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Recently I made a reply to a topic. I said that a motor that idles properly when cold but then idles too high once warmed up has probably got an intake air leak.I'm not talking about a 200-300 rpm rise. A motor in good mechanical condition can have a a few hundred rpm rise or drop just because of elevation,outside temperature,etc, I'm talking about larger rpm increases. A few of you are not too sure of this and one person wondered how this 'phenomenon' can happen. First of all,any motor with a carb can develop a high idle because of a sticking slide or throttle valve. Also,a faulty ignition advance can give you a high idle. These things happen but more common is an intake leak and using my own experience as an example,I'll try to explain.
I bought my bike new and it ran perfect for one year. Then one morning I started it,the same way as always and could tell it was acting a little cold-blooded and sluggish. I figured it would clear itself as soon as it warmed up. After about 20 minutes I came to a stop and noticed the tach read 1800 rpm's. 10 minutes later it read 2200 rpm's. I did'nt know what to think.(I was 22 at the time,not too much mechanical knowledge)So I turned the idle knob and set the idle down to 1100 rpm. I continued riding and all was OK. Then I went home and parked the bike overnight. The next morning my bike would'nt start, using the normal routine.I had to give it more choke than normal and it barely started. It would not keep an idle until I turned the idle knob back up again. I knew this idle up and down thing was'nt right so I took it in to the shop.They found 1 intake boot o-ring was bad and they said the air leak caused the high idle.They were right. Later, I read about fuel mixtures and what happens when they go into a cold motor and a hot motor.
This is my understanding of how an intake leak effects your motor. You start a motor that has an air leak.(The leak may be hard to notice until it gets bigger. Aging rubber parts are usually to blame. Sometimes the damaged rubber won't allow air in until it becomes warmed up and is more pliable.) As you crank the motor,it draws fuel and air through the carbs and into the combustion chamber. This atomized mixture needs heat to help it vaporize. Since the motor is not hot yet,the mixture will not vaporize well. So to promote combustion we use the choke. As soon as the motor starts warming,we no longer need the richer choke mixture. Now as the motor gets hot and combustion is good,the intake volume increases and the intake leak is more noticable. This leak results in a lean condition.The mixture becomes "over vaporized"and ignites too easily,causing the motor to run erratically at a higher than normal rpm. So that's it. More air=lean condition=erratic combustion=higher rpm's.
You can try to live with it and keep turning the idle up and down,but you're asking for trouble. The fuel in the mixture helps cool the motor.With too much air in the mixture,your motor runs lean and hotter. This can lead to actually melting the plugs,burning valves and burning holes in pistons. As a side note:when starting a cold motor that is in GOOD mechanical condition,do not open the throttle,this only by-passes the choke/starter circuit and will actually make starting more difficult.
I bought my bike new and it ran perfect for one year. Then one morning I started it,the same way as always and could tell it was acting a little cold-blooded and sluggish. I figured it would clear itself as soon as it warmed up. After about 20 minutes I came to a stop and noticed the tach read 1800 rpm's. 10 minutes later it read 2200 rpm's. I did'nt know what to think.(I was 22 at the time,not too much mechanical knowledge)So I turned the idle knob and set the idle down to 1100 rpm. I continued riding and all was OK. Then I went home and parked the bike overnight. The next morning my bike would'nt start, using the normal routine.I had to give it more choke than normal and it barely started. It would not keep an idle until I turned the idle knob back up again. I knew this idle up and down thing was'nt right so I took it in to the shop.They found 1 intake boot o-ring was bad and they said the air leak caused the high idle.They were right. Later, I read about fuel mixtures and what happens when they go into a cold motor and a hot motor.
This is my understanding of how an intake leak effects your motor. You start a motor that has an air leak.(The leak may be hard to notice until it gets bigger. Aging rubber parts are usually to blame. Sometimes the damaged rubber won't allow air in until it becomes warmed up and is more pliable.) As you crank the motor,it draws fuel and air through the carbs and into the combustion chamber. This atomized mixture needs heat to help it vaporize. Since the motor is not hot yet,the mixture will not vaporize well. So to promote combustion we use the choke. As soon as the motor starts warming,we no longer need the richer choke mixture. Now as the motor gets hot and combustion is good,the intake volume increases and the intake leak is more noticable. This leak results in a lean condition.The mixture becomes "over vaporized"and ignites too easily,causing the motor to run erratically at a higher than normal rpm. So that's it. More air=lean condition=erratic combustion=higher rpm's.
You can try to live with it and keep turning the idle up and down,but you're asking for trouble. The fuel in the mixture helps cool the motor.With too much air in the mixture,your motor runs lean and hotter. This can lead to actually melting the plugs,burning valves and burning holes in pistons. As a side note:when starting a cold motor that is in GOOD mechanical condition,do not open the throttle,this only by-passes the choke/starter circuit and will actually make starting more difficult.
it feels kinda wierd when i'm riding and i clutch coast and the engine drops to 1500 RPM...i'm used to hearing/feeling it idle higher. i guess i'm just adjusting a good running engine. 
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