Why change oil after a decarb?

[btravlin2]

I'm asking this as an academic question so I can understand what's going on inside the motor.

I assume the reason for changing the oil is because the motor oil becomes contaminated with Seafoam (thinned petroleum distillates) and dissolved carbon. But the only way for this stuff to get into the oil is by leaking past the rings. Isn't it? But rings are pretty tight fitting.....that's how compression is created. So I would think that only a tiny amount of anything is getting past them into the crankcase.

The other way of thinking is that a carboned-up cylinder may have stuck rings that are allowing blow-by, hence, dissolved junk gets into the oil. Is that how the oil becomes contaminated?

 

[chawk_man]

Yes. One of the main things a decarb does is clean carbon and junk from the ring seal seats and valve seats, allowing for better seals, thus contaminating the oil.

Personally, I've never had to decarb my BF 225. However, I have decarbed several old two-stroke outboards and a lot of small engines, including my 18 HP Kohler on a riding mower. Growing up in a garage, I observed my father and uncle decarb many car engines, especially the old Chevy 235's and Ford flat-head eights. Sometimes, results have been spectacular.

 

[johnnygjr]

I read a post on this forum that decarbonising a four stroke injected out board engine will cause a runaway super high RPMS and engine failure. Any truth in this fellow members??

 

[chawk_man]

Well, I certainly don't consider myself an expert on the subject, but that seems a bit far fetched. Basically you are introducing an oil-based solvent into the fuel stream that is absorbed by and dissolves gum, varnish, and carbon residues and deposits from injectors, intake valves, pistons, and combustion chambers. Sea Foam, for example, is supposedly a mixture of pale oil (AKA machine oil), naphtha, and isopropyl alcohol. I have no idea what is in YamaLube Ring Free, but it's probably not a lot different. When you mix that in a decarb solution of 5 parts gasoline to 1 part Sea Foam, and run it through the fuel system, it seems difficult to believe that could cause a runaway engine.

Also, on carbed engines, many folks have simply sprayed or poured unmixed Sea Foam into the carburetor directly while the engine is running. When the engine starts to smoke out of the exhaust, they turn off the engine and let it sit for 30 minutes or so. Then they repeat the process one or two more times.

A while back, I think Scooter did a Sea Foam decarb on his 225 with good results in improved compression and smoother running.

 

[btravlin2]

I read a post on this forum that decarbonising a four stroke injected out board engine will cause a runaway super high RPMS and engine failure. Any truth in this fellow members??

Seems impossible. Yesterday I sprayed Seafoam directly in the intake of my 4 stroke, 225 Honda. RPM's went down, not up.

 

[johnnygjr]

Thanks Chawk----man and btravlin2 I have gotten the same results as you two have.

 

[jgmo]

A gasoline engine, carbureted or fuel injected, CANNOT "run away" by the very nature of how it is controlled. You can pour all the excess fuel you want into one but, unless you hold the throttle plate wide open to add air for the fuel to ignite, the engine is more likely to stall.

A diesel engine, on the other hand, CAN run away and will if fed too much unmetered fuel. That’s because they do not use a throttle body for intake. The intake is a fixed, calibrated opening and engine speed is controlled by precisely controlling the fuel flow to the cylinders either through an injection pump or calibrated unit injectors.

So, simply adding too much engine oil to a diesel can cause it to overspeed (run away) if the oil gets splashed up into the combustion chamber.

This is also why a diesel doesn't create an engine vacuum that can be utilized for control and operation of accessories.

 

[chawk_man]

The professor has spoken, and you can bank on it. Thanks for the clarity, Jimmy.