Yamaha outboard motor (9.9 HP/ 7.3 kW, T9.9EXHP, 6G8 UL 784628, 1991) starts immediately and runs well when cold. When warm, misfires are evident.
If stopped, deliberately or by the misfiring, after running for about
20+ minutes, the motor won't start again until cooled.
The motor has an electronic ignition. From my minimal experience and reasoning, the misfiring is more likely from faulty ignition than from fuel supply. The service manual shows these parts in the ignition.
What is CDI? I don't find it explained in the manual.
Any tips to identify faulty operation of one of the above components? I prefer to identify a specific fault rather than replace parts until the motor runs properly.
Capacitive Discharge Ignition - Hotter, but shorter spark than a standard inductive system.
If you can get to the coil, check that for cracks or visible damage. In my experience (limited), 90% of heat-related ignition faults are typically caused by a cracked coil.
If you can, obtain a spare spark-plug and proceed as-follows:
a) when the system is hot and will not start - remove the spark-plug wire, install the new plug, ground it to the engine and see if it sparks when cranked. Note the color of the spark as well. If no spark, likely the coil. If a weak spark, the same. If a strong spark, then you have a fuel issue. b) IF a fuel issue, start with the fuel filter, then check the carburetor for clogging, varnish and so forth. I expect that this is the 'first start' of the season as well.
Story: We have a small Mantis 2-stroke tiller that we purchased well-used. I replaced the spark-plug, fuel-line, filter and cleaned the carburetor via the overnight soak method. Ran like a champ for three years until this spring. We use 'Real Fuel' such that a quart lasts the entire need each spring and we do not have to worry about over-winter storage. Well, this spring, I replaced the (cracked) rubber grommet to the fuel tank. After which, it would start, but only run under load for a few seconds. >LOOKED< like plenty of fuel in the primer bulb. Turned out that the new fuel line that came with the new (generic) grommet was just a hair too long, and it crimped the line within the tank. Enough fuel for starting and idle - not enough for under load. I took two inches off the line, and all is good!
I don't know that much about outboard motors and I support what Peter W. said. I do however have a story to tell about my 9.9 and since it had similar symptoms to yours, what was finally the solution.
I could start the outboard and it would run fine on slow speed and occasionally on high. It exhibited the symptoms of sucking air in the gas line at high speed. I took to putting new clamps on all the gas line connections and it still experienced the same symptoms. Finally I looked at the fuel pump (not a very complex one), and it was distorted. It is simply a flat piece of rubber that has an attachment for moving it in and out. It was stretched and not flat anymore. Replacing it solved a problem that I spent years trying to solve since I did not use the motor very much. It all made sense after finding the problem.
It sounds like a thermal intermittent - starts acting up when it gets hot. I would spray the suspected components (ignition module and crank pickup) with freeze spray (or a canned air duster turned upside down) one at a time to see if the problem clears up. When using freeze spray on a big heavy component, give a few minutes for the cold to "sink in" before moving to the next component because heavy parts tend to heat up slowly and cool down internally slowly.
One more possibility is an obstructed tank vent. Easy to check, just loosen the cap when it starts to misfile.
Another easy check is a spark tester, basically a visible spark gap placed temporarily in series with the plug. If the checker stops flashing as the engine warms up something's wrong somewhere in the ignition system. hth,
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