1973 Mercury 500 50HP no power/speed when in water!

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PatTrumps

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Hey guys, i'm fairly new to the world of boats. I have a 1973 Merc 500 50HP that runs fantastic on a hose, but is horrendous in the water. I've got good compression, cleaned out my carbs, checked the timing, rotor and cap looks good, replaced fuel lines (in engine, and the fuel line with a pump)....i'm at my wits end. Any ideas? Thanks in advance.

Pat
 
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PatTrumps

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Re: 1973 Mercury 500 50HP no power/speed when in water!

Took the carbs apart, sprayed them out with carb cleaner. Let them soak for a bit, then sprayed em out again. They're really clean.
 
merc850

merc850

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Re: 1973 Mercury 500 50HP no power/speed when in water!

Could be fuel pump, incorrect idle mixture 1 3/4 turns out while running in water in Forward (600 rpm or so) - or wrong pitch prop, or motor angle in too steep.
 
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PatTrumps

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Re: 1973 Mercury 500 50HP no power/speed when in water!

When its on a hose, it revs up perfectly. Would the wrong pitch prop really hold the engine back like that? I'm gonna pull the fuel pump off tomorrow and inspect it. I'll post my findings.
 
merc850

merc850

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Re: 1973 Mercury 500 50HP no power/speed when in water!

Yes a high pitch prop would be like trying to move a car in 3rd gear from stop which might be your problem. What is the size of the boat and approx weight.
You can find the pitch Nr. on the barrel of the prop, look for Quicksilver 49-######-19P
that's an 19 pitch prop. Or it might be at the nut end.
Also make sure the motor is at a slight angle outwards in relation to the transom.
 
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PatTrumps

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Re: 1973 Mercury 500 50HP no power/speed when in water!

Problem solved! It was a bad diaphragm in the fuel pump. Runs like a champ now. If anyone else has a similar problem, rebuild your fuel pump.
 
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