Sneezing from underneath flywheel

[Keegan7]

Hey all,
I recently acquired a boat with (allegedly) a 90hp Mercury Force. I cleaned the carbs thoroughly and got it running, had to adjust the throttle cam to get it idling.

It will rev up just fine but sneeze a puff of smoke from beneath the flywheel which is very odd. I’ve heard of the “lean sneeze”, but I thought it backfired from the intake, not the flywheel area.

I inspected the upper crank seal and it appears to be in good condition. I put fresh RTV on the plate that the oil seal sits in. Ran it again and the issue persists.

I included a video of what it’s doing. Any advice or experience is appreciated! Thank you!

View attachment IMG_9939.mov

 

[Mc Tool]

That looks to me like its being blown rearward from the top of the air box. The 2 screws for the top carb appear to be missing from the air box ,Im wondering how long they are

 

[racerone]

Motor does not sound healthy at all.-----Post the actual compression values.

 

[jerryjerry05]

You sure it's coming from under the flywheel?
Looks like it might be from the top port cover and not the flywheel????
On the top carb: where's the air screw set at?
Under the pack(never saw one that looks like that) the top port cover(1 of 3) might have a blown gasket?? The gasket is thin and can blow out when the motor backfires/ coughs or stutters.
Like Racer suggested? compression??
The motors a 90hp, a later model from the looks of the cover. Any stickers with the model and serial#? Post a pic of the lower unit? It shows it has the Mercury ignition added in the later 90's.

 

[Keegan7]

Thanks for the replies! I only had two screws in the air box for testing purposes. Unfortunately, the serial is no where to be found on the motor. It appears the tag was taken off when somebody repainted it.

Not sure why I overlooked the simplest troubleshooting step and forgot to do a compression test. Here are the values:

Top to bottom:
Cyl 1 - 120 psi
Cyl 2 - 120 psi
Cyl 3 - 0-5 psi

This is undoubtably the issue, but what are my next steps to fixing it? Pull the head off and inspect the cylinder?

 

[racerone]

There you go.-=---Pull the head off.----Lucky you if it is a blown head gasket.

 

[Keegan7]

Luck was not in my favor today. The head gasket did look terrible. However, the issues go much further than that.

I attached pictures of each cylinder. Of course 1 and 2 look fine, but cyl 3 is rough. Theres is significantly more play in piston 3 and it looks like somebody threw ball bearings in the motor - I assume from this cylinder running lean.

Aside from the butchered piston, what is the grinding noise while I’m turning the motor over in the attached video?

Am I better off getting a new-used motor or rebuilding?

View attachment IMG_9965.mov

There you go.-=---Pull the head off.----Lucky you if it is a blown head gasket.

 

[racerone]

As suggested / hinted at in post #3 -----This motor is JUNK as it is.---Cylinder #2 is also scored.------Needs to come apart for complete inspection / overhaul.-----Perhaps a rebuilt powerhead if any shop still does them.

 

[racerone]

When looking for a used / rebuilt powerhead, make sure it is a SPORT JET unit.----A FORCE outboard powerhead may not be a direct swap.