9.9 Mariner 4-stroke won't run with hood in place, runs great with out it?

[300mag-hunter]

I recently picked up a walleye boat with a 1995 or 96 Mariner 9.9 4-Stroke motor. Motor had been sitting a few years so I cleaned out the carb and put in a new water pump impeller. Motor was used in Colorado and Wyoming fishing high altitude lakes, don't know it that may make a difference or not.

I started it up today and it wouldn't stay running just idle for a bit and die off. ended up asjusting the carburator screw on top the carb out to 5 turns out and it would idle perfect even down to a very very slow idle speed. left it run at normal idle speed for a few minuets hit the throttle a couple times and it responed great. Went to put the motor cover (hood, cowl or what ever you want to call it) back on and it slowly lost rpm started to just bearly keep running at a extremely low idle and then would die. tried it several times with cover on and off and same results. when it was just about to die I could unlatch the cover and lift it up about 4 inches and it would run fine again. Nothing wrong with the cover, the air intake by the handle on top of the cover is open not birds nest or nothing like that. I've tried adjusting the carb screw on top of the carberator from the 5 turns out where it runs good with out the cover all the way down to to 1/2 turn out & every where inbetween and even out more and same every time. I can even have the idle set very high at about 2000 rpm with out the cover on and as soon as I put on the cover it instantly drops down the rpm and will die off within about 15-30 seconds. Is there something else I should be checking on or adjusting. Is possibly the float height off in the carburator, when I cleaned it looked perfectly level when needle was against the seat. Never messed with any of those adjustments. Any info would be appreciated.

Thanks.

 

[grandpaclint]

Re: 9.9 Mariner 4-stroke won't run with hood in place, runs great with out it?

sounds like the cowl is shutting off the air supply, or exhaust gas is going directly into the cover .look for a clean air intake somewhere

 

[300mag-hunter]

Re: 9.9 Mariner 4-stroke won't run with hood in place, runs great with out it?

Thanks grandpaclint that is exactly the info I needed. I already knew the intake was just fine but never considered exhaust gas getting under the hood. Ran the garden hose in the lower cowl to watch for exhaust leak, and sure enough there was one. One of the mounting bolts for the lower cowl had vibrated loose at some time, leaven a bolt hole open right into the exhaust manifold. Went to the store got a bolt, stuck it in and runs perfect with and with out the cover on the motor. Was even able to set the carb screw to 3 +/- 1 turns out like the manual says. Never would have guessed that was the problem.

Thanks again.

 

[grandpaclint]

Re: 9.9 Mariner 4-stroke won't run with hood in place, runs great with out it?

Thanks grandpaclint that is exactly the info I needed. I already knew the intake was just fine but never considered exhaust gas getting under the hood. Ran the garden hose in the lower cowl to watch for exhaust leak, and sure enough there was one. One of the mounting bolts for the lower cowl had vibrated loose at some time, leaven a bolt hole open right into the exhaust manifold. Went to the store got a bolt, stuck it in and runs perfect with and with out the cover on the motor. Was even able to set the carb screw to 3 +/- 1 turns out like the manual says. Never would have guessed that was the problem.

Thanks again.

you are very welcome