Briggs and statton 14.5hp

Good day ,
I bought a John Deere X120 with a 14.5hp intek Briggs.
There was a slight knock in the engine which I thought was the big end bearing.otherwise it was running great.
After opening it up I found that it was the balancing weight links that was worn.
Replaced the links,connecting rod ,rings and all seals.
Once I started it up it sounded as if the exhaust valve was not closing.Suspect the decompression device.
open up again and found that the spring on the decompression device was broken.
Bought a new cam shaft .Install but still the same.
Replaced the exhaust valve--Properly seated---Still the same.
I do not know anymore .Please help.
As I have said it sound like the Exhaust valve is open when it ignites. Exhaust also heats up very quickly to very hot.
What else can be wrong???? Timing marks and valve clearances checked multiple times.

Because you are there with the mower in front of you & I am here with a screen in front of me, please expand your description of "Sounds like the exhaust valve is open when firing "

It is not uncommon to get a bit of carbon or other debris stuck under the exhaust valve however that usually burns off pretty quickly.
Apart from that a too rich mixture will still be burning when the exhaust opens, or unburned fuel burns in the muffler when it is mixed with oxygen from the air.
A spark that is too late ( timing key broken or magneto coil gap too big ) will also have the charge burning when the exhaust valve is open.
Not enough valve lash can also have the valve open
And because it is generally the last thing done that courses problems, are you dead sure the cam you bought was correct and that you have installed it correctly.

It will also be of enormous help if you post the full engine numbers.
Briggs have made a lot of different 14.5 Hp engines.

Good day,
Thanks for the reply.
Found that the timing key was sheared off.
Replaced the key and engine was running great.
Again many thanks for the help!!!!!!!!!!!!

Excellent.
Good call on your behalf, the engine was in fact firing when the valve was open.
When you replaced it did you dress the edge of the slot to get rid of the little burr ?
If it goes out of time again make sure the edges of both the crank & the flywheel have no burrs.
The taper is what holds the flywheel in position, not the key.
The key is just there to make sure the flywheel is in the right place when you tighten the flywheel nut/bolt.