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MG MGB Technical - Mgb misfire

Hi all. I've got an issue with misfiring. The first journey in the car is faultless runs beautifully. This is after giving the engine time to warm up. Upon the second journey, with considerable time to cool down, misfires like hell with a few exhaust pops. I have to pull over and let the car idle for a period of time which normally eradicates the problem. The engine also misfires if I give it a few revs whilst being stationary. The other day I had taken the car to my local garage to borrow his lift to change brake pads, steering gaitor etc. That was at 7am and ran fine. The car was off and didn't move until 2:30pm and misfired until I got stopped by rush hour traffic. 30-40 minutes stuck in the traffic was enough time for the fault to sort itself out. I'm scratching my head with this one as to why. I've never drove the car from cold on first journey as it misfires a touch then but 10-15 mins warm up and it runs like a dream. Even if the engine is still slightly warm after the first journey I still have to go through the same procedure to cure the misfire. Any ideas what could be the problem???

M Pounder

car, year, ignition, coil, recent history / work / maintenance ???
Roger Walker

A vacuum leak that closes when the engine is hot?

H J Adler

Your gas cap may not be allowing air in to replace the fuel that's becoming consumed. RAY
rjm RAY

The car is an1973 mgb gt. Coil looks fairly new, was thinking maybe fitting electronic ignition would cure the problem. I've already tried with the fuel filler cap off and does the same thing.
M Pounder

Don't assume, "Coil looks fairly new". There is a lot of really bad "new" product out there.
Intermittent problems are often difficult to diagnose without seeing/hearing. Whether temp, time, demand is affecting it can be misleading as your "solutions" might just be coincidence.
Backfiring can be a weak mixture problem as well as ignition. i.e., manifold air leaks, insufficient fuel supply, sticking SU pistons or rubbish in fuel lines, etc..
Check fuel delivery quantity, in excess of a pint a minute I think, do when the misfiring occurs if possible. Also look to see if there is any air bubbles in the fuel. You can often hear manifold leaks.....they whistle!
Take the dampers out of the carbs, filters off and check that the pistons will lift to the top easily and fall quickly, in unison, with a clunk as the buffers hits the venturi.
Ignition woes are many and varied. Does the Tacho start misbehaving when the misfires occur? This is the usual indicated of Lucas problems!
If so just go through the system swapping and setting but check all the wiring and terminals. Spades can look ok until you push back the cover and find one surviving strand of wire! The earth wire on the dizzy plates is a prize one for failing, now and then, as is the condenser.
Allan Reeling

If the tach jumps about then it's an ignition LT problem i.e. from the ignition switch to the fusebox and the coil on white wires, through the coil to the points (white/black), then through them and the distributor earth wire.

If the tach is steady then it's HT, fuel or condenser. Condenser is easiest to check although they usually fail and stay failed. However new ones are often faulty of fail very soon, so it's better to use one as a test that you know works. Temporarily connect it between the white/black wire at the coil and earth.

Do you use the choke? Letting the engine warm up y idling is not good for it, you should be able to drive off straight away. You have to learn how much yours needs, both mine need full choke to start but then the 73 roadster with HS carbs can be pushed back half-way immediately, then progressively the rest of the way as it warms while driving. The 75 V8 with HIF carbs needs full choke for a couple of seconds, and only progressively pushed back from there.

The fuel delivery check described should deliver a minimum of one Imperial pint per minute and in practice double that, in a continuous series of pulses, with minimal bubbles.

HT is probably the hardest to deal with unless you can reproduce it with your head under the bonnet. My V8 would fail to start unless I changed the plugs, even though they had little mileage on them. I always had to be somewhere so never had time to fully diagnose it, until one day when by clipping a timing light onto the coil lead and plug leads I discovered no HT was reaching the plugs. The cap was breaking down, but only on cold starts, as the voltage needed then is slightly higher than normal running. Could also be rotor breaking down, or coil lead. A non-starter is easier to diagnose than a misfire wioth a timing light, but even then if you watch the flashes when the misfire occurs you should be able to see if the flashes become irregular at the same time or not. HT lead problems can often be seen by reproducing the symptoms in the dark, and looking for flashes.

Fitting electronic ignition now without diagnosing the problem may simply disturb it, for it to arise again later.
Paul Hunt

This thread was discussed between 18/10/2015 and 19/10/2015

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