Guys
My friend John Merton , a lurker here, sent me the following tips from a very similar problem that he experienced.
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I had a similar problem which took me years to sort out.
Basically, with two spark plugs not firing on one side he should check whether or not the alternatives on the other bank are not firing as well. This is because, if it's a carburettor model like mine, each carb feeds two on one side and the alternative two on the other and there is no mixture balancing between them. He should pull a lead from one of the dud cylinders and check with engine idling whether there is any spark. If there isn't the source of the problem is obvious, if there is, it's a carburettor problem irrespective of how damp the plugs look.
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I hope that helps you
