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9.9 merc, propulsion issues. Needs help!

G

Gbucci24

New member
I bought this 96 mariner this past offseason. Ran fine in tank when bought. Took it out on the river yesterday on my rib boat and had a ton of trouble starting. When I got it started all gears worked and idles great.

Problem em came when throttled up. It just churns water and propels me about 5 mph. Ran up the river 2 miles like this thinking it just needs run. Water got choppy and all the sudden the motor started working right! You could tell immediate difference in the wAy it worked. I was going about 20 at full throttle.

however, when powered down back to low rpm and isle, the engine went back to just churning water when put back on full throttle. Went on and off like this 3 times. Need help!

I called a mechanic and he thinks it's a dead cylinder, carb prob, or prop spin. No clue. Thoughts or even on how to test?
 

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Could be a bad sparkplug.-----Look at them ( post close up pictures of the gap ) on here.-----Use a timing light and see if spark stays steady on both cylinders.
 
Have a couple of questions -

you say it was running up the river at maybe 5 mph then water got choppy and the motor just "took off".

Now, took off as in "seems like the prop finally bit in" OR you had the throttle cranked and it was obviously revving low then the rpms shot up and she took off?

next question(s) - is this a long shaft or short shaft motor?
have you measured the transom on the boat?
is the cavitation plate on the motor level/in line with the bottom of the transom/hull?
do you have the motor tilted down to the lowest pin setting on the bracket

(if the rpms are constant it could be a severe cavitation problem caused by either the motor is too high or tilted too far out of the water - the "choppy" water might have allowed the prop to bite which in turn raised the bow/lowered the stern which helped reduce the cavitation further - when you slowed, the boats profile changed again to more "level" and the prop once again might be breaking the surface)

This is potentially a cheap/easy fix if it's simply adjusting the motor.
 
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Thank you for the reply. Everything with the way the motor was positioned was correct. By "taking off" I meant that in about a 1/4 mile of pretty choppy water( wind picked up) the engine suddenly started working as it should. I noticed a little higher rpm but not as much to offset the bad performance before. Then, when I throttled down as well as put in neutral, it went back to lacking propulsion.

Its raining today so I'll have to test the motor tomorrow and pull each plug individually while running to see if it is a cylinder problem. If it's not, then has to be something with the shaft or prop. It's a short shaft motor.

here is a pic of it performing correctly.
 

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Okay, but was it running okay when you tried this test? You need to do this when it's not running right.

Jeff

PS: I suspect you have a bad ignition coil.
 
There is nothing wrong with the ignition, cylinders. To me it seems like a shaft or prop issue. I'm going to tear apart tonight, change the water pump and take off the prop and see if any signs of slippage.
 
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