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Ask Dominick Your Own Question
Dominick, Marine Mechanic
Category: Boat
Satisfied Customers: 1009
Experience:  25 Years Experience as Mechanic, 20 Yrs Boat Yard Owner/Mechanic, Factory Certified
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I have a '73 (I think) Mercury 500 50 hp motor. When I

Customer Question

Hi I have a '73 (I think) Mercury 500 50 hp motor. When I attempt to open it up in the water it dies. It will only go about 1/3 throttle or so. This happened in the past and it was the switch box. In fact, it's been replaced twice and the one on there has very few hours on it. I cleaned and rebuilt the carbs, rebuilt the fuel pump, replaced the fuel tank and fittings. I replaced the stator because it was burned to a crisp and installed a voltage regulator. I noticed when checking compression that one cylinder had moisture on the plug. Compression was 105,105,105,85. I'm getting tired of replacing switch boxes and wondering if you have any thoughts?
Submitted: 1 year ago.
Category: Boat
Expert:  Jeff G. replied 1 year ago.

Can I get a serial number?

Customer: replied 1 year ago.
The last number is ***** little dinged up. It might even be a j, but I think it's a 1.
Expert:  Jeff G. replied 1 year ago.

Ok couple things. If you are getting any water in the cylinders, as evidenced by dropplets on the spark plugs then the engine will not turn up, poor idle, loss of power. Essentially you are running on only 3 cylinders, at best. The leading cause of this is the inner exhaust baffle plate corroding or the gasket deteriorating and leaking.

Run the engine on a flushing cups. Run only for about 3 or 4 minutes at a fast idle, no higher. Then quickly remove the spark plugs and look for water on the plugs.

In addition your compression is borderline on the bottom cylinder. I would run some of Mercury's Power Tune through the engine, then do another compression test and see if the compression improves.

You can check and monitor the ignition with a timing light. Easy diagnosis. Here the biggest problem is poor grounds. Make sure everything is well grounded.

Try these tests and post back your results, we'll go from there.


Customer: replied 1 year ago.
Thanks. I only have access to it on the weekend so I'll have to get back to you next weekend.
Expert:  Jeff G. replied 1 year ago.

That's fine let me know.

FYI the number in the serial should be a 1, no letters.

Customer: replied 1 year ago.
There is a lot of water on the plugs after running for just a few minutes. Can I remove the exhaust cover without pulling the powerhead?
Expert:  Jeff G. replied 1 year ago.

No you don't have access to the bottom bolts. You need to remove the powerhead. You'll also get to check the adapter plate under the block and replace the gasket, check for corrosion between the water cavity and exhaust.