Mercury 150 started misfiring after runnning perfect for about an hour.

snowbrd84

Petty Officer 1st Class
Joined
Sep 23, 2012
Messages
215
Took out the boat for the first time yesterday with the new to me '86 Mercury 150 XR2. I was leary about the oil injection, not knowing if it was working. So just in case, I mixed the gas in the tank at 75:1. Boat fired right up and ran great. Took it out on the water, it was scary how fast we were going. Motor was running beyond my expectations. So we set off and after about 15 minutes at 3/4 throttle, i start hearing a clanking/banging sound and immediately kill the engine. Try to restart it and the starter could not move the flywheel. It seemed to be seized. So I pulled the plugs, the #6 plug was fouled, the rest were dirty with some grit. I sprayed some wd40 in each cylinder and bumped the iginition, starter was able to spin the flywheel. So i put it all back together and tried again, it started but there was the same clanking sound. But it just sat there and idled in neutral. I put it in gear and it would go, but giving more throttle would just kill it, so i limped home.

Now i go start it, and it runs fine in the driveway, so sounds, revs up great. Any ideas? I am stumped.
 

schematic

Lieutenant Junior Grade
Joined
Jan 12, 2008
Messages
1,102
Re: Mercury 150 started misfiring after runnning perfect for about an hour.

Have you checked for water in fuel?
Have you check leg oil condition?
 

snowbrd84

Petty Officer 1st Class
Joined
Sep 23, 2012
Messages
215
Re: Mercury 150 started misfiring after runnning perfect for about an hour.

Brand new fuel tank, so brand new gas. Changed the lower unit oil on friday. It was doing it in gear and in neutral, so i dont think it is the lower unit...
 

snowbrd84

Petty Officer 1st Class
Joined
Sep 23, 2012
Messages
215
Re: Mercury 150 started misfiring after runnning perfect for about an hour.

Just did a pressure check, 150-120 on 1 through 5, then 0 on #6. Pulled head, piston is destroyed. Cylinder walls look good, but feeling with my finger, i can feel scoring.
 

schematic

Lieutenant Junior Grade
Joined
Jan 12, 2008
Messages
1,102
Re: Mercury 150 started misfiring after runnning perfect for about an hour.

Ouch!
 

wired247

Lieutenant Commander
Joined
Oct 8, 2011
Messages
1,557
Re: Mercury 150 started misfiring after runnning perfect for about an hour.

Its a steel sleeve block. You should be able to bore that cylinder and plop in a new piston for under a few hundred bucks.
 

snowbrd84

Petty Officer 1st Class
Joined
Sep 23, 2012
Messages
215
Re: Mercury 150 started misfiring after runnning perfect for about an hour.

pulled the flywheel and carbs, removed all of the bolts but can not get the back half of the motor off to access the pistons
 

wired247

Lieutenant Commander
Joined
Oct 8, 2011
Messages
1,557
Re: Mercury 150 started misfiring after runnning perfect for about an hour.

Is it off the mid section?
 
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