I have just purchased a 2002 18' PolarKraft aluminum bass boat with a built in 27 gal gas tank.  It came with a mid-80's Mercury 115 hp outboard.  Of course it is neither 4 stroke or oil injected.  Therefore any gas that was used is pre-mix before it goes into the tank.  The tank is almos 3/4 full.  I plan to repower the boat with a four new stroke, fuel injected outboard this winter and won't be able to run out the gas that's in the tank.  So, I will be pumping out the mixed gas.  My question is:  How do I get out the residue of the gas and oil mixture that adheres to the interior top, bottom and sides of the built in tank?  I have some 100 octane E0 gasoline available to me and am wondering if I can use five or ten gals in the tank and then do a back and forth dance with the boat hooked to my vehicle to slosh the mixture free gasoline throughout the tank and pump that out, do a refill with regular gas, and have clean enough gasoline so as not to do any harm to the new four stroke?

Any input would be appreciated.

 Fred

I wouldn't think there would be enough residue of mixed gas to be an issue once it is pumped lit.

I agree with Joe, I don't think it will hurt your new motor. Even if you added some oil, all it would do would be smoke some but the gas left in the tank will be so little, I don't think it will smoke any. Fill it up and gas it !!!!

Unless it has sat for a long periods of time where really bad gas was left in it, probably no problem.  If you are really concerned, pour you a gallon of denatured alcohol in it, shake or tow it a little peace to mix it in and then just let it sit for a couple of weeks and then tow it around again to shake everything up, then pump it out. 

Get as much out as you can and then fill it up when you get the new motor on. I really doubt it'll do anything at all.

If the old gas concerns you go get a 12 volt electric pump, plumb it onto the fitting the fuel line going to the hand primer bulb hooks to and pump it out.  The little bit left will make no difference since it will be mixed up in the new fuel you will pump in.  There are pumps as well as oil change kits (that is a bucket with a pump mounted on it) available at places like West Marine.  This will assure you that the new engine can not be damaged by the mixed fuel currently in your tank.  It would be a small investment in piece of mind.