Reed valves on a 4-stroke.

[Blown 331]

Anyone ever see a 4-stroke with reed valves? What do they do? I'm sure this will be an interesting discussion but seriously I have no idea. Weird stuff.

[bigredhead]

someone was mentioning something about the XR 650 on a few threads down... don't know if that was a joke or what tho.... dont see why or how..

[hondatrikesrule22]

Some of the big bore 4 strokes have these. Their purpose is to keep the fuel and air from shooting back into the carb. The fuel goes in, but doesn't come out.

[bigredhead]

I thought that's what VALVES were for ????.. hmmm

[slothminx]

why would you have air coming back out the carb ?? maybe if you had a big overlap on the cam or the duration of the opening being very long there might be some. but i cant see there being any advantage to using them unless you are looking for everybit of duration without loss

[Wickedfinger]

The big XRs did have them .... Billy (HondaATC) has a motor with them in it - his old 500X project maybe?. It was just there to prevent "Spit Back" like has been said.

[hrc85250r]

yes, the reed valves were used because of the overlap and duration of the intake lobe on the cam, if there wasnt a reed valve there some of the a/f mix would come back into the carb... boyesen actually made aftermarket reeds for them too...i thought that was cool...

[Blown 331]

I have a bunch of old 500cc bikes. None of them have reeds! As far as the Honda's go I have three 1982 XL500R's and one 1984 XR500R. So it looks like in 1982 the XR's had them and the XL's didn't. 1982 was the last year that they didn't have RFVC. My brother has a 1983 XR500R and it has reed valves. My 84 is the exact same bike but no reeds. On the 83 they bolt into the side of the head, not in the carb boot. My 84 just has a plate over the spot where they are on the 83 and the cavity does not go into the air way, they cast it shut. So I'm guessing if you have them you can just take them out?

[Mobular]

The reed valve helps with the bottom end response in the big bore bikes. If you have them in your bike, leave them alone. They are there to do a specific job.

[OldSchoolin86]

The reed valve helps with the bottom end response in the big bore bikes. If you have them in your bike, leave them alone. They are there to do a specific job.
Any idea how that works? I would have to guess that this is not true.

[Mobular]

Any idea how that works? I would have to guess that this is not true.


Your guess is incorrect....


This was taken from Thumper talk:


REED VALVE ASSEMBLY
XR500
The xr 500r has a power reed valve assembly in the intake port of the cylinder head. The reed valve helps the engine to achieve higher torque in the low to medium engine speed range. On the 4-stroke engine, part of the fuel/air mixture from the carburetor is forced back into the intake port and manifold during low to medium engine speed. This occurs because the fuel/air mixture enters the cylinder at a low velocity and the piston is trying to push it back before the intake valve has closed completely. The reed valve is a one-way valve, It allows the fuel/air mixture to go past the reed valve into the combustion chamber. The reed valve prevents the fuel/air mixture from reversing its flow, thus keeping more of the furl/air mixture in the combustion chamber to be burned. 1979-1982 500cc engines pages 88 and 89 clymer #m339 :D

[hrc85250r]

it might help low end, but its gonna help you no matter what, and im guessing the other 500 models didnt have the reeds because the cam wasnt as crazy...