Merc 9.9 4 stroke oil burning

[blackboat]

S/N 0H015765 1998 Motor has ran perfectly since new except...last couple times out,I noticed that it started smoking and used a quart of oil in 14 hrs of fishing. Questions...What would be suspect since it never has used any oil before.Motor still runs good. I see that Mercury parts for this motor are mostly obsolete. If I decide to freshen it up,will all the Yamaha parts for their corresponding motor work? Thanks!

[EuropeanAM]

That's a LOT of oil (like every last drop the oil sump will hold).

Leakdown Test and look at the valve-train (guides, seals, etc) if the leakdown is decent.

Also make sure the prop is correct, and in good condition.

[blackboat]

Yeah,Don...a lot of oil.First day it started smoking,1/2 qt low,2nd day after 7 hours,down another half qt. Will drag the boat out of storage and do a leakdown. What about the Yahama parts?
I actually like this motor over a newer Pro Kicker. Although they are bigger and heavier than the Pro,they run at a lower rpm for the same speed and seem to be much quieter.

[EuropeanAM]

Yes, Yamaha components would be suitable (certainly for the powerhead).

[blackboat]

update...Been 2 yrs but did a leak down originally and it tested good. Changed oil twice and the motor never smoked or used any oil the rest of the year. Fast forward to last spring..Same exact thing. First trip out this spring,again smoked like crazy but the oil was OVER full,and loaded with gas. From what I can determine,the only places gas could enter the crank case would be the fuel pump or carb leaking gas with it going past the rings into the crank case.The motor still runs and starts good.The plugs have the normal deposits but not oil soaked. Any suggestions will be appreciated.

[EuropeanAM]

Fuel Pump or Carb. Breached fuel pump diaphragm will dump fuel into the crankcase.

[blackboat]

Thanks Don! I will stick on a new pump and work the carb over. Hopefully the diluted oil didn't do much damage. These are tough little motors and it has thousand(s) of hours on it. Hope for the best but it doesn't owe me anything. Thanks again.

[blackboat]

Update #2...Same thing this spring,new fuel pump and carb kit. Smoked up a storm and the oil was diluted and milky looking,changed oil 3 times till July and then the motor quit smoking and ran great the rest of the season. It is really low on compression now so time for a refresh.Got the motor on the bench and was sizing up the project when it hit me...the thermostat! It was stuck open,ran bad in cold water,got better when the lake warmed up. Anyway,the cylinders look good and are within spec. The question(s)...What grit hone would you recommend? The service manual says 160 psi compression. Seems high to me but is this correct? Only 60 now. Thanks!

[EuropeanAM]

Just use a traditional fine-stone hone, just enough to take the glaze off the cylinder walls and leave a nice cross-hatch pattern.

Compression specs were normally done on an ELECTRIC start model (you'll be hard pressed to make those numbers if using a rewind starter).

[blackboat]

Thanks again,Don!! It is electric start and it whirls over good...a little too good...

[EuropeanAM]

Make sure the engine is up to temp, both plugs out, and throttle plate WIDE OPEN when compression testing (makes a big difference).

[blackboat]

Last update (I promise)....Have most everything I need except the piston rings. Mercury part number 39 8257121 or 39 825712t1 are NLA and Yamaha 6g8-11610-02-00 are 3 months out if they ever become available. The dealer said my best and maybe only chance is to call around and see if anyone has any laying around.Sierra would be OK too.I need 2 sets.Any help on this would be greatly appreciated!! The old rings look good but I hate sticking them back in since I have gone this far.BTW,I had the valves and seats touched up by a local high performance head expert and they look great.

[EuropeanAM]

You have a PM with a lead.