Mercruiser 3.7 165 Running Rich?

ImperialV184

Petty Officer 3rd Class
Joined
Aug 4, 2011
Messages
99
I was doing some annual checks on my 1987 Mercruiser 3.7 165 and pulled the plugs, there was a good deal of black soot on the plugs, definitely not oily, black soot I believe indicates carbon fouling. From what I understand carbon fouling indicates it is running rich. Engine was completely overhauled and rebuilt 3 years ago these are the original plugs form the rebuild. I have a good deal of black smoke until the choke opens up during start up but this goes away once at temp. I would think that if this was only a start up issue plugs would clean up during normal operation. How can I tell if it is running rich at temp, would I still see the black smoke? Or is this fairly standard for older engines?
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,306
if you see black smoke, your running rich.

adjust your choke
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,468
I would think that if this was only a start up issue plugs would clean up during normal operation. How can I tell if it is running rich at temp,

Ayuh,..... Carbs have several circuits, 'n the color of the spark plugs reflects the conditions in the minutes just before shut-down,.....

Obviously, yer choke is outa adjustment,......

If the motor idled for several minutes before ya shut it down, the idle circuit is rich now,.....

If ya run it Hard, then chop the throttle, 'n as soon as it's safe, shut the motor off, 'n pull the plugs, that would show the conditions with the high speed circuit,.....
 

ImperialV184

Petty Officer 3rd Class
Joined
Aug 4, 2011
Messages
99
Thank you for the input, I pulled the plugs after an idle run on the muffs. So I could totally see that issue may be at idle. I had no idea the timeline for the fouling. Definitely have to do that test on the water. I have felt choke was an issue. I set the choke to barely close when cold. When I start choke flutters, but I believe stays closed then opens up at temp. I always have to pump at least once at start up, I hear choke close, so it sticks open when it cools. then I give it about 3/4 before start. Does that sound right. Not sure how to make sure choke is functioning properly. Choke seams to stick but usually sticks open.
 

Attachments

  • photo303875.jpg
    photo303875.jpg
    2 MB · Views: 0

stonyloam

Vice Admiral
Joined
Mar 13, 2009
Messages
5,827
That plug is not too bad. Could be your idle mixture needs to be adjusted. At idle turn the mixture screw in until it starts to stumble ( lean) then out till it stumbles (rich), then back inuntil it is running smoothly. Do it slowly and keep track of the number of turns, listen for the rpm change. Some black smoke on startup is normal until the choke opens all the way. Once up to operating temperature take off the flame arrestor and make sure the choke is all the way open.
 

ImperialV184

Petty Officer 3rd Class
Joined
Aug 4, 2011
Messages
99
Thanks for the input,I checked the idle mixture and set the screw as specified above, it is right in the middle, i did not get a chance to make sure the choke is open all the way at temp but will check that as well. I did notice i have the choice set pretty tight so it may be staying closed a little too long at start up. Going to take a little pressure off the choke spring (not sure if i am using the right term there) to see if that helps start up. getting cold here so may be spring until i get to it, i do love how easy this engine is to winterize. Going to also get a new set of plugs while i am at it.
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
....I always have to pump at least once at start up, I hear choke close, so it sticks open when it cools.....

The choke isn't sticking. It's being held open by the idle speed screw (against the 'fast idle cam'). This is completely normal, and exactly how the design of the system works. With the engine cold, as soon as you start to open the throttle (and the idle speed screw comes off the 'fast idle cam') the bi-metal spring element in the choke mechanism flips the choke plate to the closed position....

Chris......
 
Top