Suzuki outboard motors 1988 2003 repair manual., Page 64Get this manual
4-2 FUEL SYSTEM Fuel vapor pressure is measure of how easily fuel sample evaporatesMany additives used in gasoline contain aromaticsAromatics are light hydrocarbons distilled off the top of crude oil sampleThey are effectivezit increasing the research octane of fuel sample, but can cause vapor lock on very hot dayIf you have an inconsistent running engine and you suspect vapor lock, use piece of clear fuel line to look for bubbles, indicating that the fuel is vaporizingOne negative side effect of aromatics is that they create additional combustion products such as carbon and varnishIf your engine requires high octane fuel to prevent detonation, de-carbon the engine more frequently with an internal engine cleaner to prevent ring sticking due to excessive varnish buildupBesides aromatics, two types of alcohol are used in fuel today as octane boosters, ethanol and methanolAgain, alcohol tends to raise the research octane of the fuelThis usually means they will have limited benefit in an outboard motorAlso, alcohol contains oxygen, which means that since it is replacing gasoline without oxygen content, alcohol fuel blends cause the fuel-air mixture to be leanerRECOMMENDATIONS
Reformulated gasoline fuels are now found in many market areasCurrent testing indicates no particular problems with using this fuelShelf life is shorter and, because of the oxygenates, slight leaning out at idle may be experiencedThis slightly lean condition can be compensated for by adjusting idle mixture screwsFuel recommendations have become more complex as the chemistry of modern gasoline changesThe major driving force behind the changes in gasoline chemistry is the search for additives to replace lead as an octane booster and lubricantThese new additives are governed by the types of emissions they produce in the combustion processAlso, the replacementadditives do not always provide the same level of combustion stability, making fuel's octane rating less meaningfulIn the search for new fuel additives, automobiles are used as the test mediumNot one high performance two cycle engine was tested in the process of determining the chemistry of today's gasolineIn the 1960's and 19703, leaded fuel was commonThe lead served two functionsThe lead served as an octane booster (combustion stabilizer) and, in four cycle engines, served as valve seat lubricantFor two cycle engines, the primary benefit of lead was to serve as combustion stabilizerLead served very well for this purpose, even in high heat applicationsToday, all lead has been removed from the gasoline processThis means that the benefit of lead as an octane booster has been eliminatedSeveral substitute octane boosters have been introduced in the place of leadWhile many are adequate in an automobile, most do not perform nearly as well as lead did, even though the octane rating of the fuel is the sameTHE BOTTOM LINE WITH FUELS
If we could buy fuel of the correct octane rating, free of alcohol and aromatics, this would be our first choiceSuzuki continues to recommend unleaded fuelThis is almost redundant recommendation due to the near universal unavailability of any other type fuelAccording to the fuel recommendations that come with your outboard, there is no engine in the product line that requires more than 89 octaneMost Suzuki engines need only 86 octane or lessAn 89 octane rating generally means middle grade unleadedPremium unleaded is more stable under severe conditions, but also produces more combustion productsTherefore, when using premium unleaded, more frequent de-carboning is necessaryRegardless of the fuel octane rating you choose, try to stay with name brand fuelYou never know for sure what kinds of additives or how much is in off brand fuelOCTANE RATING