1996 150 HP 2-stroke "sneezing"

[doyle007]

Guys, a lot of times over the past year-and-a-half that I've had my boat, my engine will quit after a very loud "pfft". Basically, I get a sound that can only be somewhat imitated by you saying "puff" or "pfft" with some force and air behind it (think about saying "puff" in a normal manner, and then trying to say it and blow out a candle that is a foot away from your lips).

This will happen mostly when I've just started the engine, and do it for a few times, but other times it will do it after I've run the engine for a while. It only happens at lower RPMs (I don't know how many, as my tach is broken), but it has never happened when I'm up on plane.

The engine is a 1996 Mercury 150 HP Offshore 2-stroke carb.

I'm wondering if the engine is running just too rich. I've noticed that there is definitely an oil sheen on the water once the engine starts to run. Yesterday when I fogged the carbs, I ran the lower unit in a bin of water. When I was finished and about to dump the water, I could see globs of oil in the water. I know that some of this is the fogging oil, but even if I didn't fog the engine, the sheen would have been in the water. To me (a mechanical neophyte), this leans towards to rich of a mixture.

Any ideas?

 

[Faztbullet]

Re: 1996 150 HP 2-stroke "sneezing"

What you are describing is a lean idle spit. This could go from various items:
bleed line split,intake gasket leaking,carb adjustment off or a bad thermal air valve. I would start with the valve,as this richens mixture untill engine reaches about 115 degrees then opens. It is located below #3 plug and has a hose attached to one end and hose goes all the way back to carbs. It restricts air to idle circuit so its not lean and warms quicker.

 

[doyle007]

Re: 1996 150 HP 2-stroke "sneezing"

fatzbullet,

Thank you for the reply.

Is the thermal air valve an easy to replace part, or will i get in over my head quickly? I'm pretty handy, and I have the Mercury Service manual, so I'm sure that I could get through the process, but wanted to hear from someone that may have done this before.

Also, where would I look for a split in the bleeder line? Obviously on the bleeder line, but I'm not sure where the bleeder line is located.

 

[Faztbullet]

Re: 1996 150 HP 2-stroke "sneezing"

The air valve is on the starboard head and take about 10 minutes to remove and test. The bleed lines are small 1/8 dia. hoses that come from carbs back to fittings on the rear block area(5 or 6), usually the line will be split where it sliudes on fitting. Study your manual for testing instructions on thermal valve.