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It sounds more like water in the fuel to me. I have exerpienced a similar situation many years ago. I have never actually done this, but I understand that you can pour a little bit of pure alcohol (not vodka - something like isopropylalcohol) into the fuel. This will mix with any water that is in there and allow it to be burned. I think you can also get something like this from Witworths or BIAS boating. Make sure that your water seperator is not full and if you don't have one. Get one. I consider them to be essential.
Splash you can have 2 filters, water trap one close to your motor,but not in front of your manual pump,remember to keep an eye on your filters as you do not want to load up your fuel pump trying to suck fuel through 2 dirty filters. Polky. And yes your spark seems ok. Another way to check spark and leads have motor running on idle pull 1st lead of plug motor should loose a few revs, do this to each lead seperatley, if when you take one of the leads off and the motor dose not loose revs then that plug or lead are shot, becarefull you have moving parts Polky.
Matey check to make sure your choke is not sticking, take guard and larage dust filter of front of carby block, helper start motor visual check that the butterfly valves open and then close, maybe the carbys are sucking in to much air. Polky.