I would like to hear a detailed theory from some of u who concluded that an airleak could have caused an extreme revving of the engine?? If an airleak can cause a hanging idle, then what is it that determines whether the air leak will cause a slight increase in rpm such as a hanging idle, versus the leak causing a huge gain in rpm such as the one the op described.
Please break it down in detail. thanks
Welcome to the forum. You should consider engine speed and air flow velocity on this one, as well as your future on the forum...
If We have an pinprick airleak, let’s say between the reed cage and the cylinder, where they usually occur, this could cause a hanging idle at - you guessed it, idle! At idle, anywhere between 500-1000 rpm, the engine is drawing air at a very low velocity, because it is turning over slowly. You are also running on your pilot jet and off the pilot screw, which is usually set a tad rich. So you have low air velocity, low atomization of fuel and a fairly large ratio of fuel to air. This is a healthy condition, normally because it allows the motor to stay cool while idling. Now add the airleak and you interfere with that setup. If your idle is hanging, it means its is lean, meaning you have too much air in the idle system. Now if we extrapolate that to higher engine speed, we end up with much higher airflow velocities and a higher "suction" force on the intake system. We also have higher atomization of fuel and a lower, more accurate air to fuel ratio. We all know the Stock blaster intake system is restrictive and that air flow will always follow the path of least resistance. If the motor is sucking to get air from the filter, and another source becomes available, it will use it! This being so, a small pinprick airleak can become a major factor when the RPM picks up. Coupled to this, your needle and main jet are usually set a bit leaner than you’re idle to avoid bogging and spluttering on the rich side, but also not too lean to cause dangerous conditions on the lean side.
So basically, a small leak causing a hang at idle, can develop into a source of a lot of unaccounted and unjetted air entering the motor. This will cause a dangerous lean condition when the motor starts revving into power band/peak output ranges. We have no way of knowing how much air is unaccounted for, but it’s safe to say that the higher you rev, the more it will pull from the leak. Ride it like that and you will be peppering your fellow riders behind you with molten aluminium.
Let’s also not forget your premix in this story. Your jetting is a function of your fuel viscosity. Let’s say we have two scenarios. 1st scenario: 1 part oil with 50 parts fuel will be less viscus than Scenario 2: One part oil with 25 parts fuel, which will be thicker. Now if we have a 300 main jet, and want to pass fuel through it at high engine speed, when airflow velocities are at a maximum, you will be able to pass the same volume of premix for both scenarios through the jet. However, scenario one will be richer in fuel than scenario 2 and will require more air than scenario 2 to get the air/fuel ratio the same. Don’t get confused with the difference between the two types of richness here. Scenario 2 has a richer premix, because it has a higher oil to fuel ratio than scenario 1. However, in application, Scenario 1 will have a richer air/fuel ratio when running in the motor simply because it has less oil and more fuel than scenario 2.
This is why jetting is such a personal and bike specific thing. It is possible to jet a bike to accommodate an airleak, but due to the unconforming nature of airleaks, it is often not possible to do this accurately.
Bottom line? Test regularly for air leaks, fix them, jet properly and don’t argue about it, your wallet will thank you, and so will we.
