90 HP Mercury Oil, mix in gas or separate tank?

[lmccollum4]

Hi, I have inherited a 90 hp Mercury, it is a 1978 model outboard that runs great. I am familar inboards, 4 cycle motors, but not outboards. The problem is my father inlaw once said you don't have to mix the oil in the gas on this motor, but if you don't put oil in the gas, where does it go? I think he implied there was a tank somewhere just for the oil and the oil mixed with the gas. After looking around on the boat, the only tank is a gas tank unless there is a tank built into the engine itself. I took the cover off the engine on one side the gas line comes in, connects to somekind of a black box and then goes up to the carbs. Also connected to the black box is a small hose that goes over to what appears to be the engine block. Any ideas or should I mix oil in the gas?

 

[andrewkafp]

Re: 90 HP Mercury Oil, mix in gas or separate tank?

You may have a autoblend (Oil injected) motor, but 1978 seems a bit old for that. Make DEAD sure that you have this system or not before starting it. The tank that contains oil should have a clear area or an indictor somewhere on it to show the level.If it turns out to be a pre-mix (most common format) mix the marine grade oil 50:1 no more and no less. A photo with the cover off would be handy

 

[ZmOz]

Re: 90 HP Mercury Oil, mix in gas or separate tank?

Yeah...I don't think a 78 would have an automatic oiler. If it did there should be a pretty obvious oil tank to fill up. Wether or not you have the automatic oiler, you can still premix, and just leave the oil tank empty. 40:1 is much better for premix...

 

[lmccollum4]

Re: 90 HP Mercury Oil, mix in gas or separate tank?

After looking at the engine some more, I am conviced it does not have a separate oil tank. What my father inlaw thought I am not sure. I am going to premix the oil in the gas. This boat has a nicely built aluminum tank that fits at the back of the boat, holds probably 15 gallons or more.

 

[DJ]

Re: 90 HP Mercury Oil, mix in gas or separate tank?

No oil injection on a 78 Merc.

 

[monolithic]

Re: 90 HP Mercury Oil, mix in gas or separate tank?

Mix it an a 5 gallon tank and then dump it in , 50;1 good oil

 

[JasonJ]

Re: 90 HP Mercury Oil, mix in gas or separate tank?

Run 50:1, no more, no less. Running 40:1 does nothing except waste money, inhibit combustion, and skank your plugs up. Even permagap plugs get fouled. Good luck...

 

[ZmOz]

Re: 90 HP Mercury Oil, mix in gas or separate tank?

Yes, using extra oil can foul plugs and make your motor run worse, but using something as lean as 40:1 will NOT have such an effect. If we were talking about 25:1 that would be one thing, but we're not. You should clean your plugs and decarb your motor frequently anyway...

 

[lmccollum4]

Re: 90 HP Mercury Oil, mix in gas or separate tank?

Thanks to all for your help! I know what to do now.

 

[JasonJ]

Re: 90 HP Mercury Oil, mix in gas or separate tank?

Z, lord knows no one blames you for being cautious, burning up a motor tends to do that to a person. You may be best off just making your suggestion and be happy with how you do things. Not everyone agrees with everything everyone does, so maybe burning less energy trying to convert everyone to the ways of 40:1 is the way to go. I gave up trying to get everyone to do it my way a long time ago, and I am so much more at peace now. Take care and have a ton o' fun with that 1400 when you set it up....

 

[ZmOz]

Re: 90 HP Mercury Oil, mix in gas or separate tank?

I really don't care what ratio people use in their motor, but I will continue to suggest 40:1. What pisses me off is when people like you try to say that it's a bad idea, because it isn't. Again, you can use 50:1 all you want, but I'm going to continue to get pissed off when someone specifically say that 40:1 is a bad idea.