CObarry

I went with the strong recommendation of a local marina/shop and changed the oil in my 502 MPI's from the Mercury 25W/40 to Valvoline 40 wt racing oil and lost some top end. I'm obviously going back to the Merc oil, but has anyone else seen this? BTW, the straight 40 wt oil is like molasses, even at 75 degrees...

DrNautica

That would be a major drop in RPM, too much for just an oil change. Could it be weather related instead??? If it were true, then you should all be able to pick up 3-4 MPH by going to a full synthetic like Mobile 1.

deboatmon

That's just about unbelievable. Not that your speedo reading is down in MPH, but rather that an oil change caused it. I would spend my time looking at something else, like maybe the pitot tube. It's probably not possible that you lose that much power simply from an oil change. Look elsewhere

robyw1

How many RPMs did you lose?

Iggy

You think that's bad try 50 wt in January.
You can hold the bottle upside down and it won't budge.
That's why they don't run straight weight oils any more. Picture how long it'll take to pump up oil pressure when the oil is cold. In the right conditions you could break the oil pump drive shaft.
Hope you have good bearings.

CObarry

It's the only thing that changed, and max RPM's also dropped by 200. I'm changing the oil just to know for sure and I'll report back. It's cheap enough, and it will be an interesting test.

mopower

Ditto

Strip Poker 388

iTrader: (6)

I had a friend that over filled one and it did that . He also asked me why the oil was blowing out the dip stick tube also.

He is a dip stick

I also run the 40w racing val and never had any problems and Thats what teague said he runs also.

Its never cold here to have to worry about cold starts with the straight oil .I have ckd the bearings for this also.

Rob

CObarry

It's hard for me to believe also, but it's the only logical conclusion I can make. BTW, I put the oil in myself and I know they're not overfilled.

Prior to the oil change, both engines would run 5100 + RPM, the speedo was buried at 75 (GPS said 73). Now, both engines top out at 4850-4900 RPM and the speedo will barely break 70. If only one engine was down on RPM I'd say that something else happened coincidentally, but since they both are down it's got to be the oil....

Steve_H

Barry , when was the last time you ran the boat before the oil change? its amazing how much faster boats run when its cold out.