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MG MGB Technical - mgb misfire

hi everyone,i'm sure this problem has been on here many times before but i'm at my wits end.basically the car starts and idles fine,but when i try to drive it,it misfires and wont pull,i changed all the usual parts,points,condenser,dizzy cap,rotor arm,plugs,i've topped up the dashpots with oil.i took the fuel pump off and cleared the breather,it pumps well over 1 pint of fuel in 1 minute,also fitted a fuel filter.the car is a 76 with a recon motor from heritage which has done about 30k,i phoned heritage to get info on timing as the original engine numbers have been replaced with their own but they no longer had any info for these engines because they were only sold for a few years.so i've set the timing using haynes manual but its only best guess.vacuum advance seems ok too.also,if i start the engine then stop it after a few seconds then it wont start again,i have to let it run for a few mins,the plugs are always black with soot too,i altered the mixture but it didnt make any difference to the misfire.although it's mot'd i dont put the car on the road because its so undrivable.any ideas would be most welcome.
dd edwards

Hi, as you've done all the obvious culprits, we'll need think outside the box a little...
Whilst the soot on the plugs could be a mixture problem it's more likely to be as a result of the misfire eg if you miss a combustion stroke the next fuel charge will add to the unburnt fuel in the cylinder and as a result it will be rich.
It's also highly likely this will be an electrical issue but we realy need to know if the misfire is caused by a low or high tension fault.
Try these in order
1, replace coil
2, replace HT leads
3, remove the wire on the +ve side of the coil and replace it with a test wire from any brown cable you can get to on the fuse box. This eliminates all the 12v side of the ignition eg switch, ballast resistor, any relays etc. BUT only leave it connected for a short time (no more than a couple of minutes at a time) and remove it to stop the engine.

My guess is the coil is breaking down when off idle so hopefully it'll be fixed on 1 :o)

MGmike
PS. where are you in Northumberland as I regularly travel between Newcastle and Edinburgh and could drop in.
M McAndrew

As you say you have done everything including the points, check that the gap has not closed up. this has happened to me in the past. I had not tightened the screw enough and the points closed giving some of the symptoms you discribe.
Trevor Harvey

With one exception I wouldn't replace anything yet until you have done some diagosis, it's expensive and frustrating when it doesn't help.

Firstly, does the tach jump about when the misfire occurs?

If it does then the problem is in the ignition LT circuit - ignition switch, ballast resistance, coil, points, distributor earth wire and the connections between them.

If it doesn't then it is in the HT circuit or fuel or could be the condenser, and this is the exception. A condenser is cheap and can be clipped between the coil -ve and earth without removing the existing condenser as a quick test.

Black plugs and not restarting for a few minutes could well be very rich, but that would be more visible at idle than running, as a regular beat-type misfire. Does it rev at a standstill or cause the same problem?

If it revs OK run it and rev it hard (maybe away from the neighbours if you can) then check the plugs again. If it's rich the plugs should still be black, but not if the sooting is caused by the misfire.

If it misfires while revving then clip a timing light onto the coil lead and each plug lead and watch the flashes to see if they are erratic or regular.

Even if the points gap is correct you should check the dwell as a double-check that there isn't something else wrong.
PaulH Solihull

This thread was discussed between 23/06/2012 and 24/06/2012

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