Kawasaki STX-15F/STX-12F Hot Start button mount

Summary

The STX-15F and 12F have a well documented issue where they have a hard time starting after you get them hot and let them sit for a while before trying to start again.  The issue is that when sitting the water slowly drains out of the exhaust manifold.  This results in the exhaust manifold temperature sensor reading the ambient temperature of the manifold instead of the water temperature inside the water jackets in the exhaust.  The will then cause the ECU to think the engine/water is much hotter than it actually is and will not provide enough fuel to start the ski.  The fix is to install a normally closed momentary contact switch in parallel with a resistor to give the ECU a lower temperature reading momentarily so it provides enough fuel to start the ski.  Once running you release the button and the sensor reads normally.

Parts Needed

• Normally Closed momentary contact switch: https://www.amazon.com/dp/B00KJXBELC?ref=ppx_yo2ov_dt_b_fed_asin_title

• For 15F a 100k Ohm 1/4 watt resistor.  For 12F a 1M Ohm 1/4 watt resistor

• 2x Stainless Steel M6x1.0 bolts 35mm in length

• 2x Stainless Steel M6X1.0 nuts

• 4x Stainless Steel M6 or 1/4" washers

• Roughly 10' of tin coated copper wire

• Sumitomo 6187-2171 connector and Sumitomo 6180-2181 connector.  Ready made connectors can be purchased on eBay which greatly simplifies this part. https://www.ebay.com/itm/395512073437

• Heat shrink tubing of various sizes

• Plastic wire protector (optional)

• Stick on zip tie cable mounts

Tools Needed

• Soldering iron and solder

• Heat gun

• wire strippers

• Basic hand tools

Printing

• Use a high temperature material like ASA, ABS, or PETG.  It gets hot inside the hull.

• Print with enough walls that  the main plate prints solid.

Assembly

1. Cut 2x 5' lengths of wire

2. Strip both ends of the wire

3. On one end of the wire install them into the screw terminals on the momentary contact switch

4. Install you resistor in parallel with 2 wires going to the switch.

5. Use heat shrink tubing on the switch side to seal everything up.

6. Install the switch into the bracket and tighten down the nut.

7. On the Sumitomo connectors slide a piece of heat shrink tubing over one of the red wires and then solder it to the red wire on the other connector and then use your heat gun to heat shrink the tubing over the solder joint.

8. Slide some heat shrink tubing and over one of the wires going to the switch and then solder it to one of the 2 Sumitomo connectors and do your heat shrink tubing on the solder joint.

9. Repeat step 8 for the other wire going to the switch.

10. Slide your split wire protector over the 2 wires going to the switch.

Installation

1. Remove the glove box.

2. Loosen the bolt closest to the stern of the jet ski on the starboard mirror mount.

3. Slide bracket in and over all 3 bolts that stick out on the underside of the mirror bracket inside the hull.

4. Tighten down the bolt you loosened in step 1.

5. One at a time remove the other 2 bolts on the bracket replacing each one with the 35mm M6x1.0 bolt.  As you bolt these down place the nut and washer on the underside to help you get it started.  If you tighten these all of the way down before putting the nuts and washers on it will be nearly impossible to get the nuts on afterwards.

6. Tighten the 2 nuts with a 10mm wrench

7. Route your wiring so it goes onto the intake manifold side of the engine

8. Connect the female Sumitomo connector to the exhaust manifold sensor

9. Connect the male Sumitomo connector to the wiring harness

10. Use stick on cable tie brackets to neatly tie of the wire to the switch so that it does not rub on anything or get pulled.

11. Re-install glove box.

12. Test run your ski to make sure there are no issues