90% of the time this type of issue is due to gunk in your carb, but before you do a carb rebuild (it’s not that hard) remember this.
Rule of thumb, you need compression, spark and fuel to make an engine run.
Most 9.9hp are two cylinders, this procedure works for two and four stroke engines. Take out the spark plugs. Put the spark plugs back in the spark plug boot (make sure they’re in there good and tight) ground the exposed tips of the plugs on a piece of metal (I usually wedge them against one of the bolts) and make sure it stays there, do not touch the spark plug and pull the starter cord and look for spark. If you see whitish blue spark then that’s not the issue.
If you only see spark on one spark plug, swap the plugs. At this point you should trace each the spark plug cord to coil and remember (or mark like I do) which is which, I usually number the coil for the top cylinder 1 and bottom 2. With the plugs swapped to the other boot/coil, make sure they’re grounded and pull the cord again. If you see only one spark and it’s in the same boot/coil (with the other plug) that fired before, then you have a bad plug. If you only see one spark (with the other plug) and it’s in the boot/coil that didn’t fire before then you probably have a bad coil and need to replace that.
Lets assume you have spark in both plugs.
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Remove the spark plugs from the boots and put them in your pocket or on the ground. Press your thumb into one of the spark plug holes and pull the pull start (or ask someone else to) and if you feel a solid puff of air, you have compression. Repeat this with the second spark plug hole. If you feel a good puff of air on both cylinders then you have compression. If you don’t feel a good puff of air with either cylinder (or both) you probably have lost compression and need new rings in the cylinder(s) without compression.
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Remove the spark plugs from the boots and put them in your pocket. Press your thumb into one of the spark plug holes and pull the pull start (or ask someone else to) and if you feel a solid puff of air, you have compression. Repeat this with the second spark plug hole. If you feel a good puff of air on both cylinders then you have compression. If you don’t feel a good puff of air with either cylinder (or both) you probably have lost compression and need new rings in the cylinder(s) without compression.
Lets assume you have compression in both cylinders.
Like I said, 90% of the time it’s a fuel issue.
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Be prepaired to watch a few videos on removing and cleaning the carb, before you do it, I’ve had small Yamaha’s like yours, it’s not that hard to do if you have a few tools (or maybe it’s a good excuse to buy a few?) here’s a good video of a guy cleaning a carb while on the boat! https://youtu.be/LbEM5BNU1Xw
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Be prepared to watch a few videos on removing and cleaning the carb, before you do it, I’ve had small Yamaha’s like yours, it’s not that hard to do if you have a few tools (or maybe it’s a good excuse to buy a few?) here’s a good video of a guy cleaning a carb while on the boat! https://youtu.be/LbEM5BNU1Xw
90% of the time this type of issue is due to gunk in your carb, but before you do a carb rebuild (it’s not that hard) remember this.
Rule of thumb, you need compression, spark and fuel to make an engine run.
Most 9.9hp are two cylinders, this procedure works for two and four stroke engines. Take out the spark plugs. Put the spark plugs back in the spark plug boot (make sure they’re in there good and tight) ground the exposed tips of the plugs on a piece of metal (I usually wedge them against one of the bolts) and make sure it stays there, do not touch the spark plug and pull the starter cord and look for spark. If you see whitish blue spark then that’s not the issue.
If you only see spark on one spark plug, swap the plugs. At this point you should trace each the spark plug cord to coil and remember (or mark like I do) which is which, I usually number the coil for the top cylinder 1 and bottom 2. With the plugs swapped to the other boot/coil, make sure they’re grounded and pull the cord again. If you see only one spark and it’s in the same boot/coil (with the other plug) that fired before, then you have a bad plug. If you only see one spark (with the other plug) and it’s in the boot/coil that didn’t fire before then you probably have a bad coil and need to replace that.
Lets assume you have spark in both plugs.
Remove the spark plugs from the boots and put them in your pocket or on the ground. Press your thumb into one of the spark plug holes and pull the pull start (or ask someone else to) and if you feel a solid puff of air, you have compression. Repeat this with the second spark plug hole. If you feel a good puff of air on both cylinders then you have compression. If you don’t feel a good puff of air with either cylinder (or both) you probably have lost compression and need new rings in the cylinder(s) without compression.
Lets assume you have compression in both cylinders.
Like I said, 90% of the time it’s a fuel issue.
Be prepaired to watch a few videos on removing and cleaning the carb, before you do it, I’ve had small Yamaha’s like yours, it’s not that hard to do if you have a few tools (or maybe it’s a good excuse to buy a few?) here’s a good video of a guy cleaning a carb while on the boat! https://youtu.be/LbEM5BNU1Xw