Yanaha making oil

I have a friend with a 05 Yamaha 70 4 stroke. He had parked his boat after a fishing trip and a week later he checked the oil and it was up to the top when he removed the dipstick plug. One mechanic told him to change the oil and run it hard with the motor trimmed down for 45 minutes and check the oil. With gas going in the oil, it would scare me to death to run it that long with the oil getting that thinned out with that much gas in it.
Any suggestions as to the cause and if running it that long without checking the oil is a good idea?

When I first bought my 50 used a few years ago it did same thing. Did some research and found that they will do that on occasion. The cure I found was change oil and filter, change spark plugs. Then just run it. Found out I had a bad plug and what those engines will do especially if you do a lot of low speed trolling with a bad plug will let that gas bypass the rings and go into crankcase. It also helps to run some seafoam through on occasion or Yamaha ring free. Since I figured that out I Change plugs every year. I pull cranks with big motor and haven’t had any more problems with it. And mine is a 04.

I am on my second Yamaha four stroke...As recommended by Yamaha I mix one ounce of ring free to every 10 gallons of gas...no problems with carbon sticking the rings

Probably wouldn't hurt to unplug the fuel line when you park it, or install a fuel cutoff switch.
I would not be happy to just "Accept it as the way it is", no no.
That issue happens on occasion with small gas engines that have their fuel tank at higher elevation than the carb, whereas in such cases no fuel pump is necessary. When the problem occurs there, fuel cut off switches is the fix.
I remember finding gas in the oil of a pressure washer once, I installed a fuel cut off switch and changed the oil - problem solved.
I don't want to deal with such as that more than once.

This may be way off track, but I've often wondered about the intense pressure that builds up in fuel tanks which have no vent. I think this may include all the new portable tanks. I try not to tighten my fuel filler cap very much so that excess pressure can escape, but at times I've noticed the fuel tank swollen immensely. That pressurized fuel has to be seeping somewhere.

The tank has a vent and was open.

these two quotes are my new mission statement.

I would not be happy to just "Accept it as the way it is", no no.

I don't want to deal with such as that more than once.

Love it

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He changed the plugs and all filters and ran it 8 miles down the lake without the oil level changing any. He did not idle it as he does running trot lines though.

I had the same problem with a new Suzuki 140. I called the dealer and was told to change the oil then take it somewhere where I could run hard for 2 hours. I did that and have not had the problem again. The dealer said that the engine has to be ran pretty hard to seat the rings. My son has a Yamaha 115 that we need to do the same thing to.

It's the manual fuel pump plunger. In valve cover.

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Originally Posted by Mbsbeek

It's the manual fuel pump plunger. In valve cover.

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How about going ahead and explaining that. What causes it and how do you fix it.

The primary low pressure fuel pump is mounted on valve cover , has a plunger that runs on cam lobe. It has a o-ring to seal oil leak. The plunger seal will start a slow leak at first and will make oil just a little at first, but when it totally fails . oil will come out dip stick. Fix , replace fuel pump.

I thought it was Suzuki. But pretty sure it has same kind of pump.

Quote:

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Originally Posted by Mbsbeek

The primary low pressure fuel pump is mounted on valve cover , has a plunger that runs on cam lobe. It has a o-ring to seal oil leak. The plunger seal will start a slow leak at first and will make oil just a little at first, but when it totally fails . oil will come out dip stick. Fix , replace fuel pump.

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Thanks for the explanation. Now it makes sense. Hopefully this will help someone else down the road.

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Originally Posted by Mbsbeek

I thought it was Suzuki. But pretty sure it has same kind of pump.

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Wouldn’t think there would be much difference.

Thanks guys for the advice. I know he has changed the oil, filter and plugs and has run it twice but I have not talked to him since that.