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MG MGB Technical - power loss

My mgb gt has a problem with power loss. When engine starts from cold it runs great, you can give it full throttle and it shifts pretty quick. But after about 15 mins constant driving when the engine reaches proper running temperature giving the car lots of throttle when changing gear or pulling away from a corner say, the engine just seems to die. It doesnt splutter or cough, just as if ther no power there at all. But if you just give the engine slight throttle it seems to run fine. And the occasional time the engine cuts out if you stop and you have to wait about 20 mins for the engine to cool down before it will start again.

Any help will be greatly appreciated.

Thanks.
steven macleod

Hi Steven,
Maybe the mixture is somewhat rich. It would be able to tolerate the rich mixture when it is cold, but not so well when it is warmed up. When it stops, it may be too rich also, causing it to "flood out". After a period of time it would start again if that were the case. Remove the spark plugs and note the colour. If they are black and sooty it is a rich mixture causing it. They should be a tan colour. Also the idle will sound as if it is hunting for its speed, speeding up and slowing down slightly in a quick rhythmic cycle. If that is the case, I always like to check the air filters first, and adjust the tappet clearances before playing with the carb mixtures. It minimizes the chance that you will be "going in circles."
By the way, where is the Isle of Lewis?

Good Luck,
Ralph
Ralph

Hi Ralph, hope you read this. Well the engine is running a bit rich, but i didnt think that would cause it. The isle of lewis is the most northernly island on the west coast of Scotland. Its quite a remote place but very nice to live in.

Thanks very much for your help.

Steven
steven

Steven.
Please contact me at esane@msn.com
My maternal grandmother was from south Galson Stornoway. Macay/Macleod.My wife and I have been there.
Sandy Sanders
conrad sanders

Steven,

I would also suspect the ignition coil, and check to see that the points (if you still have them) are opening far enough. A weak coil often shows problems as the temp increases, and can also have the characteristic of being able to take a little throttle, but not a lot.

Charley
C R Huff

All of the above is possible, but check that the carb pistons are free to rise and fall, and that the piston hits the bridge with a clunk when it falls. It's best if you remove the air filters and lift the pistons with your finger with the dampers removed. They should also take the same time to fall! Also check that the dash pots have oil. The oil damped pistons are what prevents the mixture going too lean when you floor the throttle.
Allan

Oh, yes Allan; oil in the dampers. How could we have all missed that? So obvious I guess we overlooked it.

Charley
C R Huff

I don't know what year your GT is or what carbs you have on it. On my GT with HS4 carbs I had one choke jet that wanted to stick in the down position and that created a problem that was similar to yours.

-Jared
Jared Snider

Steven..
What's the ;latest on your car power loss problem?
Sandy
Sandy Sanders

Had the same problem with an MGA many years ago. Check the fuel flow from your SU pump. Filter "screens" are fitted to the inlet and some fouling may starve it for fuel. Another point to check is venting of the fuel system (you don't mention the year of your car). If the tank can't replace the fuel drawn out with air, it creates a vacuum that reduces or totally stops the flow of fuel. I'd have to look it up, but somewhere in the early 70s, the gas caps changed from vented to unvented with ventilation coming to the tank through the carbon cannister (1973?). Did you replace your gas tank cap recently?
Rick Penland

UK cars (always assuming being in the UK Steven has a UK car) never had the charcoal canister which meant they kept the vented cap right through, so a valid point if an unvented cap has been fitted. If you remove the filler cap when this problem starts happening and you get a large gasp, then it could well be the cap. However even vented caps have a spring-loaded valve and will give a small gasp immediately after running, so you have to be able to distinguish between large and small gasps!

Any tank or pump problem would be revealed by doing a delivery check, remove a pipe from a carb and direct it into a container (watch out for a spurt if the ignition has been on recently). Turning on the ignition should deliver at least one Imperial pint per minute and in practice closer to two, in a steady stream of pulses with negligible bubbles.
Paul Hunt

This thread was discussed between 19/04/2009 and 13/05/2009

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