First, gas burns reasonably well over a surprisingly wide range of afr.
Second, the range is narrower on the lean side of ideal than it is on the rich side.
Third, most folks can identify a slightly lean condition without fancy equipment but most folks can not do the same about slightly rich.
Fourth, while many think there is no harm in over rich, it can dilute oil,wash down cylinders, build carbon, foul plugs, waste fuel, and reduce power.
Fifth, most home jetted bikes are jetted over rich because of fear of leanness, belief that it causes no harm, and because few can detect a slightly rich mixture without specialized equipment. Most of the over rich bikes are over rich on the Iidle circuit. In my opinion this is about the worse place to be rich. A bit rich on the main is not necessarily a bad thing. The main does not influence the lower speed circuits, it only comes into play when wide open or very close to it. A bit rich here will not affect performance at lower speed running and will provide additional cooling.
Sixth, don't assume that bikes are delivered lean in the idle circuit or the main because often they are not. They are usually lean in the area controlled by the needle, however. Pilot jets as delivered are usually more than capable of handling the fuel requirements of a stock engine but the mixture screws are almost always set leaner than ideal as delivered. Carb tuning is more than changing brass. The engine must be fully warm as in ridden, before mixture screws can be adjusted. Rich mixtures are less fussy below optimum temps and folks searching for a bike that runs well off choke and not warmed up will end up with less than an ideal mixture. The bike will end up rich if final tuning is done before a few miles of actual riding.
Seventh, richer mixtures tend to burn faster and have a different ideal Timing requirement. Faster, more turbulent combustion chambers can run leaner mixtures without noticeably acting up. Because afr affects burn rate ideal timing can change with it as well. Different fuel additives can change burn rate as well and therefore can change ideal timing. Ideal timing is key to getting the most out of a combination. A degree or three off can account for several hp sometimes more.
Eigth, reductions in intake restrictions, can have more influence on jetting than most think and reductions in exhaust restriction as opposed to pipe wave tuning alterations have less influence than most folks think.
Ninth, a mismatch in cam and pipe tuning can create a jetting nightmare. An example of this is a cam with a lot of overlap combined with a large diameter, short drag pipe.
Second, the range is narrower on the lean side of ideal than it is on the rich side.
Third, most folks can identify a slightly lean condition without fancy equipment but most folks can not do the same about slightly rich.
Fourth, while many think there is no harm in over rich, it can dilute oil,wash down cylinders, build carbon, foul plugs, waste fuel, and reduce power.
Fifth, most home jetted bikes are jetted over rich because of fear of leanness, belief that it causes no harm, and because few can detect a slightly rich mixture without specialized equipment. Most of the over rich bikes are over rich on the Iidle circuit. In my opinion this is about the worse place to be rich. A bit rich on the main is not necessarily a bad thing. The main does not influence the lower speed circuits, it only comes into play when wide open or very close to it. A bit rich here will not affect performance at lower speed running and will provide additional cooling.
Sixth, don't assume that bikes are delivered lean in the idle circuit or the main because often they are not. They are usually lean in the area controlled by the needle, however. Pilot jets as delivered are usually more than capable of handling the fuel requirements of a stock engine but the mixture screws are almost always set leaner than ideal as delivered. Carb tuning is more than changing brass. The engine must be fully warm as in ridden, before mixture screws can be adjusted. Rich mixtures are less fussy below optimum temps and folks searching for a bike that runs well off choke and not warmed up will end up with less than an ideal mixture. The bike will end up rich if final tuning is done before a few miles of actual riding.
Seventh, richer mixtures tend to burn faster and have a different ideal Timing requirement. Faster, more turbulent combustion chambers can run leaner mixtures without noticeably acting up. Because afr affects burn rate ideal timing can change with it as well. Different fuel additives can change burn rate as well and therefore can change ideal timing. Ideal timing is key to getting the most out of a combination. A degree or three off can account for several hp sometimes more.
Eigth, reductions in intake restrictions, can have more influence on jetting than most think and reductions in exhaust restriction as opposed to pipe wave tuning alterations have less influence than most folks think.
Ninth, a mismatch in cam and pipe tuning can create a jetting nightmare. An example of this is a cam with a lot of overlap combined with a large diameter, short drag pipe.