What happens with motor performance when ageing? Mercury V6 135 hp

[Scand]

Hi

I just wanted to post this question to make sure I don?t change "the wrong parts". A compression test showed approximatey 120 psi on all 6 cylinders with very small differences inbetween.

Motor is equipped with a Laser 2 prop, 23P. Boat is an english speed boat, a fletcher 16ft with a deep V-bottom. I guess the motor can be placed a hole higher on the transom, the cavitation plate is exposed on both sides of the V-skaped keel.

The boat has a top speed of about 41 knots (47 mph) with 2 persons. I calculated the total weight and got 1700 lbs (765 kg) including crew and all equipment.

-Is this a normal top speed ?
-If not, should I suspect that the motor is not performing 100% ?
-Is it really logical to expect 135 hp from such an old motor (1988) ?
-Do old outboards loose power due to age ?

Very difficult questions i assume, but I still wanted to get this off my mind.

Best regards

Hansen, Sweden

 

[Texasmark]

Re: What happens with motor performance when ageing? Mercury V6 135 hp

My service manual states that below 120 one could expect to have problems. You're ok there. Got clean carbs, linkage is tight, lower unit oil is clear and fresh, fresh gas, a little Sea Foam if you are inclined. Why worry. Be happy.

If you want to diddle with something you might change out your fuel pump, fuel filter, and get a new fuel line and Quicksilver OEM squeeze bulb. If running the gray OEM hose (the one from the tank to the engine) it deteriorates over time. Happened to me at about 10 years and to others on here. The fuel pump valves might get worn and you might not be able to pump adequate fuel at WOT. If you can keep your rpms up then don't worry about the pump.

Mark

 

[CharlieB]

Re: What happens with motor performance when ageing? Mercury V6 135 hp

Boat speed can vary depending on the set up.

The critical # is WOT RPM.

State of tune, clean carb, correct link and sync, correct mounting height, prop selection, all are critical to achieving top performance of the boat/motor combination.

As the motor ages and compression reduces then this WOT RPM and boat speed will begin to reduce.

You can have the heads shaved a bit to increase compression and add power. Do NOT increase compression to more than 145 unless you plan to ALWAY run higher octane fuel. 135 to 140 is a good safe range. Your 120 is an average factory number.

So there are a few questions:

What is your WOT RPM?

Trim the motor such that the propshaft is parallel to the boat bottom, is the propshaft at or 1 inch above the boat bottom? The mounting height should allow the A/V plate to 'just' skim the water at WOT (ideally) any higher is too high and the prop will lose 'bite' and vent on turns. All this will change with how the boat is loaded and how much fuel is on board, so any testing should be done with an 'average' load.

 

[Scand]

Re: What happens with motor performance when ageing? Mercury V6 135 hp

Hi CharlieB

Actually, I have had some difficulty getting the motor to full WOT even with a 19P prop. It stays at 5300 rpm. I have some experience with 2-stroke motors, and my first feeling was that it was running rich on fuel, but it is more difficult with boat motors since they are much bigger, watercooled and difficult to test. The owner before me was from Scotland and he bought it 2 years ago from his friend, a boat mechanic.

The owner before me ran both oil injection and premix, so I am also a bit suspicious about carb deposits on pistons and rings. Mayby the motor needs a proper cleaning...and decarb.

When I checked the spark plugs (NGK BU8H) they were quite black. I will borrow a videoscope and look into the cylinders at piston heads.

The A/V plate is 0,5 to 1 inch above the lowest point of the keel, but transom is also V-shaped, so the water stream cuts above the A/V plate at the sides.

Regards/Hansen