reed valves

[Oregon_Native]

i really haven't had a lot of experience with reed valve engines so i am wondering what an engine would act like if the reeds needed to be replaced would it just not run or would it just not build rpm's and would it try to breath out the carb when pulling it over slow.

i am having a hard time getting the carb tuned on my sp125 and was wondering if it could be the reeds.

 

[TimberPig]

If the reeds are completely broken, then yes it wouldn't want to run. If the reeds are chipped, cracked or frayed, then poor throttle response is the most common symptom. They will often tend to load up when the reeds are on their way out as well. A reed that isn't sealing properly will allow bottom end compression to flow back into the intake, so yes it will show backflow of air into the carb. This also translates into poor flow of the air fuel mixture from the crankcase into the cylinder, which is a cause of the poor response and loading up with fuel that you may be seeing.

 

[Oregon_Native]

i pulled the reeds from the saw and they look great but they stand open at rest just a little i dont know if this is normal or not.

 

[TimberPig]

You will often find that the reeds do have a slight gap visible between them and the reed block, but it should be very small. The pressure in the crankcase easily seats them to seal the case. It normally is less than about 2mm, beyond that you may encounter sealing issues. Unless the gap is large, it sounds like your reeds are not the source of the problem.

 

[Oregon_Native]

i flipped the reed any way because the sealing edge was rough it had also left marks in the reed block. i have not hade a chance to run it yet but it might make a diffrence.