Mercury 4.5 HP - Need Help Please!

[muskyvic]

Hi all -

I own a 1976 Mercury 4.5hp. The motor starts on the first or second pull EVERY time, even after sitting for an entire winter. It runs as smooth as possible. The motor also pisses a strong stream of water, so it is apparent that the water pump is working fine.

The motor will run strong for 45min-1 hour, then it will slowly cut out on the other side of the lake (You can hear the motor start to slow down). It will start right back up, but it will die again in 30 seconds. If the motor sits overnight, it will start right up in the morning, run strong for about an hour, then kill again.

I have tried changing the gas and spark plugs, but it has not made any difference as I have had this problem for the past 3 years.

Does anyone have any advice as to what this problem may be?!

Thank you for your time!

 

[Laddies]

Re: Mercury 4.5 HP - Need Help Please!

It sounds like a ign failure, something heats up in the system and the engine quits and after it sits and cools down the engine will restart

 

[dennyb123]

Re: Mercury 4.5 HP - Need Help Please!

Make sure your gas tank vent is working. I once had a ATV that would run great for about a half hour, then would cut out for no reason. I eventually traced the problem back to the gas cap. The vent was plugged.

 

[muskyvic]

Re: Mercury 4.5 HP - Need Help Please!

Thank you for your replies. However, the gas cap is working fine...we tried the motor with two different gas tanks and still had the same problem.

Any other ideas??

Thank you!

 

[emckelvy]

Re: Mercury 4.5 HP - Need Help Please!

After the next failure, pull out the spark plug, stick the plug back in the plug wire and lay on the block. In Neutral, give the flywheel and stout clockwise spin with your hands and see if there's a strong spark at the plug.

If you see an issue with the spark, one thing I'd check is the condition of the points, under the flywheel. They can be fairly burnt-looking and still work ok, but must have the proper gap (.020"). If they look really nasty and the gap is wrong, pull them out, clean to bare metal, reinstall & adjust gap.

While you have it apart, replace the white plastic insulator for the points wire (where the wiring goes thru the breaker box to the outside). This insulator is notorious for developing cracks which lead to High-Voltage leaks and loss of spark.

It's entirely possible this insulating block is breaking down from heat and causing loss of spark.

HTH & let us know what you find.............ed