Hey all... I have 2003 CH25 that has been oil fouling the left plug.  Several months ago I did a leak down test and the left cylinder was leaking through the valve guide.  I pulled the head and the valve guide was worn (the valve stem wobbled), so I replaced both heads (valves, etc)  as the mower has several thousand hours on it.  Put it all back together and it still fouls the left plug.  I thought maybe the rings were bad on the left cylinder and I just missed it because the valve was leaking so much.  So... I did a compression test:  left cylinder is 170 psi and right is 165 psi.  Did a leak down test and both cylinders well in the green at 20% each.  All tests were done cold.
 
I've read that the reed valve can cause plugs to oil-foul, so I pulled the cover and the reed valve "looks" fine to me.  I don't have a manometer or vacuum gauge that will go low enough to test the reed valve, so I just ordered another reed valve, gasket, and the rubber hose that goes to the breather.  
 
The reed valve should be here in a few days, but I was hoping that someone here could give me some advice.   

You can have good compression (good compression rings) but have a worn/ carboned up oil ring.
 
That would allow oil to pass into the combustion chamber...
 
When you removed that head, was there excess oil deposits inside the combustion chamber?  
 
 
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Scott... Here's a pic of that head.  I don't think I've ever seen just the oil ring being bad or stuck.  Does this happen often?  I guess I could try some Marvel Mystery Oil.  Unless you have some better advice?  Thanks.   

No pictures posted but yes I've seen good compression but the oil ring(s) are worn...
 
If carboned up MM oil couldn't hurt to soak a couple of days but the compression rings would get carboned up first, (closest to the combustion chamber) but you have good compression. 
That tells me those rings are very likely fine.  Oils getting past them due to the likely worn
oil ring.(IME)
 
I gather IF there was an valve oil seal(o-ring) for the valve(s), that / those got replaced...
 
With the high hours you have on the machine, that wouldn't surprise me..
 
As you were able to R&R both heads, I'd suspect you could pull the engine, and 
ultimately the pistons out for a re-ring job..  
 
You likely could get by with just hone of the cylinder as your compression is good..
 
Pic of my 18 hp Briggs,(valve seat let loose, broke the valve, destroyed the head and piston) after honing for re-assembly:
 
 

Also, 15-20% leak down is about the acceptable limit. 
 
Did you have any leakage into the crankcase or just the exhaust?
 

Here is the pic... I hope. 

It sounds like you’ve done a thorough job diagnosing the issue so far. With good compression and leak-down results, the rings are likely fine, so your focus on the reed valve makes sense. If it’s not functioning properly, it could definitely be the culprit for oil fouling that left plug. Since you’ve already ordered the reed valve and related parts, replacing them is a smart next step. Hopefully, that resolves the issue, but if not, it might be worth rechecking the new head installation or considering if there’s an issue with the crankcase ventilation. Good luck, and let us know how it turns out!

One more time... hopefully the pic will upload this time.  

Thanks for the advice, SamualYu.  I hope the reed valve does fix it, but as the current one looks ok, I'm not confident it will.  But fingers crossed.  It's very strange to me... I can't ever remember a fouling plug having good compression and leak-down test.  20% on a leak-down test is about as good as it gets (IMO).  When I replaced the heads, I installed new valves, seals, push rods, head gaskets... pretty much everything.  I can't figure out how oil can get into the cylinder other than maybe the reed valve.  And were it the reed valve... seems a little strange that only the left cylinder would foul plugs.  But sometimes intake manifolds can favor a cylinder. 

SamualYu
It sounds like you’ve done a thorough job diagnosing the issue so far. With good compression and leak-down results, the rings are likely fine, so your focus on the reed valve makes sense. If it’s not functioning properly, it could definitely be the culprit for oil fouling that left plug. Since you’ve already ordered the reed valve and related parts, replacing them is a smart next step. Hopefully, that resolves the issue, but if not, it might be worth rechecking the new head installation or considering if there’s an issue with the crankcase ventilation. Good luck, and let us know how it turns out!

The valve seats are filthy..  Is that pic before you used valve lapping compound and lapped the valves?
 
 
In any event, where I circled, is there a hole there, in the head?