Intake deposit

[Kerrville]

My A&P friend and I borescoped the pistons on my lycoming io360. Two of the pistons had some carbon deposit on the intake valve and piston crown. The exhaust valves looked relatively normal. Usually I hear a discussion of exhaust valve isssues, but what is the likely cause of intake valve deposits?  

[Andy95W]

Lead in the fuel, plus normal carbon deposits.

[EricJ]

Carbon deposits are not unusual.   If the compression is good it's not a big deal.     If the compression isn't good, carbon that's keeping the valve from closing completely can often be treated by "staking" the valve, which is basically just giving it a few whacks with a mallet with the rocker removed to try to break the carbon loose.   That's often effective and is pretty easy to do.

If you want to minimize deposits, run it a bit closer to peak EGT and/or at higher power levels.   Leaning on the ground helps, too.   Whatever it takes to get it closer to burning more of the fuel charge and not running too cool.

[Kerrville]

I do a lot of pattern work. That must be the culprit. Low power…

[N201MKTurbo]

The lead bromide is a gas like steam. It condenses on cold things. Like your pistons and valves if they are not hot enough to vaporize it. It will trap any carbon That happens to be there. 
 

If you run hot and LOP, it will eventually vaporize all that and blow it out the exhaust. In reality, it is harmless, so don’t lose any sleep over it.

[Kerrville]

I try to do some good high power lop runs now and then, but the majority of my flying is done at very low power settings. I keep it very lean, but obviously it doesn’t help too much.  Glad to know it is not a huge worry. I have good compressions…  I know our planes aren’t really meant for the kind of flying I mostly do, but I really like it for the occasional trip.