Question about primer start system on Mercury 2003 4stroke 90hp

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Aug 16, 2018
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Question about a primer start system on a 2003 Mercury 4stroke 90hp (it’s really a Yamaha powerhead). I purchased the boat about 4 years ago and it always started with just a touch of the key. In January, my fuel pump died. Replaced the pump – the motor started but ran like crap. The thought was when the motor failed, some of the internal components of the pump broke apart and made their way to the carbs. Rebuilt the carbs and the motor ran great. The only issue was if the motor was cold – it was a pita to get started. At the time, checked compression and spark -- everything was good.

My starting procedure is:
  • Pump the fuel bulb
  • Move throttle to mid throttle position, in neutral position
  • Depress ignition twice to engage choke
  • Turn key to start
After reading about the primer start on Yamaha’s –I’m not sure pressing the ignition key to engage the choke does anything.

My understanding is when the key is turned to engage the starter and the flywheel is turning, the wires to the primer start deliver the current. As the flywheel continues rotating the wires to the primer start continue to deliver the current which melts a wax bead allowing the pin/needle on the end of the primer start to retract. This results in a leaner mixture to the motor as the motor warms up. When starting a cold motor, the fuel to the motor is automatically enriched. And as the motor warms, the primer start unit automatically adjust to a leaner fuel mixture.

There seems to be a love/hate relationship dealing with the primer start device; some boaters love it, others loath it and the difficult starting problems.

I’d like to confirm that depressing the ignition does absolutely nothing.

If the motor does not start and you continue to crank the motor, is it possible the wax pellet has melted and you’re trying to start the engine with a lean fuel mixture.

What pre start sequence do you find to work the best. It’s been suggested to keep the throttle at the idle position when first trying to start the motor. Given this primer start unit, is it possible to flood the motor?

My next step is to remove the carbs and troubleshoot the two primer start units.

Any help appreciated.

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