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MG MGB Technical - Misbehaving B

Since getting my 1978 MGB back on the road after winter it has started to misbehave

It will drive fine until it gets hot after a few miles and will first start to cough and lose power just as I increase the revs after pulling away from a stop or when slowing for tight corners etc. It will at first start to clear when I increase the revs further, but it rapidly gets worse until it won't pull away at all without cutting out. It will generally start up straight away, but as soon as I put any load on it will cut out again.

I have replaced the points, condensor and the carb dash pot oil is ok. When it has cut out there is still fuel in the clear petrol filter under the bonnet and when I disconnected the fuel pipe from the carbs it seems to be pumping through ok. Also there is no "gasp" when removing the fuel cap so I assume thats ok and starting the engine without the cap on doesn't make any difference. Spark plugs cleaned and regapped - colour looks ok. All fuel pipes, leads etc were renewed less than a 1000 miles ago when the car was restored.

The first time I started the car after winter fuel ran out of the front carb overflow pipe, so there was probably some dirt in the carb (recon carbs less than 1000 miles ago) but flushing the carb through with fuel stopped it and its not happened since - don't know whether this is anything to do with the current problem?

Last year I also had an intermittant problem with the indicators, heater and Tach stopping working whilst I was driving along. I understand these are all supplied from the same power source, but indicators and tach seem ok at the moment.

I would very much appreciate your help with this problem. Thanks.
Debbie
D Callaghan

Sounds daft, but try wiggling the ignition key - I had a misfire which was impossible to locate, turned out to be the ignition switch...
N
Neil22

I would wonder if there still may be some dirt or varnish in the carbs causing the symptoms you describe. Maybe a poor sealing needle valve leading to extremely rich mixture. Plugs have been cleaned, but have they been checked for fouling since the onset of symptoms? Maybe needle valve is sticking and depriving fuel to one carb? This fits with the fact that the engine runs good cold (when extra mixture is helpful) but worse when hot. It may be worth pulling the carbs off and checking out the float chambers (HIFs I would guess?). If HSs, may not need to remove carbs to do this.
When the engine is sputtering, is the tach needle bouncing wildly, or simply registering engine revs accurately? If bouncing alot, it would suggest a low-tension ignition problem. Sounds like thats been taken care of though.

Hope this helps,

Erick
Erick Vesterback

Failing when hot is a common trait of a bad ignition coil. It may be the condenser even though it's new specially if it will idle but not accelerate.
Last years electrical problems may be this years problem. Double check the wiring connections, pull apart and clean the bullit connectors if necessary, check all the ground connections.
John H

First thing to do is look at the tach when it starts misbehaving. Is this flicking all over the place? Or only changing as the engine revs change?

If the former then it is an ignition LT problem, if not an ignition HT or fuel problem.

In the tach *is* flicking all over the place, then look at the ignition warning light as well. Is this flickering on? If so it is a loss of power to the ignition system, which could be the ignition switch or ignition relay. In this case you will lose the indicators, heater fan etc. at the same time.

A single fuse feeds the tach and other gauges but not the engine, so you could lose these and the engine will keep going if the problem is to do with the fuse or its circuits.

Once it starts happening, iuf you can reproduce it with ypour head under the bonnet revving the engine by pulling on the accellerator cable, clip a timing light onto the coil lead and each plug lead in turn. If the flashing on the coil lead remains regular and consistent but that on the plug leads gets intermittent as it misfires, then the distributor cap or rotor is probably breaking down. If the coil lead also gets intermittent then it points to the coil. Remember the coil lead flashes at four times the rate of any plug lead.

If the plug leads flash consistently when the misfire occurs it could be fuel. You have already eliminated the filler cap it seems, so turn off the ignition when it starts happening, bring the car to a halt, then turn on the ignition and listen to the fuel pump. If it chatters away rather makes a single click or no click at all then it is a pump problem. If no click then carefully remove a feed pipe from a carb making sure any spurt of petrol doesn't get onto the hot exhaust. If there isn't a spurt of fuel then again it is a pump problem. If there is then it's possibly a carb problem where one of the float valves has stuck shut and its float chamber is faulty.

If fuel is dropping on the ground from an overflow pipe now when the ignition is on then it is a float valve not closing, although in my experience that doesn't affect running as much as you seem to be experiencing. It could also be a float valve stuck closed, to check for that remove the air cleaners, blow gently into each carb vent/overflow pipe in turn, and you should see fuel bubbling up from the jet. That indicates the float chambers are full which they should be.
Paul Hunt

This thread was discussed between 05/06/2008 and 06/06/2008

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