Yamaha 115HP saltwater series V4 two stroke missing at idle

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Misfire & idle problem

I had a problem with my Yamaha V4 115CETO Saltwater outboard missing & coughing at idle and surging from 500RPM to 800RPM then at times stalling out and being hard to re-start.

After the previous owner had spent $600 on a service, carby clean & tune-up, twice, the problem soon came back to haunt me as the new owner.

After reading the online version of the Yamaha workshop manual I thought everything looked good with the air / fuel mixture screw carby settings at 5/8 turn out for the 115C until I got to page 154 and read that 115CETO base setting on the air mixture screws was not 5/8 of a turn out but 1-1/4 to 1-1/2, the 115C & 115CETO are two different engines in settings and not to be confused.

AH-HA I thought, problem solved, the two motors are drastically different when it comes to air fuel mix screw settings, that fixed the surging problem but the missing & coughing at idle problem remained I found after a few minutes into an on water test.

I pulled the four spark plugs when I got home from the lake and they looked very black, glossy & oily, but it is a two stroke, I thought that the plugs may be carbon & oil fouling and shorting down the ceramic nose of the electrode causing the miss so I went to the next hotter range of NGK spark plugs.

Problem fixed, the Yamaha V4 ticks along now like a Swiss watch for hours on end at a steady 500RPM without skipping a beat if need be and the plugs look clean and the correct colour of tan / grey even after a high speed 30 minute run back to the ramp.

It makes a huge difference one heat range hotter of $15 NGK sparkplugs & the correct air fuel mixture, I was just about to spend $20K on a new 4 stroke motor.
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