Mechanic help on a merc 250

My b.i.l. has an older merc 250 (1995) with very few hours on it. Only gets used a week or two a year. He’s lost about 8 mph on his top end. RPMs have remained the same and his prop is still the original 4 blade and is in perfect condition. There’s no hesitation and liuttle to no fluctuation in the rmps at any throttle setting. Just looking for direction on where to start looking for the loss of top end perfoirmance. Unfortunately, there are no local marine mechanics (that are dependable) around him out here in Colorado. If it becomes more than a DIY, he bring it to WI on his next trip back. Hopefully someone knows of the silver bullet for this

Sounds like a fuel problem –

what is he burning in the Motor? Non – Ox 91 is the only way to go. Otherwise he will continue to suffer with performance problems.

Dog

In addition, he could have a bad spark plugs or the wrong spark plugs –

If it only gets used a couple of times per year, I would start with fresh (came out of the pump today) gas.

If he is still getting the same RPMs and the prop is good the problem most likely is the hull or in the loading of the boat. Something may be causing him to get more slippage such as too much trim, or something putting bubbles in the water in front of the prop.

Can’t say mercury for sure, but on a yamaha, that sounds like it needs new fuel pumps.

could be same problem I had, weight gain via equipment accumulation in boat.

Is he using a GPS for speed or speedometer with water pickup?

I spent years on my old Merc 200. I’d start by checking the compression on each cyl., then check plugs for color variations and oil deposits, next check the resistance on each coil and look for discrepancies. You would think that a bad plug, coil or burnt rings would cause more vibration, but they sometimes don’t.

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He’s lost about 8 mph on his top end. RPMs have remained the same and his prop is still the original 4 blade and is in perfect condition.

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If RPMs are the same, it isn’t the motor. This is a “direct drive” system, meaning that at whatever the RPMs are, is what the prop is turning. If you lose speed and the rpms are the same, then it is prop/hull/weight issue. I even challenge the weight issue, as additional weight will pull down RPMs, along with speed.

Verify speed with GPS.