Compression readings change from 160 psi to almost nothing, carbon under valve??

[200XMichigan]

Was trying to get this 150 started, got spark, got fuel. So I tested the compression. First reading was around 90 psi, sprayed some Sea Foam in to break loose the carbon. The original owner also kept the original spark plug, and it was covered in carbon. After spraying with Sea Foam, then some carb clean, into the spark plug hole, turning the engine over with the spark plug out, the compression got up to 160 psi. Went to start it didn't want to start, compression was back at 60 psi.

The only thing I can figure is that there is a loose piece of carbon getting under the valves causing them not to seal all the way. Any ideas on how to get it out without tearing the cylinder off? I have heard of people pouring water through with the engine already turning via the starter, and the spark plug out but I am a little hesitant to try that. Blowing out the cylinder with an air compressor, rotating the engine so I blew out the intake and exhaust valves. Any idea on how to get the carbon out or is it time for a tear down?

[MrGiggles]

I'm going to guess the Sea-foam was acting like oil and making the rings seal better. When you tried to start it most of it blew out the exhaust pipe and it stopped helping the rings seal. If you have a leaky valve you can take off the carb and intake tube and feel for any air leaking out during the compression stroke.

[Dirtcrasher]

IDK what the issue is?, maybe the valve is hanging up in the guide or is there a decompression involvement issue here??

I cannot be convinced that carbon is on a valve seat and sometimes hangs the valve up, and other times it allows them to seal properly.

More info needed

[200XMichigan]

I'm not sure what is causing I but got the thing and it fired up, smoked really bad, started hard, but ran. The oil was like black tar. Air filter pretty much non-existent. I drained the gas, changed the oil, cleaned the carb. Went to fire it up and it just sputters. It has good spark. So I checked the compression and it was only 90 psi. The results were pretty much never the same. I sprayed the into cylinder with carb cleaner then sea foam and cranked it over a few times. After a couple times doing this. I got 5 compressions tests in a row of 160 psi. Tried to start it, no start. Compression dropped to 30 psi. Didn't figure sea foam spray would be thick enough to increase the compression. There's no decompression mechanism. Held the air comp spray fitting into the spark plug hole and the air leaked out from the exhaust valve.

The guy never changed the spark plug and it was the most carbon fouled plug I have even seen. Just was guessing maybe some of the carbon broke loose and caught under one of the valves.

Probably just tear it down. Lap the valves call it good. Would have been quicker than all the messing with it today.

[Vealmonkey]

Compression tester is faulty? Not holding throttle open while testing? Holding the air compressor valve by hand to head is not a definitive test, you could have had either valve slightly open. Valve stem bent?

[200XMichigan]

I tested with my finger and when the compression tester is reading more PSI, there's a lot of pressure on the spark plug hole. I'm really thinking with as much carbon that was on the spark plug its just jamming up the valve. When I held the air comp to the spark plug hole it would turn the engine until it locked into a position with what seemed like both valves closed. Also I had the valve cover off so I could watch what the valves were doing.

Think its just tear down time.

[Erics350x]

Spraying any liquid in the cylinder will give you a false high reading. Its time for a rebuild

[MrGiggles]

I tested with my finger and when the compression tester is reading more PSI, there's a lot of pressure on the spark plug hole. I'm really thinking with as much carbon that was on the spark plug its just jamming up the valve. When I held the air comp to the spark plug hole it would turn the engine until it locked into a position with what seemed like both valves closed. Also I had the valve cover off so I could watch what the valves were doing.

Think its just tear down time.

That sounds good. Gasket sets are pretty cheap, get one and take the top end off.

My 110 had poor compression when I got it. Turns out the intake valve was badly pitted and 3.00 in grinding compound with some elbow grease it was back up and running.