Lapping Valves

This requires some special training. These aren't like your typical auto valves or a Continental or Lycoming engine.  

You don't lap these valves they are hardened. You can clean them and if they are sticking in the valve guides and have a lot of debris accumulation and you have not been trained to clean them and don't have the special tools then you'd be far better off sending the heads to a service center so you don't ruin them. If you ruin them it will get expensive. 

Thanks! I have a Lycoming O-235 in my other airplane and lapping valves is not an uncommon process but also not something that is needed often. I see in the MMH for the 912/914 (ed. 2 Nov 2025) on PDF page 226 (72-30-00 Page 16) Step 4 it mentions lapping the valves to touch up small variances with "valve lapping paste" but there's no mention of a metric like 25% on the pressure differential in the manual. Oddly, in step 2 on that same page it mentions "touch up paste" but nowhere else is "touch up paste" mentioned in the MMH or what that is. "Valve Lapping Paste" is described on page 63 in the PDF (00-00-00 Page 43).

Have you gone out and run it for 10-15 minutes and then tried to do another compression check to make sure it isn't the rings?

Your valve issue could be either something trapped under the valve lip or possibly the valve guide sleeve needs cleaning out. Your valve top can be cleaned. I was shown years ago in Nassau to use a soda blaster like a sand blaster. The soda doesn't do any damage and cleans the valve accumulation off.

If running the engine and doing another compression test fails the I'd pull the heads and look under the valve edges and make sure the valve smoothly can move up and down.

No my engine is running fine, all at or above 78/80...Someone was telling me that they had to lap their 912 valves because of low compression and that the manual said to do it. I didn't believe them that the manual said this, that's why I'm curious.

If your engine is running normal leave it alone. Don't open a door you may regret.