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1998 Mercury 90hp outboard won't start

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Deebo863

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I have a 1998 Mercury 90ho outboard that won't start. I have 120 compression in all 3 cylinders, it has good spark and spark plugs, I cleaned the carbs and the fuel pump. I have tried spraying fuel directly into the cylinders and directly into the carb and I get nothing. It's doesn't sputter, it doesn't fire up at all. When I put the gas tank on, I can hear the gas flowing to the carbs and when I check the carbs are full. When I try to run it for a few mins, and check the spark plugs they are wet and have fuel on them. I even tried using the choke and opening the throttle while giving it gas and nothing happens. I checked all the fuses and nothing is blown, I checked that the kill switch is working, by turning off the switch and there was no spark and when I turned the switch back on, I had spark. I also made sure the motor was in neutral and it still won't start. If anyone has any other suggestions, it would be greatly appreciated. My only other option is to take it to a marine mechanic.
 
Air, fuel, compression, ignition and timing. I'd be looking at those things again. Also in a two stroke, you have to look at air pumping as well. I have seen engines that will run fine with water muffs on but won't start when the prop is submerged.
 
Load test the battery and check all connections for starting, may not be spinning fast enough to make spark under compression.
 
racerone said:
Check the flywheel key.-----And flywheel magnets.
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Yesssssss! You have everything but "is the timing correct"? On some 4 stroke lawnmowers I've had, just a "partially sheared" flywheel key is enough timing error to keep them from starting. On your engine, you may have loose or lost flywheel magnets glued to the underside of the flywheel.
 
On older motors with breaker point magnetos , a sheared key means NO SPARK.-----On many modern outboards a sheared key means you have wonderful spark.----Just at the wrong time !-----And no you can not find the sheared key with a timing light.
 
racerone said:
On older motors with breaker point magnetos , a sheared key means NO SPARK.-----On many modern outboards a sheared key means you have wonderful spark.----Just at the wrong time !-----And no you can not find the sheared key with a timing light.
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In my instances of discovering the damaged key was the source, I removed the flywheel nut and peered down from the top of the flywheel to inspect the key....easily done.
 
Update: after looking over the motor, I saw that some wires coming from the flywheel and CDI box have melted at the connectors and are melted together. I will replace the connectors and see if that works and it it doesn't, then I'm taking off the flywheel and looking under there. Thank you for all your help and suggestions. It is much appreciated.
 
Update, I replaced the wires that were burt. Got the motor to start, but it had a very high idle. I couldn't get it to idle down. Then if sheared the woodruff key which in the fly wheel key. I had to replace it 3 times. Then I got it to run and the idle is better. Im going to try to take it to the water and see what it does.
 
Deebo863 said:
Update, I replaced the wires that were burt. Got the motor to start, but it had a very high idle. I couldn't get it to idle down. Then if sheared the woodruff key which in the fly wheel key. I had to replace it 3 times. Then I got it to run and the idle is better. Im going to try to take it to the water and see what it does.
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Are you saying that you found the original key sheared/partially sheared (???), replaced it and it sheared again twice, then quit shearing and you achieved improved operation........that is really strange......mid boggling actually given the product and location of your testing.

Usually what shears the key is the engine is running at high RPM and the "mass" of the spinning flywheel has a lot of contained Kinetic Energy. An immediate stop, or attempt to stop of the crankshaft with all that KE in the flywheel means that the flywheel doesn't stop as fast as the crankshaft resulting in the key shearing or partially shearing. Common in lawn mowers with the blades connected directly to the crankshaft [rather than via V-Belt drive (belt absorbs the impact)] when the blade hits a non-moveable, or only partial moveable object.
 
Loose flywheel nut can also cause this. You must make sure the flywheel nut is torqued to factory specs. If you have sheared the key three times, the crankshaft or flywheel broaches may be damaged now. Also make sure the the crankshaft and flywheel tapers are clean from rust or upset metal from the shearing of the key.
 
I this is was the fly wheel nut, it was very easy to take a loose. It felt like it was only hand tight, but torqued to the right specs.
Yes, it sheared the flywheel key completely in half, I replaced it 2 more times and it did it again. Then I replaced it a 3rd time and made sure the flywheel nut was torqued to the tight specs and it stated with no issues. I also found that strange, but I think the high idle was caused by me, trying to get it to start the first time and spraying fuel in the carbs and the cylinders.
 

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Today's lesson or flywheel 101.-----The key Loates the flywheel for proper timing.-----The flywheel is driven by the locking tapers. ----You now need to use lapping compound to restore the taper fitting.....Failure to do so will leave you stranded again !!!
 
racerone said:
Today's lesson or flywheel 101.-----The key Loates the flywheel for proper timing.-----The flywheel is driven by the locking tapers. ----You now need to use lapping compound to restore the taper fitting.....Failure to do so will leave you stranded again !!!
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Thank you, I really appreciate the information.
 
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