MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - engine wont run when hot

Every time I go out in my 1976 MGB and the engine gets hot . it starts to run very badly and then cuts out all together . If I leave it for about 10 - 15 mins then it starts again as though nothing has happened and that usually gets me home again, bearing in mind its never been more than three mile away. . every ignition par has been replaced the head has been skimmed and the valves ground in , do you have any ideas thankyou Trev Walton.
trevwalton

Have you replaced the coil ? Are you running points or electronic ignition? A little more info would help...
Gil Price

yes gil the coil has been replaced along with all the other ignition parts. It is running on points. I was thinking more on the lines of a fuel problem trev
trevwalton

Trev
Is your carb heat shield in good nick?
Dave
D M Tetlow

What happens when you open the fuel filler cap?
Stan Best

You need to diagnose this from first principle. The first thing with any stopper is to look at the tach and ignition warning light when you lose power, but before you have come to a halt, or even knocked it out of gear or dipped the clutch, i.e. when the forward momentum of the car is still spinning the engine.

If the tach drops suddenly and completely thne instant you lose power, it is an ignition LT problem i.e. ignition supply, coil primary, points, or connections between them. If the ignition warning light is glowing as well then the ignition suupply has been lost.

But if the tach continues to register as the car slows down with no power, then it is either HT or fuel. All this without the car having come to a halt, never mind getting out of the seat and opening the bonnet.

I agree that with a stopper after a few miles, removing the fuel cap is a good next step. If you get a big 'gasp' then the cap vent is blocked or an unvented cap has been fitted, and the vacuum in the tank is starving the engine, especially if the engine then restarts straight away. But note that even a working cap will cause a slight vacuum in the tank, which will result in a slight 'sigh' when removing the cap.

Next, and also with a non-starter, clip a timing light onto the coil lead and each plug lead and watch it while cranking. No or erratic flashes on the coil lead is probably a coil secondary problem, but this is rare. If that's OK then no or erratic flashes on the plug leads is the rotor or distributor cap breaking down. If you get regular flashes on the plug leads then while on 1 and 4 point it at the crank pulley and check the timing is a few degrees BTDC. If all that is OK then it is probably fuel.

If leaving it switched off for a few minutes, then switching it on again results in a lot of chattering from the fuel pump, then the likely cause is a problem with the pump or the 12v and earth supply to it. But if it only clicks once or twice, and the engine starts, it wasn't the pump or supply to the float chambers.

After cranking a bit with no start remove a plug and look at it and sniff it. Wet (more likely with a cold non-starter) means it is flooded, which will probably happen anyway if there is a non-fuel cause of it not starting. Cranking with the choke closed and the throttle wide open should allow a flooded engine to catch, but be ready to release the throttle and half-pull the choke. If the plugs are dry with no fuel smell ther is probably no fuel getting through, especially on a cold engine. Normally the plugs should have a strong fuel smell but not be wet.

Problems with the heat shield after such a short time and especially in the UK in winter are unlikely. Fuel vapourisation is often mentioned as a likely cause of problems in MGBs, but is very rare. About the only time it can happen is with a missing heatshield, or a metal one with missing insulation pads, and HS carbs. Vaporisation in the jet pipes of HS carbs is the only place it can happen, and only then with a faulty heat shield *and* high ambient temperatures. These cars run in desert states without vapourisation problems.
Paul Hunt 2

This thread was discussed between 28/02/2008 and 29/02/2008

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.