self-inflicted. Also since using dwell instead of gap to check the points I've not had to adjust them in the 45D4 in the roadster, and only mildly tweak them in the V8 as they have a much smaller tolerance. I deliberately left the points in both for getting for 18k miles (the recommended change interval is 10k) and only changed both when the roadster started to take just a little longer to start from cold than normal, since when it has gone back to firing-up within 2 or 3 secs after standing a month or more.

Like many things, given the appropriate care and attention cars of this age can be as reliable as any.
Paul Hunt

The only times I've had problem with points were:

1. foreign matter cooking onto surface during spark.
This should be extremely rare, but it can happen. File & reset and all good.

2. normal wear & tear - after many thousands of miles. File & reset and all good.

The only times I've ever had problems with condensers were either bad out of the box or a wire / connector issue. Again extremely rare. After I rebuilt the motor in my MGC I couldn't get it to start for several days when it really should have. I finally traced the trouble to the new condenser I had installed. Swapped the old one back in and VAROOM!!
Rick Bastedo

This thread was discussed between 12/11/2008 and 15/11/2008

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