fabric store foam is NOT safe to use. More than likely it will dissolve around the oil/gas

just clean the plug and put it back in without bending the tang

I watch the car shows. They don't spend a dime for a spark plug until they have to.
Get it running, if you have to remove the air filter, pull it. It's not gonna ruin it to run ten minutes without one. After you Prove it IS a working pressure washer, blow the old one out. No way it's actually dirty from use unless someone used it commercially.
My pressure washer is five years old and has under three hours of use...
Your kind words are appreciated.

Sounds like the spring for the governor might be too loose or too tight.

Most small engines have a large flap and arm assembly that are attached to the carb butterfly valve that control the air intake.

When the motor is started in the "start" position, the throttle arm spring pressure holds the butterfly closed.
After it starts and you put it in the run position, the governor springs then take over.
The flap (governor) catches the air coming from the spinning balance wheel (which also has your magneto to control your spark timing) and keeps the valve at a specific angle to keep the rpms steady.
If the engine slows down the spring tension increases and the butterfly  closes, choking the engine and adding more fuel.
If it speeds up the spring tension decreases to open the butterfly and let in more air, slowing down the engine.
Does it just quit or rev up and down?

Thank you OR.

Yesterday before looking at the manual, I could not have figured in my entire life how to drain old oil? Many of the bolts which I suspected are drain ones, were difficult to access. I used a syringe and sucked all the fluid, then tilted the engine completely to make sure it's completely empty. After looking at the manual, filling hole is also a drain hole too.

Now to your questions, if I put it on to run at the highest speed, it'll run for a few mins and will just shut off. A quick search tells me there's a possibility of vacuum(gas cap need to be loosened up or even removed) or there is less oil in the engine.

I just made sure that there's enough oil and I can see it's up to the 'Full' mark, going to test gas cap too.

When it runs, runs just fine and continue for a few mins.

Thanks for all the help and support.

I did not pull the tank off, just removed fuel supply line and clean that thoroughly, as well as tank with carb cleaner.

Removed float bowl, cleaned few holes(not sure what are they though? ) Also,  tried to clean all the dirt/debris/grime from carb and main pilot/jet, I think?

I am happy to report, I just tested it for almost 10 or so minutes with input water, as well as releasing pressure from the system. Only different thing I did was to lossen up the gas cap and it did the trick, go figure!

But most importantly, I am very very and very thankful to all of you guys, every single person who posted his valuable input.

For now, I can safely say it's fully functional and working like a charm!!!

Of course, I'll consult you guys again if I have trouble/issues.

Thanks again folks  

That's so funny and hilarious. Since, I just acquired a power wash and am watching a lot of youtube videos about it.

But let me ask you guys, what cleaner/solution should I be using to clean the deck and driveway? I am thinking, dish detergent, mixed with bleach, vinegar and some baking soda.

After power washing the deck, should I stain it too or just leave it for natural aging? At the moment, there is no stain or paint or anything.

Yeah, I know it can rip skin off and can be dangerous but my question is, is it not safe to use on a wooden deck?