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Rainman Technology Pty Ltd
49 Orchard Road
Brookvale, NSW 2100


It is extremely rare that a Rainman petrol (gasoline) powered pressure supply unit (PSU) will require the Honda engine to be replaced. Honda makes the best small engines, but may eventually wear out and require replacement.

When you purchase the Honda motor from Rainman, it will arrive as a sub-assembly with the quieter exhaust option and air flow cowlings mounted at the factory.

Reasonable technical skill is required to perform this procedure. If you would like assistance with this procedure, please contact Rainman or one of our many dealers around the world.

Preparation – Before You Begin

Tools Required

A number of tools will be required to execute this procedure.

  • 5mm Allen key
  • 3/16″ Allen key
  • 8mm socket or spanner
  • 10mm spanner
  • 13mm spanner
  • Phillips head screwdriver (PH2)
  • Medium strength thread locker (eg. 243 Loctite)

Prepare The PSU

Before beginning the procedure to change the Honda engine, three steps are required to prepare your PSU.

  • Drain oil from Honda engine
  • Drain fuel from Honda engine
  • Replace travel plug into black HP pump

Procedure To Remove Honda Engine From PSU

The PSU is powered by a Honda GXH50 engine with a auto-tensioned pulley system to drive the impeller lift pump and the high pressure pump. It is all mounted onto a base plate inside the shell. The removal procedure consists of sliding out the pump / motor assembly, disconnecting the engine electrical cables, disassembling the pulley system and removing the engine from the base plate.

gasoline desalinator

Remove Pump / Motor Assembly From PSU Case

  1. Using a 5mm Allen key, take off the belt guard by removing the two screws that secure it to the pulley structure. One of the screws is behind the belt guard with limited access.
rainman watermaker 50 belt guard attach
  1. On the prefilter side of the PSU there is stainless steel aperture cover with three phillips head screws securing it. Loosen the screws and separate the aperture cover from the blue case by sliding it higher on the white prefilter hose. Note that one screw is hidden behind the white hose in the photo.
  1. The white prefilter hose feeding into the aperture is secured with a pushfit fitting. Push the ring in towards the fitting and gently pull the hose out. A 13mm spanner inside the blue case may assist in pushing the ring in.
  1. The prefilter hose that feeds the brass lift pump inside the PSU case needs to be detached. Unscrew the nylon nut and disconnect the white prefilter hose.
  1. Detach the prefilter hose that connects the filter housing to the lift pump. Slide the hose so that it is mostly outside of the blue case. This will keep it out of the way when removing the pump / motor assembly.
  1. Using a 5mm Allen key, remove all four rubber feet from the bottom of the PSU.
  1. The white base plate has four vibration isolation mounts. The front right plug in front of the engine must be removed for the pump / motor assembly to slide out of the PSU case. Remove the mount plug from the top of the base plate. Then remove the rubber spacer from underneath the base plate. Note that these are the same rubber spacers as the feet under the PSU.
  1. Partially slide the pump / motor assembly out of the PSU case so that the wires on the right side of the engine are exposed for disconnection.

Disconnect Electrical Cables

  1. Using an 8mm socket or spanner, remove the nut securing grounding wire to the engine chassis. Detach the wire.
rainman watermaker kill switch
  1. Unplug the kill switch connector.
rainman watermaker honda engine kill switch
rainman watermaker honda engine kill switch
  1. Using the 10mm spanner and 5mm Allen key, remove the low oil cutoff capacitor that is attached to the base plate.
  1. Fully remove the pump / motor assembly by holding the base plate and sliding it out of the PSU case.

Remove Engine Sub-Assembly From Base Plate

The white base plate is the main component that securely holds the Honda engine, both pumps and the pulley system in alignment.

  1. Using a 5mm Allen key, remove the tension screw and spring from the lift pump tension arm.
  1. Using a 3/16″ Allen key, remove the high tensile screw that secures the pulley bridge to the Honda engine. Expect this bolt to be tight.
    Important: Ensure this high tensile screw does not get mixed with other screws for the reinstallation process.
  1. Tilt the pump / motor assembly backwards and use a 5mm Allen key to remove the four screws that secure the engine sub-assembly to the base plate.
  1. The Honda engine sub-assembly can now be removed by sliding it away from the untensioned drive belt.

Procedure To Reassemble the PSU

The majority of the reassembly process is simply the reverse of the disassembly. There are a few points of differentiation, so the entire process is fully detailed again.

Mount Engine Sub-Assembly Onto Base Plate

  1. Position the Honda engine sub-assembly onto the base plate while aligning the small pulley into the drive belt. Use your fingers to pull on the drive belt for positioning.
  1. Tilt the Honda engine sub-assembly and base plate forward to loosely secure the four screws with thread locker applied. At this point, the engine should be mounted on the base plate, but still have some looseness for fine positioning.
  1. Align the Honda engine sub-assembly to the pulley bridge and use a 3/16″ Allen key to firmly install the high tensile bridge screw with thread locker applied.
    Important: The hole for the screw in the pulley bridge is slightly horizontally elongated. Position the screw to be as close to the centre of this hole as possible.
  1. Using a 5mm Allen key, firmly tighten the four base plate screws that were loosely installed in step 18 above.
  1. Using a 5mm Allen key, install the tension screw and spring into the lift pump tension arm with thread locker applied. Tighten it down until the screw bottom touches the top of the black high pressure pump. Then add 1/2 of a rotation to set the screw.

Honda Electrical Connections

  1. Using 10mm spanner and 5mm Allen key, attach the low oil cutoff capacitor to the base plate.
  1. Partially position the pump / motor assembly into the PSU case in preparation for reconnecting wiring to the Honda engine. Ensure the quick connect high pressure fitting slots through the aperture on the left side of the PSU case.
  1. Plug in the kill switch connector.
rainman watermaker honda engine kill switch
rainman watermaker honda engine kill switch
  1. Using an 8mm socket or spanner, connect the grounding wire to the Honda engine chassis.
rainman watermaker kill switch

Install Pump / Motor Assembly Into PSU Case

  1. Slide the pump / motor assembly back so it is positioned correctly in the blue case. The three rubber vibration isolation mounts under the base plate should be positioned over the corresponding holes on the bottom of the PSU case.
  1. Slot the rubber spacer underneath the front right corner of the base plate. Replace the mount plug.
  1. Using a 5mm Allen key, fasten all four rubber feet back onto the bottom of the PSU case.
  1. Using a 5mm Allen key, install the belt guard by replacing the two screws with thread locker applied.
rainman watermaker 50 belt guard preparation
rainman watermaker 50 belt guard attach

Set Honda Throttle

The engine is designed for fixed throttle operation which is optimised for the power requirements, gear ratio, high pressure pump, and required flow rate of the system. Once set, the throttle should not be changed. The most accurate way to set the throttle is by using a third party tachometer. Tachometers are readily available in most parts of the world. If a tachometer is unavailable, it is possible to use flow rates to set the throttle, but this is less convenient and less accurate.

Important: Before beginning this section, ensure you have put oil and fuel into the Honda engine. Also ensure the breather plug is replaced on the black high pressure pump. Use a bucket of about 10 litres of non-chlorinated water to run the PSU for setting the throttle. Put all three hoses (intake, brine discharge, product water) into the same bucket so it recirculates easily.

  1. Attach the alligator clip from the tachometer to the spark plug cable of the Honda engine.
  1. The engine throttle is the rectangular bar and moves up and down. Start the engine and keep the RO unit unpressured (valve fully anti-clockwise position). Use the throttle adjustment screw to set the engine to run at approximately 5,450 rpm.When pressurised to 55 bar (800 psi), the tachometer should read about 5,000rpm.
rainman watermaker throttle
rainman watermaker unpressurised tachometer reading

Alternate method to set throttle:

If a tachometer is not available, the throttle can be set using flow rates. With the engine running and the RO unit unpressurised (valve fully anti-clockwise), the flow rate should be 7.9 – 8.1 litres per minute (~2.1 gpm). Adjust the throttle until this flow rate is achieved and continue.

  1. Using a 10mm spanner, lock the throttle into position by tightening the nut underneath the carburettor.
rainman watermaker throttle