firecat6

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Jun 25, 2005
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I am new to dirt biking and was just wondering exactly how the reeds in the engine work? are they suppose to be shut all the time or open a little or what? should there be a gap at all or complete sealed?
 

MX851

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Jun 14, 2005
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From how I understand when you turn the throttle the cable opens a slide in the carb which allows fuel and air to flow in and into the reed cage. The reed cage is the part that slides in just behind the cylinder and the reeds are mounted to it. When air and fuel flows through it it pushes the reeds open and allows gas to flow into the cylinder.
 

RM_guy

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...almost...when the piston moves up it causes a vacuum and draws the fuel/air mixture through the reeds (they "swing" open). All of that fuel is dumped into the crank area. Once the piston moves back down it tries to force the fuel back into the carb but the reeds don't open in that direction so the fuel gets pushed up through the transfer ports and dumps it on top of the piston, all ready to be ignited to start the whole process over again.

There should be no gap between the reeds and the reed block or some of the fuel will get pushed back into the carb and you will loose some power. Worn reeds will make the engine hard to start and you will loose low end power.
 

RM_guy

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Breaking is one reason to replace them. The other is when they don't seal tight against the reed block. There should be no gap at all.
 

jdc9q8

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Mar 9, 2006
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i know this is a really old thread..but i've been researching on how reeds work for a while and this is the most informative one i've found yet...so i decided to bring this thread back up instead of repeating the questions over and over!

my question..for RM_guy or whoever wants to elaborate...

i've never taken apart a 2-stroke engine..only 4 strokes..so

what are these transfer ports...cylindrical holes drilled in the block that lead to the top of the cylinder from the crank area??...

how do i access the reeds...by taking the carburator off..or do i have to disassemble the engine?
 

RM_guy

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Transfer ports are passages in the sides of the cylinder that let fuel flow up from the crankcase into the combustion chamber. When the piston comes down it pushes the fuel up through them.

You can access the reeds by just removing the carb. They are located in a block between the carb and either the cylinder or the crankcase (depending on what size and model bike it is). It is a lot easier to get to them by removing the subframe but it can be done leaving it on.
 

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