As this B80 is in a Saracen, you need all the power you can get, and using a petrol / paraffin mix will not help, also paraffin is known to induce bore wear as it can wash oil from it. With the exhaust manifold on the opposite side to the inlet, there is little chance of it vapourising., another negative. as for slight misfire, you need to check the two sets of points are correctly adjusted and also not pitted, plugs are serviceable, etc.