Engine Compression + Gas Octane??

[100333624]

Well it looks like its the beginning of the season, finally! So I got my boat out to de-winterize it only to start off with a bit of an issue... After about 8 hours of working on it I finally found my problem, a clogged fuel line with plastic from the inside lining blocking flow of fuel from the tank to the fuel bulb. It drove me insane because I coincidentally changed the fuel filter this week and thought maybe I messed something up there. Anyways..

So I got it fixed and the engine fired right up (1995 Yamaha 115 2stroke) After running the motor for about 10 mins I pulled out my laser temperature reader and pointed on the cylinder heads beside each piston. Standing behind the engine obviously, I noticed the port side cylinder bank running about 10 degrees F cooler then the other side cylinder bank. After about another 5 minutes all the pistons seemed to have roughly the same temperature more a less (124 F) plus or minus 2 or 3 degrees, but the starboard side top cylinder was about 10 degrees hotter (132ish) The engine was still very warm to the touch (not HOT) even after an additional 10 minutes of running. I ran it in a tank not on muffs btw... Ran my bro in laws XR6 yesterday in the same tank, the block got really hot to touch in 10 minutes, and pretty much boiled the water in the tank lol... anyways moving on...

So after experimenting with temp I pulled the plugs and did a compression test. Plugs looked a little oily and good. Port side top ~125, bottom 130, starboard top ~127, bottom 125. Seems pretty good and close enough to each other. Now my main question is what is considered "high compression" in relation to running "high octane" fuel. I'm not talking about like race engines or anything but just in general. After speaking with some people I was told running high octane in low compression engine would cause damage, and running low octane in high compression engine would also cause damage. So what do I do? I was also told that running 87 octane supposedly is causing Yamaha 2 stroke V engines to break rods? And that I should be using premium only, but then a rebuilder told me its the worst thing to do. Actually he advised of adding another gasket to the cylinder heads?

I normally run 87 octane and add just a touch of seafoam in the tank to stabilize fuel and clean carbon, but I was also told even that is too aggressive for 2 strokes and will eat away headgaskets more quickly? I can live with using a regular fuel stabilizer, and I can live with putting premium in my tank. I do have a water separator installed in the boat as well. But the question is: what is the "ideal" octane for what type of engine? How do I know what octane gas to use? I know its good to use high octane in all the small 2 strokes like mowers, leaf blowers, chainsaws etc.. but outboards?? My engine runs excellent and I want it to stay that way

 

[flyingscott]

If you have never had a problem stay with the 87 and what does your owners manual say. Whoever told you the seafoam is to aggressive and will eat your head gasket away might want to take that advice with a grain sof alt.

 

[Scott Danforth]

seafoam was designed for 2-strokes.

 

[100333624]

I don't have the owners manual for mine as I cant find one online, the next best thing is for a 1997 115 and it mentions MINIMUM octane 87. I don't know how different this motor is to mine though they look exactly the same but I know something changed in those years. Also mentions spark plug torque at 18 ft lbs whereas the service manual says 14 ft lbs...

So considering it says **minimum** 87, does that mean its safe to run higher? Do I risk any damage running 89 or 91? The maximum I can get here is 94 octane. Will running 91 have any improvement to the engine performance, or compression or anything like that?

So regarding the seafoam you mean its safe? I've used it in lots of stuff with no issues but not sure why he mentioned that to me, he is a local rebuilder so I figured maybe he knows something? He did seem quite knowledgeable on the subject and lots of stuff he was saying did make sense to me... Also he seemed to be the only one I spoke to who actually knew that there was a difference between the 115 and 130 V4 engines and that its not just a "sticker". He also came recommended by someone else I know as a good local rebuilder..

I've had the engine for a little over 2 years now, never had any problems with it but never got to run it much when I got it. Last year I got to run it a bunch of times and I was overall very impressed with its performance.

 

[flyingscott]

Higher octane will not increase your power or hurt your motor just cost you more money with no benefit. Seafoam as far as I have seen has never wrecked a gasket it is safe to use. Being able to say a 115 Yamaha is different than a 130 Yamaha is common sense. Really think a multi billion dollar company would risk it's reputation on a decal

 

[fhhuber]

Octane is a :knock resistance" factor. High compression + heat of previous combustion can cause the fuel to ignite before the spark This pre-ignition or detonation can blow holes in the pistons.

For most gasoline engines made since the early 1970's, "regular" (appx 85 to 87 octane) will be fine. Engines made for racing or aviation tend to be higher compression and need higher octane.

 

[100333624]

Thats good to know seafoam is safe. I only use a very little anyway just as a "stabilizer" and to clean carbon and whatnot.. So I guess than just stick with regular 87 since there is no benefit to using higher octane gas.

Also regarding the Yamaha sticker thing, well you'd be surprised how many people couldn't tell the difference and say its just different jets or carbs... When infact there is a little bit of a difference between the two motors. even from supposedly yamaha rebuilders who didn't know the difference???

Anyway I got some good information from this thank you all for your inputs!

 

[tommarvin]

87 octane, seaform can't hurt anything, your fuel line degraded is from E-10 fuel .
You should get USCG approved the best fuel line, incase you ever have an accident out there,
You only need fuel stabilizer the last tank of the year, mix it in well or in doesn't work.
If it's within budget use 100% synthetic two stroke oil, if not walmart has a partial synthetic, never any carbon buildup.
Dump out your 10 micron spin on water separating filter at the end of boating season, light it on fire, grease gasket spin it back on for next season.

 

[bruceb58]

Dump out your 10 micron spin on water separating filter at the end of boating season, light it on fire, grease gasket spin it back on for next season.

Worst thing you can do is reuse a spin on filter. All the contaminants that are trapped on the input side of the filter spill into the out put side when you pour it out. ALWAYS use a new filter.

Speaking of spin on filters, better check your manual to see if an external fuel filter is allowed on a Yamaha. Some external filters provide enough resistance to the fuel pump to cause an engine to go lean. My Mercury four stroke with a Yamaha head specifically says not to add an additional spin on filter.