1991 9.9hp 2-stroke Mercury diagnosis

[Patriot]

The motor runs fine for half hour or so then decides to rev for about 30 seconds then runs fine again. This happens about every half hour. I had the carb rebuilt and it did this ever since.

[rolmops]

Its rearing to go.

But seriously,there probably is a small leak in the fuel line which only opens up every so often and produces a lean mixture for a few seconds. I would check all the hose connections and the fasteners on the carburetor,also the ones that connect the carb  to the block.

Edited May 14, 2015 by rolmops

[lakebound88]

Could be improper seat on the face of the carb gasket

Sent from my SCH-I545 using Lake Ontario United mobile app

[ray koziatek]

if it revs that long i might suspect a little further from the carb check yer seal at engine conn..on the other hand some pay hundreds for a speed control to change presentation.....  but the reving sounds lean which means less lub to where its needed

[Xxx]

Its rearing to go.

But seriously,there probably is a small leak in the fuel line which only opens up every so often and produces a lean mixture for a few seconds. I would check all the hose connections and the fasteners on the carburetor,also the ones that connect the carb  to the block.

EXACTLY X2 Pap.

[Chas0218]

if it revs that long i might suspect a little further from the carb check yer seal at engine conn..on the other hand some pay hundreds for a speed control to change presentation.....  but the reving sounds lean which means less lub to where its needed

Yup sounds like a lean condition but it could also be caused by excess fuel getting burned off.  Does it have a mixture adjustment? You might be a little too rich then when she leans out to correct mixture you get more rpm.  I had this happen on a weed whacker and chainsaw.

 

If you whack the throttle before the RPM rises is it slow to gain rpm or does the rpm rise up fast?

Edited May 14, 2015 by Chas0218

[Patriot]

I’m by far an outboard engine mechanic and was totally stumped. But now I have some sort of direction. I’ll let everyone know of my findings. I'll start at the remote tank and work my way into the carb.

Thanks for the replies guys.

[Patriot]

I tightened hoses and screws and I took the 9.9 out for a spin yesterday and the same thing. Maybe the carb needs another rebuild

[Sk8man]

I think I would run some Sea foam through it before going to a rebuild