MG Car Repair: Misfire After changing head gasket, head gasket, uk spec

Question
QUESTION: Hi. I have just changed head gasket on my 1975 1500 midget (twin carb, UK spec) but on starting car afterwards it has a big misfire and is running really rough. Although I did not move any of the timing I have re-timed the car but to no avail. where is the best place to start looking?

ANSWER: Hi Jon.  

Before you do anything else, could you check that all 4 spark plugs are correctly wired.   It sounds to me as though you have got two of them reversed (either cylinders 1 and 4, or 2 and 3).    

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QUESTION: Hi Barrie

I have double checked all 4 plugs are correctly wired. I have also now replaced all 4 plugs and all leads. Still have mis-fire. I have tried removing 1 lead at a time to see if there is a particular cylinder which is the problem and there is no change in the engine note when I remove lead 1. All the others result in worsening of mis-fire. Could it be distributor cap or should I try anything else first?

Thanks

Jon

ANSWER: Hi Jon
I guess I should have asked why you changed the head gasket in the first place.  
I need to know some history.  Was it mis-firing before you changed the head gasket?
Did you take the valves out and inspect them while the head was off?

Before you do anything else, beg, borrow or hire a compression tester,
take all 4 plugs are out and do a compression test.  
I would not be surprised if there was very low compression on number 1 cylinder.
If all 4 compressions are within 10 psi of each other, then the head is ok
and we can look elsewhere.

Feel free to get back to me.


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QUESTION: Hi Barrie

Thanks for your response. I changed the gasket as it had blown. I didn't remove any valves whilst it was out, and there was no mis-fire prior to changing head gasket. I did change all gaskets whilst dismantled. I will borrow a compression tester some time next week. If it shows low compression in number 1 cylinder does this mean the head is damaged and needs replacing?

Thanks  

Jon

Answer
Hi Jon.  If the gasket was leaking then usually that means the head is slightly warped and it needs skimming.  You may get away without it, but you should test it with a steel ruler to check that it is perfectly flat.  Often there is a problem with a burnt out exhaust valve, in which case you need a new valve, which must then be ground in properly.  Have you got an unleaded head?  If not, then the problem will keep happening so it would be best to get your head converted and reconditioned by a competent machine shop.  Your engine is basically a Triumph Spitfire engine, not a BMC A series unit.  They are very different, so I would approach a Triumph specialist.