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MG MGB Technical - Not revving properly

I've just bought a 1973 BGT, it is standard except for 3into1 exhaust manifold, single box exhaust and pancake air filters.

It has done very little mileage in the last few years, but it starts and runs well.

However, it doesn't seem to want to rev very high, it gets to 4000rpm in any gear and starts to missfire and hesitate as if suffering from petrol starvation. I have fitted a replacement fuel filter, which is showing clean fuel. I topped up the su dashpots which has helped a bit.

It also "runs on" when turned off. Any ideas as I intend doing some long journeys soon in the car and want it to run well.

Thanks
John
John Collins

Hi John, and welcome.

I would start with a thorough service, with particular attention to the distributor and the rest of the ignition. If it has standard ignition and the dwell is wrong (basically the points gap) it could cause this prob.

Does the plug colour suggest a weak mix ?. If the air filters were changed for more free-flowing ones, new (richer) needles probably should have been fitted, but were they ?.

Do you get a hiss when removing the filler cap just after running the engine, suggesting a blocked tank breather ?.

Running on is quite common, does the front number plate obscure the lower air intakes ?.

Don
Don

Hi John
I've just had exactly the same problems with a B I've bought for my wife. In addition to the good advice from Don, check out the fuel pump. The one on her car ticked away quite happily and supplied plenty of fuel into a bucket, but when I ran out of ideas and took the pump apart I discovered the points were almost burnt away. A new pump completely cleared the problem, so it obviously wasn't supplying enough fuel at high revs.

Only other thing that it might be is that the valve timing's wrong, but the engine would have to have been rebuilt for someone to have got that wrong......

Let us know if you need any more help - there's no reason the B shouldn't be good fun on long runs......

Cheers,
John
John Marr

I'd be for eliminating the ignition on this one as well. These components can be the first to break down under high revs so I would replace dist cap, rotor, points, condenser, leads and spark plugs. Set the points at .015" and the timing at 12 degrees btdc at 600rpm. Make sure also that the valve clearances are set correctly at .015".

If this does not cure the problem then its into checking carb needles and settings etc.

Iain MacKintosh

When I got my car, it had been fitted with an electronic fuel pump and a pressure regulator. The problem was the fuel pressure was set too low, and I was overrunning the fuel supply.
Larry Bailey

Hi all.

John Marr raises an interesting point (no pun intended).

The points on the standard SU pump do last reasonably well (as long as the points are good quality), but I believe that they should be regularly inspected (say every 20k miles) and replaced if necessary.

It isn't a particularly fiddly job if done off the car, my favourite method for this kind of thing is to have a spare unit, replace the original with the spare and re-furbish the original one at your leisure for next time (nice job for a cold wet winter's day).

Don
Don

I've also seen the situation wher the pump will deliver quite happily on a continuous basis into a bucket but could show symptoms of being slow off the mark to start up. This being the case whilst delivery was seemingly OK it could have difficulty keeping the float chambers full just because it did not respond quick enough. LIke you however I still favour ensuring that the ignition is 100% before we go down the thorny fuelling issues.
Iain MacKintosh

Always go though a full setup of ignition and carbs (in that order - 90% of SU carb problems are due to Lucas ignition) before attempting to track down any problems of this nature. A standard SU fuel pump should deliver *at least* one pint per minute into a container at the carbs, and this is enough for a V8. There should be minimal air bubbles in the fuel.
Paul Hunt 2

John,
Had the same problem with our 79 LE. I found that the points gap in the distributer was too wide, when I narrowed the gap to 37 mm the car revved out to over 5000rpm, in the lower gears, problem solved. Sad to say It was me who set the points previously. Lesson learned.
Kim R.
K Rutherford

Many thanks to all who posted.

The ignition timing was way out!

New plugs, points, condensor, rotor arm, dizzy cap & leads were changed as well,

Much, much better, runs great and run on problem seems reduced too.

Thanks
John
John Collins

This thread was discussed between 19/08/2005 and 23/08/2005

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