Engine misfiring

hannah00

Hi,

I have an MGB GT which was off the road for the last few months. When I started using it again it has started to misfire. It gets really bad if I get stuck in traffic but sort of clears itself up once I park it up for an hour or so. Does anobody know what is causing this & how it can be put right. I am guessing that the carbs need to be balanced, does this sound right?

Thanks in advance for your help

Dades

Could be something as simple as cleaning your spark plugs. And they are usually a good indicator of how your fuel system is functioning.

More experienced people here can help you better I'm sure...

Henry Ford III

Could be a lot of things.

Plugs, points, timing, or faulty HT leads. A dodgy coil. Dirt in fuel system. Out of tune carbs..........

hannah00

I have taken out the plugs before & they were black & sooty. A "mechanic" said that he tuned the engine & balanced the carbs & fitted new plugs but I took out the new plugs last night after only 1 week & they were all sooty again.

Spit62500

You'd need to make sure that you check the plugs after the engine has been warmed up properly - they can often get a bit sooty until the engine warms up.

You might want to check to see if the choke is releasing properly - maybe the mechanism has got a bit sticky and you're running on part choke. If it was running fine after the service and before being off the road then this is the most likely possibility

If its not the choke and they are sooty after a run then the mixture is too rich - you can adjust this yourself if you're handy - there's a nut at the top of the jet underneath each carb - you turn it anti-clockwise to raise the jet and lean out the mixture and clockwise to lower the jet and richen the mixture. If you adjust the nut by one flat (ie turn it enough to move the flat part of the side of the nut to its next position, if you know what I mean). Run it for a while and then repeat until you just get a clean plug. Remember to check the correct pair of plugs for each carb.

You should really have the ignition system in good working order first - carbs are the last thing to adjust...

There are other possibilities - incorrect plugs, worn engine etc

corktina

sounds like insufficent air (ie too rich) fouling the plugs but if its cutting out as you describe, I would suspect an over-heating coil.Could be loads of things though...best to methodiaclly work through every element starting with a good service, making sure the plugs are correct type and the gaps are correct.

hannah00

Thanks for your help. I will try to lean the mixture & see where it gets me.

franksm

I'd pop the spark plugs out and see if any particular one of them is a different colour than the others, or is wetter with oil or petrol.

If two plugs are darker than the other two, it's a carburretor problem (each of the two carbs feeds two cylinders). If it's one particular plug, then try replacing that plug - maybe it's just bad.

If all the plugs are the same colour, then your ought to test the coil (resistance check) or check the carb mixture (colourtune is perfect for this).

Could also be fuel pump (fit a clear-glass in-line fuel filter, and see if it's bubbling or whatever).

If in doubt, I tend to replace the plugs, fuel-filter, distributor cap & rotor arm as a matter of course. 30 quid well spent

hannah00

Thanks Franksm. I will try that.

hannah00

Just a quick update. I changed the plugs, fuel-filter, distributor cap & rotor arm & it has sorted the problem. Thanks for your help.