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Random Quote: The road of life is full of flattened squirrels that couldn't make a decision.
The owners manual on my 2000 Merc 904 stroke recommends decarbong every 100 hours. Those who think 4 strokes do not carbon up are seriously mistaken. They will carb up much less than a 2 stroke, but will still. More car dealers are recommending decarbing and cleaning the fuel injection system on cars. Just pumping up business? Maybe. My mechanic friends all say that the newer gas will build up more carbon. I did my car and truck, and it made, at least to me, a difference.
As for gas, I used the Costco cheap gas when prices skyrocketed. I did notice my plugs were dirtier, with carbon build-up. Back to Chevron and my plugs are much cleaner.
Just my observations.
I've put hundreds of thousands, probably more than a million miles, on automotive four stroke engines and I've never de-carbonized one of them. I've also never bought a "fuel injector treatment" or dumped other crap in my tank.
I've also run four stroke generators, lawn mowers and other implements, and I've never, ever had to de-carb them, and they all have run just fine.
My wife's car owner's manual says to change the oil once a year or every 10,000 miles, that is it. Can you really run a car for a full year on one oil change? As far as I'm concerned you can. Point being, I follow the service schedule recommended by the manufacturer. If your outboard says you should de-carb every so often, then I would follow the schedule.
If it is just some mechanic telling you that you should, I wouldn't trust it. If it aint written, it aint happening.
My truck has a recommended oil change interval of 7500 miles. It can't be. The oil change places say 3000. Well, I believe the manufacturer, not the oil sellers. My outboard, which is a two stroke, does not recommend de-carb treatments (Yamaha C90). They do recommend tune-ups (changing plugs, basically) and replacing foot lube, and retorque, and that's about it.
They also give instructions for putting-up that is similar to a de-carb, but no "de-carb" service.
I don't believe a lot of things I hear about stuff like this, I think much of it is legend.
Oil change businesses are in the busines of, ... well, selling oil !!! I didn't know any auto manufactor had gone to 1 year for "normal duty" oil change, but I can belive it. The additive package has been getting better and better every year.
I have long been a 5k oil change guy, regardless of the number of months (usually some where between 6 and 12). I buy whatever "name brand" is on sale at the local discount auto store and don't think I have yet paid more that $1/qt.
Similar logic should apply to 4 stroke outboards.
What kills oil is overheating, underheating (not enough run time to drive out the humidity/comdensation in the air in the crankcase ie. short trips) and dirt (operating in dusty environments
Don,
I seem to remember somewhere reading in the Yamaha tech info for Ring Free that if you used it in a 4 stroke, you should change the oil shortly after. Something about it building up in the crankcase oil (bad) and not just doing what it's supposed to do in the combustion chamber (good). Might be one of those situations where "it can't hurt" actually does hurt.
Maybe a way to smoke out the dealer and Yamaha is to get them to put into writing what the official position is.
Now I aint bustin' your chops -- I really want to know... you say you follow the owner's manual. Me too. But that makes me curious: what does your C90 manual say about the use of Ring Free? Anything at all? I'm curious if there's any appreciable difference in this regard between your C90 and my oil-injected 90.
Four Stroke.. to Decarb or not to Decarb 
