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I have a 86' Johnson vro 90hp. at startup I would get a back fire then it would idle rough. Running on the water it would hesitate a little then get up and go. I was considering a foul plug, but a friend suggested that the mix could be off. Does the brand of gas matter as well as the octane? And does mercury premium 2 cycle oil work well with johnson? Should I dump the oil I have now and replace it with new oil? I want to get some quality time off of this engine. Before 7g's on a 4 stroke. Any suggestions?
I am not positive about the oils but I am a firm believer in using the brand of oil that is the same brand as your engine.
Example:
OMC engine = run OMC oil
Mercury engine = run Mercury oil
Yamaha engine = run Yamaha oil
Backfire usually means timing is off but could be other things.
Did this just start? Did you just buy the boat?
I always like to start off with NEW spark plugs.. Champion gapped at .030 for OMC eninges.
Fresh Fuel... Generally regular octane gas works just fine for the smaller HP motors.
After that, if it still is giving you problems, time to take it to a mechanic and have it tuned up.
I agree with Joe, may be timing. I would also check out you fuel pump and carbs. The hesitation can definitely be caused by a worn fuel pump. If your carbs are filled with junk that will be a problem as well. There is a good additive you pour in the gas help safely clean the system. Seafoam sold at auto supply shops. Would be a good idea to inspect all you hoses and connections. Regular gas should be fine, if its from last season do not use it.
As far as the oil I am not as finicky as Joe. I do use the mercury premium+ in my Yamaha (the only type I would use, maybe I am finicky ). Also use the Mercury lube in the lower unit. The lube allows for easy water detection, I was told that by the Yamaha dealer
Regardless of what you use make sure it good quality oil and do not mix them. There may be additives in the oil that do not mix well.
Good luck
Jon in NJ
Many other boats and outboards in my boating history
The Whaler is the one I like the best!!!
). Also use the Mercury lube in the lower unit. The lube allows for easy water detection, I was told that by the Yamaha dealer 
