Intake deposit

[Guest]

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.

[Guest]

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.

[Guest]

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. 

[A64Pilot]

Any recip engine four or two stroke has what’s caused reversion, that is a little reverse airflow at times. It’s easily felt by removing the air filter on a lawnmower or whatever and putting your hand by the carb and feeling the pulsing air flow out, as it’s a carbureted motor it’s likely your hand will get some fuel on it too, so carbon etc build up on intake valves is normal.

Valve timing is interesting with valves opening when it doesn’t seem logical, but a lot of it’s based on the air having inertia meaning that air continues to flow even when you would think it’s not.

It’s even possible with valve timing to get to greater than 100% volumetric efficiency without any kind of compressor, rare and only at certain sweet spots of load and RPM, but possible.

Edited November 28, 2024 by A64Pilot