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2006 Mercury 9.9 Four Stroke Start Issues

T

thebubba

Regular Contributor
I bought a fleet of 9 used 2005-06 9.9 Mercury Four Stroke Motors from a local marina that no longer rents boats. I have been able to get 6 of them running perfectly with very little work. I am stuck on motor #7. I have rebuilt the carb and put new spark plugs in it. This one appears to have been run for only a short time, because it looks much less used than the rest. It will almost start every pull. I can hear fuel go into the carb when I push the prime button, so I know it is getting fuel. It doesn't seem to matter if I prime it or not, it almost starts everytime I pull it. My arm is getting tired. It has good compression on both cylinders at around 100 PSI. The spark looks a little weak to me, but I don't have a spark gap tester. I have spare motors, where should I start on this baby? What ignition component would you swap out first? Coil, switchbox. I would like to know which part is bad, so I can use the rest on another motor. Am I missing anything else? I am also looking for used cowlings for a couple of these motors if anyone knows where to find one. They seems to be in short supply.
 
Unfortunately there is no a way to squirt anything into the carb throat, because the air attentuater doesn't give any access without removing it, which is what holds the carb in place too. I could try a little fuel in the spark plug holes I suppose. I have a second motor that will run at high RPM, but not idle. I am thinking this is all carb issues. I may pull a carb off one of the good running motors and see if it runs well on this one, but I hate to mess with a motor that is running well. I finally did get a spark gap tester ($8 at oreilly auto parts) The spark is not very strong unless I pull really hard on the pull start. Then it is ok. I swapped out the power pack from another motor and it is about the same spark strenghth. I could try the coil next, which is really easy to swap as well.
 
Ok, after about wearing my arm out pull starting this thing and having it almost start everytime. I went back through the carbs again. Everything looked perfect and shiny, re-cleaned anyway, re-assembled and exactly the same thing. It would almost start every time. I noticed this motor was much shinier than the others like it had an issue early in its life. Then it came to me. Flywheel Sheer Pin. I wanted to also check the timing belt for correct alignment. The nut holding the flywheel on was not very tight. The flywheel itself came off with very little effort and sure enough the flywheel sheer pin was broken in half. I check the timing belt alignment which was accurate and put a new pin in and correctly torqued the flywheel down. She runs perfect. Mystery solved. :)
 
So what does the flywheel shear pin do and why would it keep it from starting? If broken does it prevent the flywheel from continuing to turn on start?
 
The flywheel sheer pin has one main purpose to keep the flywheel in the exact same position on the crankshaft. Having it sheared made the flywheel spin into different positions while turning, messing up the ignition timing and not keeping the flywheel spinning all the time. It also loosened the flywheel nut from the spinning. My guess is someone hit something while it was running fast and sheared the pin with the sudden stop. The reason it almost started was because the ignition was working, just not firing at the right time always.
 
thebubba, I have a 2006 9.9 4 stroke with what sounds like the same problems. I cannot find a diagram showing the location of this pin. Is it easy to spot? I could not pull up the link that rmcderm listed in his response.
 
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