One is to reduce the idle until the throttle slide is completely shut, this way on closed throttle too little air-fuel mixture will enter into the engine making combustion impossible to occur, thus even in the presence of previous fuel deposits we'll never have the conditions for combustion to arise, and consequent pipe-bang (given the fuel deposits) to happen anyway. Getting rid of the annoying pipe-bangs can be done in several ways. So it's important not only to have good jetting (neither rich or lean) at closed throttle, but also at all other throttle positions, in order to not inherit excess fuel from a previous situation, which would be impossible to dispose of even with the good closed throttle carburetor setting. By closing the throttle - even if from this moment on we had proper jetting - the engine will still have a previous accumulation of unburned fuel, and this phenomenon is self-feeding, ie the deceleration surge cycle itself (as its combustion quality is low) is also a source of stagnant unscavenged exhaust gases which cause further misfires, in a vicious circle. at part throttle (when the mixture is determined by the needle). It is not sufficient to lean (or rather to optimize, neither rich nor lean, as also lean jetting slows down combustion and is a source of misfires - in addition to being harmful for many other reasons) closed throttle jetting, because even with perfect closed throttle jetting, fuel accumulation may have occurred up to the moment before, eg. The cause therefore is not directly rich jetting, although this certainly worsens the problem indirectly, as it is the source of both (additional) misfires and unburned fuel accumulation, as well as slowing down the combustion, all things that certainly do not help the engine to run cleanly. This combustion obviously also produces new exhaust gases, which will require many engine revolutions to be cleaned from the cylinder because of the low differential pressure we have between crankcase and cylinder due to the closed throttle, and thus begins a new long cycle of misfires with the engine not able to burn the mixture, until exhaust gases are sufficiently diluted and we finally have combustion again (very intense because of all the fuel deposits accumulated in the meantime), perceived as a surge, production of new exhaust gases, and so on. Finally, again, we get back to the condition that the spark is able to begin combustion and the latter will also complete, but there are such huge accumulations of fuel (meanwhile evaporated and mixed with air, thus now in ignitable form) that the new combustion will be very intense (it will produce a lot of pressure on the piston and thus of torque, in a single engine cycle, resulting in a surge due to the increase of 500-700 RPM in just one engine revolution). Every time a misfire occurs (we get a regular spark, but the mixture does not ignite because there are too many residual burned gases around the spark plug, or the combustion begins but the excess of burned gases quickly extiguish the flame front and we get only partial combustion) and imperfect scavenging, the percentage of "fresh" mixture in the combustion chamber will however increase, engine revolution after engine revolution, while residual exhaust gases (which prevent ignition and combustion) are diluted more and more. The cause of this very intense combustion is a considerable buildup of unburned fuel, unburned due to the repeated misfires caused by the stagnant exhaust gases (still trapped in the cylinder because of poor scavenging due to the low differential pressure we have between crankcase and cylinder when the throttle is held closed, and subsequent very limited gas flow) which does not allow normal ignition for many of the next engine revolutions. This is perceived as a sudden surge, during engine braking. Instead, it is a normal but very intense combustion ignited by the spark plug, which increases the engine speed 500-700 RPM in a single engine revolution. The notorious pipe-bang is neither detonation nor pre-ignition, as it is sometimes claimed. The so called "pipe-bangs" in 2-stroke engines, especially large-displacement ones (250cc plus), are one of the most debated and unresolved topics on enduro forums.
2 STROKE TUNING VIDEO HOW TO
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