97 770 tigershark pre-mix

so yesterday i filled up my tank i think its a 12 or 13 gal tank so had around 2 gals of pre mix gas in it 50 to 1 so i added 10.5 gals to fill it up... now today i went to go and use the jetski and my motor has no power at all so i thought it was a gas problem like gas pump was gone or carbs was cloged but its all working fine/cleaned then itjust died on me and did not even wanted to start or try to start.. so my ? is now that i know its my spark plugs cuz i just got some new ones and it started right up, am i doing the right pre-mix? tigershark takes 50 to 1 pre mix so i have a pre mix cup and i first put the oil in the cup to how many gals i want like yesterday i wanted a fill up so i filled the cup of oil wich came up to 10.5 gals so then i just mix the oil with 10.5 gals of gas is that right or no? if it is right why on gods green earth am i keep trashing my plugs??? plus should i just put my oil pump back on or just keep useing premix and does anyone know what type of oil tigershark use cuz i am useing penzoil 2 stroke syn oil is that ok??? sorry for the long post