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I have a 2002 40 HP Mercury on my 13 footer. The person I bought it from suggests that I run it with oil and fuel. However, from what I can tell the engine already mixes the oil and fuel since there is an oil tank in it.

This is my engine model. https://www.crowleymarine.com/mercury-outboard/3021.cfm?mdl=9XNQCV

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I would make sure the oil injection is working properly.

To do that yourself, fill the oil tank and run the engine for awhile and then make sure the oil level is going down.

If the oil level does go down, then you should be able to run straight fuel instead of the Fuel/Oil mix.

If not, then someone may have disconnected the Oil pump system.

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If you really want to be safe, forget about the oil injection system and add oil to each tank full at a 50:1 ratio. Be sure the oil tank is empty.

If you TRY the oil system and it doesn't work, you'll destroy the engine the first time out.

The engine is almost 20 years old and the injection systems weren't 100% reliable even back in the day. That said, some will advise you differently. Are ya feelin lucky?

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Use premix until you know the engine oiling system is functioning it may be a little smoky but won't hurt the engine.

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Just want to make sure we are all on the same page here.

He is already running Pre-Mix fuel/oil.

To check to see if the VRO is working, fill the VRO Oil Tank.
If the oil level goes down then the VRO is working.

Then, and only then, can he use straight fuel.

The above should be performed after any disconnection of VRO lines, tanks, pumps, etc.
You should always run pre-mix first to test out any new repairs of the VRO system.

Yes, as Mark says, it may smoke during the test and may also foul the plugs.

For those that haven't read this, check out the "VRO equipment" statement here in my article.
https://www.whalercentral.com/article...ticle_id=9