1973 Mercury 20HP Starts Easy...Dies After 90 seconds

[BRANDEL]

I've got an old mercury 20 horse. I ran this motor all summer and into the fall of 2012 pretty much every weekend with no issues.

Replaced the pull cord when it broke, and while I was at it did fuel lines, Impeller, and replaced the serviceable parts in the fuel pump (integrated with carb). Also disassembled the carb, at least the easy parts, cleaned it all out and reassembled it.

Thing starts after three pulls and will idle for about 1-2 minutes then sputter and die. Fuel is fresh.

Tried working with the fuel mixture screw.

I'm at my wits end...please help.

 

[Texasmark]

Re: 1973 Mercury 20HP Starts Easy...Dies After 90 seconds

Start in your usual manner and while running just keep the bulb firm, whatever that takes. What happens?

Mark

 

[BRANDEL]

Re: 1973 Mercury 20HP Starts Easy...Dies After 90 seconds

The bulb stays hard all the way through. I even tried pumping it as the motor begins to sputter. So I don't think it's the bulb or fuel line from the tank to the motor.

Coffee is brewing and I'll be back at it soon.

 

[Don S]

Re: 1973 Mercury 20HP Starts Easy...Dies After 90 seconds

May not die from fuel starvation. After 90 seconds, the engine could be just loading up (excess fuel). Does it smoke extra and slow down gradually?

 

[BonairII]

Re: 1973 Mercury 20HP Starts Easy...Dies After 90 seconds

I wonder if once warmed up(couple minutes)...that an ignition component is failing.

Will motor start right back up? Or maybe after sitting for some time?

 

[BRANDEL]

Re: 1973 Mercury 20HP Starts Easy...Dies After 90 seconds

I think I may have a bad coil.

I pulled the plug wires off the coil. One of them looked relatively good and the other had some pretty substantial corrosion. Also, I shined a flashlight into the coil itself, and could see more corrosion.

Also, the exhaust is much more smokey than usual, and a lot of oil is accumulating in the barrel.

So I think I may be running on only one cylinder. Would that explain the extra oil in the exhaust, rough and jerky idle.

The corrosion on the coil seems pretty obvious too. If you guys can reaffirm this I will go ahead and order new coils and plug wires.

Thanks so much for all your help!

 

[BRANDEL]

Re: 1973 Mercury 20HP Starts Easy...Dies After 90 seconds

The motor does start right back up after a couple pulls.

 

[Laddies]

Re: 1973 Mercury 20HP Starts Easy...Dies After 90 seconds

Check the work on the fuel pump for a bad or improperly placed gasket.

 

[quicktach]

Re: 1973 Mercury 20HP Starts Easy...Dies After 90 seconds

After it dies, do you choke it to restart? Sounds like a fuel starvation....you're maybe running the carb bowl dry ...points to a float needle/seat issue

 

[BRANDEL]

Re: 1973 Mercury 20HP Starts Easy...Dies After 90 seconds

So after a lot of blood, sweat and tears, I figured out that I had blocked the vacuum supply for the fuel pump by incorrectly placing the carburetor gasket.

Good news is it runs like a champ and has 120 psi compression on both cylinders. Going on a five night camping trip in the Sierras for Memorial Day and planning to burn lots of gas!