I was driving home last night when I lost power and started getting a knocking sound.Couldn't get above 50 and barely made it home.
Checked it out today and the knocking sound is coming from the back of the engine and if I pull the plugs on 3 and 4 it makes no difference to the way the car runs.Spark is there and no oil in the radiator water.
Possible blown head gasket ?

95% probability.. YES
Put your thumb over each of these 2 plug holes in turn and crank the motor over. No compression on both these cylinders, it's guaranteed 100% a head gasket.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

could also be a stuck valve or burnt valve, easily checked with the rocker cover off to see if any of the valves are sititng lower on thsoe 2 cylinders.
but yes can be 90% head gasket i would say.
sorry doc had to allow 10% for crappy fuel and burnt seats etc.

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Heads off and it is indeed a blown gasket.

http://www.users.bigpond.net.au/drekkus/gasket.html

atleast it you did it well it had a meal of that gasket

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there is a lot of black around the gasket.
I would not use a copper gasket in the new one i would go for the ACL black gasket for the future after the head has been checked etc.

well done for finding it.

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