Any suggestions on what might ail this Honda?

On a punt, I bought a mower principally for spares but with hope that it was a runner as described by the owner. It is an alloy body re-powered with a Honda GXV120. My son was doing business down the line and collected it for me and delivered it last Friday.
The first thing was to check the oil and spark; it all checked out well, all it required was the oil top up. So on with a starter from my other Honda.
On the second pull it fired up with some small smoke for a small fraction of time. I shut it down and rested it then started it again, easy, no problem. This time there was hardly any smoke and it ran well.
On the strength of that I decided to give it a good clean. I removed the cover and gave it a kero wash with an old brush to loosen up the gunge, a good thorough water blast and blow down with the air.
All went well, it came up a treat.
Then I tried to start.. no joy.
On pulling the starter this time the mower was lifting as if the motor was locked. It would turn but every often it locked in this fashion. I pulled the plug and wound it over; free as a bird and no locking.
Retried and the same result.
There is spark.
Before posting this thread, this morning I removed the plug and gave it a few pulls and still nothing coming that would point to a hydraulic problem. Yet on replacing the plug it behaves as if there is.
Do I need to remove the plug and tip the motor forward to ensure the cylinder is clear? Is this advisable?

Thanks for any insight or direction that can help with this matter.

Just a guess but you may want to check your valves adjustment.

I overlooked to mention that while water blasting, the air filter cover was removed and I had stuffed a rag into the airfilter intake. This should have prevented water ingress but maybe not....

Even though I had not experienced this locking up prior to cleaning the mower it could explain why the previous owners starter was toast. I will give him a call.

Boobala said:

Just a guess but you may want to check your valves adjustment.

Click to expand...

It is a worthy suggestion Boobala, thanks.
0.004" and 0.006" right? While I am at it I'll check the compression.

Just got off the phone with the previous owner. The problem was with a broken starter spring and the shop scrapped the complete starter - morons.
So, on to further investigation.
One question before I go; the spark plug is a Champion RC12YC which doesn't appear to cross reference. Is this plug suitable for the GXV120?

Update:
Before checking out the valve lash I pulled the HT lead and turned the motor over by hand just past compression. One decent pull on the starter and away; I did this twice to confirm my suspicion. Those previous starts before water-blasting must have coincided with the past TDC crank position. Blind luck.
So off with the valve cover. The valve lash was out something horrible, maybe 0.015" on the exhaust, the inlet wasn't far behind. A bit of fiddling to get the correct settings then back together and a test or two.
Success!!
Thanks for the advice and insight on this problem Boobala. That out of limit valve lash was enough to upset the decompressor?
Or is my thinking arse-about-face?
How far does the decompressor lift the valve off its seat?

The engine cowling paint was rubbish with a lot of underlying rust so into the caustic and on into the electrolysis.
Next is some prep and paint.
So while I prepare, can anyone suggest where NOS or replica decals can be purchased for this GXV120?

Thanks.