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115 HP runs rough

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  • 115 HP runs rough

    115 HP Yamaha 2006 fuel injected 4-stroke

    At Trinity lake, 3000 ft elevation. The motor ran excellent for about two hours before it sounded like the motor was being choked….fuel rich. Like the choke plate was partially closed….if it had a choke plate. We ran down the lake about 10 miles. The engine ran labored and rough when starting to plane. Semi smooth when planed out. Once I stopped, put the engine in neutral and rev’d to about 3000 rpm. I put it in gear and took off. The engine seemed to be “less fuel rich” after doing this…for a while. The rough, “fuel rich” operation did not subside. We travel 10 miles back to the dock at medium speed…engine was still operating rough. Like it was running on three cylinders when under load, very rough and lacking power. Idle, slow speed was mostly smooth, but still rough.

    Before the above happened, I prepared the boat for the first run of the season. I put fresh gas in the tank and added Sea Foam at the recommended mix. One oz per gallon. I removed and cleaned the gas filter bowl and reinstalled the filter (not new). Nothing more than fine silt in the bottom of the bowl. The motor has run great since new.

    After the rough running I installed new plugs. No change. The old plugs when pulled ......#1 was dark, #2 was gray, #3 was gray, #4 was semi dark. None were wet.

    Any ideas? My motor is at a certified Yamaha service shop. They have had the motor for a couple of weeks and are not certain what the problem is.

    I truly want to help the service shop if I can. Running a 115 Yamaha down the lake and fishing is one of the special things in life.

  • #2
    I want to give feedback as to what caused the engine to suddenly run rough. The certified Yamaha shop checked everything; compression, fuel pressure, electrical, spark, hoses, timing, etc. The answer was that California gas tends to gel even with the red Stabil added. Adding Sea Foam caused the globs of gel to soften and flow into the injectors plugging the injectors. Replacing the four injectors cured the problem. The northern CA Yamaha rep helped with the resolution. My advice is use a good quality gas treatment designed for ethanol gas. Thank you to the shop and to Yamaha. Yamaha continues to be the best outboard out there.

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