They problem is, there is a pressure release valve internal to the pump. When it gets to a certain pressure it's supposed to bypass. This allows the engine to continue to run. If the engine is getting killed during operation, it means the bypass isn't doing it's job.

EDIT: Sorry, realized I answered correctly, but incompletely.

I believe everything is internal to the valving, meaning there's no real adjustment for it. If it isn't doing it's job, it means it isn't mechanically working correctly. Not much you can do for it (to my knowledge) other than replacing valve body/relief valve.

References:

• https://guttervacuumsystems.co.uk/2016/06/08/how-a-pressure-washer-unloader-valve-works/
• Cut-Away Valve Demonstration: https://www.youtube.com/watch?v=3PxPngJfmM0

How a pressure washer Unloader Valve works

A pressure washer unloader valve diverts the water flow through the bypass when the trigger on the gun is depressed. The Unloader valve is designed to respond to an increase in pressure or a change in water flow. The unloader valve can also be used for adjusting the pressure of your machine.

The trigger on the gun and unloader valve together make up a two-part valve that directs water into the bypass of the unloader and then back in to the water inlet side of the pump or alternatively back to a header tank. The trigger shuts off the water flow, causing the unloader valve to re-circulate the water back to the header tank or alternatively back to the inlet side of the pump. This is sending the water into bypass. Trapped pressure unloaders are opened by the increase in pressure when the trigger is released on the gun. Flow-actuated unloaders divert water to bypass when there is a sufficient drop or stoppage of water flow into the unloader.

How a trapped Pressure Unloader Works

A trapped pressure unloader valve opens when there is an increase in pressure, so when the trigger on the gun is released the pressure increases and opens the unloader valve sending the water back to a holding tank or back to the inlet side of the pump.

A trapped pressure unloader is a simple valve with a spring mechanism mounted in a Brass body. There is A channel within the body that gives water access to the back of a piston. When the trigger is released the pressure against the piston increases and pushes the spring open to allow the water to pass into the bypass. If the trigger is closed, then all of the water will flow into bypass and back to the header tank.

How a flow-actuated unloader works

The flow-actuated unloader responds to the water flow stopping, This will also create an increase in internal pressure. When the pressure-actuated unloader has the check valve in the discharge port, which helps divert water to press the piston down and open the ball valve allowing water to bypass, the flow-actuated unloader has an orifice.

When the trigger is depressed and the water flow stops, the system pressure is increased rapidly at all points in the system past the orifice. A small channel in from of the orifice allows the increased pressure to travel up the channel and in to the piston assembly, pushing the piston down. The rest of the water is sent through the bypass and back to the headder tank or pump inlet.

Overheating in Bypass:

A pressure washer with a bypass sent back to pump inlet should not be run in bypass mode continuously. When a pressure washer is in bypass mode the temperature of the water in the closed loop increases rapidly due to the friction in the pump, which increases the temperature of the water. It is not good for the pump to handle water over 140 degrees Fahrenheit 60 celsius. Occasional discharge at the gun replaces water in the loop and prevents pump damage from high temperatures. Or having the bypass return to a holding tank where the pump is always getting a fresh supply of cold water.

Using the Unloader Valve to Regulate Pressure:

Trapped Pressure and flow-actuated unloader valves can both be used to control the pressure of a machine by adjusting the tension on the spring that holds the piston in the valve. Tightening the spring on a flow-actuated unloader reduces the pressure and on a trapped pressure unloader tightening the spring increases the pressure.

When the adjustment bolt on a flow-actuated unloader is loosened, this causes the piston that controls the valve to rise, allowing more water through the gun and consequently a higher operating pressure.

When the adjusting bolt on a trapped pressure unloader is loosened, the tension on the spring is reduced and therefore causes the piston assembly tension to lessen meaning that it can open into bypass at lower pressures, allowing less water to be forced out of the gun and as a result a lower operating pressure.

Adjusting a trapped pressure unloader valve

Trapped pressure unloaders must always be adjusted with the pressure washer operating and the trigger pressed so the water is flowing through the gun. The initial adjusting position should be loose or screwed away from the body and with little tension on the spring.

Release the trigger to see if the unloader is working. Press the trigger, and allow the system to develop its operating pressure at that adjustment. Take note of the pressure gauge reading.

Press the trigger and tighten the adjusting bolt. Repeat until the desired pressure is achieved. Use the gauge to monitor the systems pressure rise with each adjustment.

All adjustments must be made with water flowing and pressure in the system. Release the trigger to check spike pressure. This adjustment procedure is exactly the opposite of the procedure for adjusting the flow actuated unloader.