Reading spark plugs Mercruiser 120 (pics)

[italianstal27]

Hopefully this helps out other people!

We've been maxing out 3500 rpm in our mercruiser 120 (1973). Boat is 18' holiday restored with 3/4" plywood and pink foam below the deck. Really not too much extra weight. Gas tank is 13 gallons in the stern and 2 batteries.

Here's spark plug #3



And here's our fuel filter



And it's not properly adjusted! We there's a return spring on the throttle cable that is kinda interfering with the choke fully closing. Jets are at 1 1/2 turns out.


We've ordered a new fuel filter and i'll update this thread on whether we gain more RPM.

Symptoms include: Bogging past throttling at 3500 RPM. In neutral, we can rev the engine to 4500 RPM in the water (don't do this outside the water). Engine is timed at 8 degrees BTDC. Carb was cleaned fairly thoroughly with carb cleaner and compressed air in february.

thanks to all of you who are active on this thread. We wouldn't have been able to get this boat to where we are without you!

 

[Scott Danforth]

that crud you see in your fuel....... has gummed up your carb.

time to pull the carb apart

 

[Rick Stephens]

You have a lot of water in the fuel. If your local fuel has ethanol, then the fuel in your tank probably is no good. So to add to what Scott said about rebuilding the carburetor, you need to consider getting everything out of the tank and making sure any other filters are clean as well.

When water gets in fuel with ethanol, some of the water and ethanol will bind together and drop to the bottom of the tank. The ethanol accounts for 15-18 points of the octane rating in the fuel. When separated the fuel no longer has enough octane to burn correctly. Pump the old gunky looking fuel out and any water and rebuild your carb and the motor will return to normal operation.

 

[italianstal27]

Are we supposed to have another filter in line besides the water fuel separator?

We've probably put 100 gallons through this boat over the last 3 years, and only once used ethanol when we were in a pinch. Is ethanol one of those "don't even try it once" type of things? This is the first time we've replaced the water/fuel separator.

When we restored the boat 3 years ago, we drained the tank and swirled it 3 times with new gas to clear all the crud out, so I know it was clean when we reinstalled. We're curious how the tank could have possibly gotten this bad or if this was because we didn't change the filter in time?

 

[Bondo]

This is the first time we've replaced the water/fuel separator.

Ayuh,..... This should be done, at a minimum Annually,..... or anytime the boat is down on power,......

 

[Rick Stephens]

Might be a filter in the fuel pump. Not sure what they had on a 73 even though I had a Steury with a I6 in it that was just a few years newer than that year. Really good you have that aftermarket separator! Change it every year and pour it out like you did this time every time you change. Tells you a bunch about what's in your tank. After three years, you have no idea what kind of water build up was recent.

Might be a filter in the inlet to the carburetor. Sounds like you need to rebuild the carb and maybe take a peek in the fuel tank to see if there is a bunch of water. Fuel with ethanol runs fine. Just doesn't do well with water. I don't put any in my boat, but many places you have to. I posted the above just so you would know what to think on and look for.