I have a karcher 4.6 pressure washer that developed the usual fault.... drop in pressure and leaking from the cylinder head. I have replaced the cylinder head and the attached control chamber. The pressure is now good again and the leak is fixed. Unfortunately now when the trigger on the lance is released the the machine does not switch off automatically. I have bled the machine by letting water run through it

SOURCE: honda pressure washer cuts out when lance trigger released.

You most likely need to set your valve clearances. If it is hard to pull over or the recoil snaps back after starting your valves need to be set for sure. This may not be the entire problem but it is a good start. Also change the spark plug, a bad plug will cause odd problems and is too cheap not to eliminate as a source. In addition check your fuel, pour a little out into a white container (I like spray paint can caps for this) and look at the color. If it is yellow to orange it is bad. You can pour it into you car, the car electronic sensors will compinsate for it quality but a small engine can't. Use a NGK BP6ES or BPR6ES plug.
You need:
Feeler gages
10mm open end wrench
14mm open end wrench
1 long shaft screw driver or rod
1 spark plug socket and ratchet

Remove the spark plug and the valve cover. Pull the recoil over slowly until the intake valve opens (intake is closes to the carb and it will go down when it opens). Now put your long screwdrive down the spark plug hole. Slow pull the engine over until the screwdriver handle is at it highest point, this is top dead center. It may take a few attempts. Now check your valve clearance between the rockers and the top of the valve. The intake should be .006 and the exhaust .008, both can be +/- .001. To adjust: hold the large nut with the 14mm then loosen the small one with the 10mm. Lefty loosey. Now turn your large nut until you get the correct setting and then tighten the small one the lock it in. It may take a few attempts and alway reverify your setting after you lock it down. Once done, find top dead center again and reverify. Now your valves are set.

SOURCE: no high pressure no leaks motor runs off and on when trigger is released

check filter in wand and clean out nozzle hole for starters

SOURCE: Karcher K3.91 compressor run for a few seconds every 10-15 second

They have a problem. I removed both valves and lubed with lithium grease as I was told. Unit is now working for how long who knowes.

SOURCE: When I switch my karcher k4.97 washer on it just

you have an internal crack in the out let head of the machine very comon in thease machine and not expencive to fix

SOURCE: When I switch my karcher k4.97 washer on it just

Well, first the easy bit... the pulsation is caused by the leak, because the machine can't hold pressure when the trigger is released.  The difficult bit is: what to do about it?  Unfortunately, there are several different places where the leak can occur and, even with the cover off, it's usually impossible to be sure exactly where the water's coming from.  In the repair shop where I work, we almost always replace every part that might be causing trouble without even trying to identify the exact source.  To do otherwise will rack the labour costs up so high as to render the job uneconomic.  We reckon that if you can't complete the job in 45 minutes it's not worth it.  Sad really, but such are the realities of our economics.
Anyway, unless you are a really competent handy-person, this is probably a job you won't want to tackle for yourself.  So your choice is either to live with it (and do whatever you can to limit the pulsating time - keep it working or switch it off immediately) or take it to a repair shop.  If you are in the UK, Karcher head office can direct you to a repairer, and I guess the same applies in many other countries too.
If you do want to have a go at it yourself, post another message here and I'll give you as much guidance as I can (but there will be a limit to the amount of detail I can provide, as I can't get in to writing a complete service manual!).