Kohler Command CH25 Fouling Left Plug

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.   

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.   

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

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!

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!

Scott, that head has been sitting out under my shop's lean-to for about 6 months, so yeah, it's filthy.  As I said, I replaced both heads... the pic is of the old head that had the loose exhaust valve guide.  I just kept them to practice welding on.  That's not a hole, I think maybe a tiny spider web. On these Kohler Command Pro heads, the valve guides are pressed in at the factory while the head is really hot... then it shrinks down to clamp them in.  So you can't replace the valve guides... just replace the whole head.  I have not removed the new heads as I feel they are probably fine as the leak-down test is good.  Thanks for the reply.