Need to replace the pull cord assembly, because something that tension the spring is missing. Tried substitute cord but found after reassembly there was no tention on the cord something is missing.

Remove for the n-th time the pull start assembly. Confirm the engine will in fact rotate by hand. Turn the flywheel a full turn or more ensuring that the engine is free. You should expect to feel some resistance from the compression cycle.

Holding the hand crank assembly attempt to pull the pull-cord and observe that the hub rotates. When released the spring returns the cord and handle into the wound position.

If the cord will not pull or operate as expected then remove and reassemble the cord into the pulley making sure that the knot retaining the cord is small enough and does not interfere with the operation. U-tube has several videos showing how to assemble the cord to the hub.

When fitting any pull start spring, you must first pre wind the spring. Once the spring has been at least 1/2 way wound up or tensioned, wrap the pull rope around the spool. When you assemble it and pull the cord, the spring should wind up further to its fully wound position and now retract as normal. If you don't prewind the spring, there will be little or no tension to rewind the rope.

How did any of the advice given help what was asked? How to you replace the pull cord on this pressure washer?

I assume that you are speaking of a pull start mechanism.
I can not imagine how to put a put pull starter back on "wound tight". Your pulling on the cord tightens a spring which pulls the cord back in. They go back together with the cord fully wound onto the spool.
Usually, I only take these apart to replace the cord. Wind the new cord onto the spool without handle. Reassemble, then pull out on the cord a little when attaching the handle to the cord. This is the only way to get tension onto the spring at the fully retracted position because the catch for the spring is on the machine and the spring itself is on the underside what you are assembling.
If you do not wish to shorten the cord or replace it, then remove the handle, assemble enough to catch the spring, wind one turn tighter then feed rope through hole and reattach handle. Easier said than done.
If your spring is broken, then you will be needing a new one.
:o)

is it the pull rope? if so you will need to remove the four to six bolts around the shroud. once shroud is off turn up side down, remove old rope. you will need to wind it up and if you feel tension then the spring is still good, if not then it is broken. if the spring is good wind it a few times, do not let go, and feed cord back through, tie a knot and burn the end, let go of rope and it will self wind, pull a few times to ensure the knot is going to hold. then reassemble, hope this helped Tim

Take off the whole assembly, find and replace the spring, manuually wind up the reel until it is loaded with tension in the direction that will retract the cord. THEN put new cord in through the feedhole then the assembly, then tie a small figure eight knot at the very end of the pull cord, pull it tight so it stays in its recess and then let the cord retract by releasing the tension. - you now have a fully charged retractor assembly. put it on tightly with the three 10mm bolts, reattach the plastic cover and git mowin.

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Remove cowling to acces pull start assembly. Have new cord on hand. Remove old cord fragments. Wind rotor 4-5 revolutions to tension spring and hold while threading new cord into slot. Knot new cord and slowly let it wind back into assembly.

Good job on the cord replacement. Most people take the cover off the spring and then find out it is nearly impossible to get the spring tension set. I have personally repaired dozens of these cords that have been messed up by owners that did that.
If anyone has this problem, just do what this post says...remove the assembly and turn the pulley until you can get to the part of the cord that is broken...grab the end that is broken after you get it out of the opening with a vise grip and pull the cord out fully and lock the recoil spring with a vise or vise grip (but don't crush it)...then replace the cord and hold the end of the cord pull fully extended and remove the lock and allow the spring to recoil...and reinstall the assembly.
Simple, but THINK before you take the housing apart. It is REALLY TOUGH to reset spring tension properly.

Remove the housing covering the clutch. The cord spool can then be accessed. Replace the cord. Be careful that you maintain the proper spool spring tension. Check operation before reassembling