OPINION ALERT: The following are opinions and do not reflect the opinions of the management of the SOHC4 forum.
I like both products (SeaFoam and Marvel Mystery Oil).
SeaFoam Part 1:
I have had disturbingly good luck with SeaFoam. It got so ridiculous that I joked with my brothers that all I had to do was show the can of SeaFoam to my car and the "check engine" light would go out. The following is admittedly anecdotal evidence... I have "fixed" my Honda snowblower, Dodge Ram (360), Honda Civic (twice), Yamaha 9.9, Evinrude 18, Toro Lawnmower. When I say fix, I mean that they ran like crap, barely ran or had a check engine light until I added some SeaFoam to the fuel. In order to qualify for the "fixed" category, the engine had to run noticeably better (my opinion of course) or turn off a check engine light. One of the things that needs to be considered is that a carbureted engine is likely to run badly when not run for a while (many of my experiences are based on this scenario). Just running it will often cause it to run better over time probably because gasoline by itself is a very nice "cleaner/solvent". In other words, when you got something that runs like crap, often just running it will make it run better. It's my belief that SeaFoam aids or accelerates this "process". Sometimes, however, no amount of running or fuel additives will "fix" the problem. In fairness, I did have one "failure" in my attempts to fix things with SeaFoam. I have a weed whacker that takes pleasure in confounding me. Since new, it was much more cold blooded than a K8. It needed to run for a full 20 minutes before I could completely disengage the choke. Starting it is always a challenge and after years of fighting this evil device, I've moved on to a battery operated weed whacker.
SeaFoam Part 2:
I was in the boat storage/maintenance business. We used Stabil (and topped off tanks) at the end of the season to preserve the fuel in boats for the off season (with good success). That was several years ago. I think Stabil is the same stuff it was years ago but our fuel isn't. I would avoid using Stabil in any of my gasoline engines today. I have read on other forums and heard from folks in the fuel industry (Chemical Engineering types) that recent blends of our gasoline can have issues with Stabil. Instead, I add SeaFoam and have not had a problem with bad fuel. This weekend, I used fuel in my outboard that was 50% 1 year old and 50% 2 years old. Both batches had SeaFoam in them and tanks were sealed pretty well (important I think). The engine started on the second pull and ran like a dream. Is that evidence that SeaFoam is a fuel preservative? No, but that anecdotal evidence and many other similar experiences will have me using it for the rest of my days in all fuel I put in gas cans and tanks ('cause fuel is often stored much longer than I intended and I believe SeaFoam stabilizes the fuel well).
SeaFoam Part 3:
There's a process called "de-carbing" in the boating business where you cycle through high concentrations of "SeaFoam per Gas" until you stop getting white smoke (usually three 10-15 minute cycles). The white smoke is from carbon deposits. Carbon deposits are common in 2 stroke outboards and can cause serious damage (ring cracking). Most boat forums have instructions on how to "de-carb" your 2 stroke engine. It is a good thing to do (opinion). There may or may not be good effects keeping our combustion chambers clean using SeaFoam as a fuel additive. I'm not sure I'd recommend running high concentrations in our bikes.
Marvel Mystery Oil:
I use it in my bikes as a matter of habit. I am told that it has a cooling effect on the combustion chamber and lubricates the valves (I am especially interested in this because of the potential exhaust valve guide issues we experience). I have less evidence that MMO does good things but I believe in it and use it.
