I have a karcher Hds750 pressure washer and the heater quit.

SOURCE: trouble with Karcher Pressure washer

Dear Pgarciax,
It is important that you must have mentioned the model number of your Karcher. Just the same I can surely say that your machine's fault lies in the "SPILL VALVE"
When the gun is open and pressure is being produced, the spill valve is in a relaxed position. Also it has at the bottom a valve, which when pressure is produced it is closed. If this does not close due to dirt or corrosion, the pessure goes down. Now when you close the gun,pressure forces the spill valve down to close the electrical system. Now if the "o" rings of the spill valve are damaged, the machine will start going on and off while in stand-by. So if you know how to locate the Spill Valve, all one has to do is change it with a new one.
In some Karcher machines there are 2 spill valves. One regulates pressure and the other one is for switching off the machine when the gun is closed.
Low pressure could also be due to faulty suction or delivery valves. Most probably the suction valves.
But when saying that there are short interruptions while in stand by, I stick to the fact that the spill valve needs renewing.
Regards,
MG.

SOURCE: Katcher pressure washer w/ 5hp Honda Engine

pour a bottle of carburettor or fuel injector cleaner in the fuel tank, replace the spark plug and set the spark pluglug gap at 0.7mm. Replace the air filter element. Also change engine oil if necessary (mobil 10w30 synthetic.

SOURCE: karcher 2600 psi pressure washer wont stay running

Remove and clean the carburator. Sound line a stuck needle on the float. 

SOURCE: my karcher pressure washer has lost its water power

I have the same problem. With mine I have to remove the tip that I am using then pull the trigger and then replace the tip and pull and pressure should return. I believe that one of my valves is sticking causing me to reprime the pump frequently. I am trying to clean or replace my valves to fix the problem.

I hope this helps.

SOURCE: Unloader Valve - Karcher Pressure Washer

where is the unloader valve locaed on a Briggs Stratton pressure washer?