Reed Valves

I am in my second year with KA100. This is the first reed valve motor I have run. New reed valves were installed after a rebuild over the winter and all seems good. However, I have some reed valve questions.

I think I understand the basic principle of the reed valve is to seal the motor on the pistons down stroke which keeps the fuel charge in the engine and I assume also has an effect on compression.

If I am correct with this… What would be the effect of reeds that are either damaged (cracked or chipped) or worn out (not sealing tightly)?

How many hours do you typically get out of a set of reeds?

As always, your input is appreciated.

Correct in that a smaller air/fuel volume moves into the combustion chamber and less pushing out of the exhaust gasses before the pressure wave returns some of the exiting gasses back to the combustion chamber and the exhaust port closes for compression. The volume of air trapped when the exhaust port closes is what will affect compression most, but a smaller fuel charge could affect power.

First sign of not sealing could be fuel spitting out of the carburetor as the escaping gasses flow backwards. Second and back to the point above, less power as less fuel/air charge is being combusted. Third, you are sending shrapnel through your engine.

I would probably suggest inspecting them half way between top end rebuilds to be safe and for sure replacing ever time you do a top end. From what I have seen, the layers will start to separate before they chip or break and usually near the corners of the petals.

First sign of cracked reeds is a hard-to-start engine… then as the damage progresses you’ll lose power of course. But at first it can be hard starting.

Seeing fuel coming back out the carb at times is normal, so I’d be cautious to use that as an indication unless you have a good baseline of what looks normal for your engine vs not.

Always carry them as spares in case they crack but the good news is the fiberglass ones we run in the US have proven pretty durable.

Doing with a top end depends on how long you try to stretch that top end. I’d do reeds every 6-8 hours as cheap insurance to make sure the engine is running as optimally as possible.

1 Like

As mentioned above,it is pretty difficult to start an engine with cracked reeds but a quick test is …remove the spark plug turn the engine with a key,at the sprocket side,when the reeds are good you feel a slight resistance when the piston goes down,and can you hear a plop sound,when the reeds are cracked there is no resistance,this works nine out of ten times.
You can prevent cracks by sanding the edges of the reeds length ways,with no.400 sandpaper.


In a two-stroke, there are 2 points of compression; primary and secondary. The primary compression area is in the cases, the secondary is in the bore.

The dynamics of blood flow suggests, to me anyway, that the reads are only effective in the lower RPM ranges. I believe that in a certain part of the RPM range, the read valves remain open all the time. I came to this conclusion analyzing the power curves of breed engines on my dyno.

Broken reads will effect the power in the lower RPM ranges. They will lower the compression pressure in the lower compression area which should affect the secondary compression area i.e. the piston compression.