Scooterotica

Hello all. Mechanical numbnut.. Can someone explain to me what a reed valve is/ does ( I know it's to do with carburation- is that a word??) can you fit them to any carb/ manifold, and what benefits do they provide? Just looking for knowledge

the simple way to describe a reed valve is a one way valve that allows negative crankcase pressure to draw in a fuel and air mixture, but when the negative pressure inside a 2 stroke engine turns positive, the valve closes, so it doesnt dispel the mixture drawn in, any clearer ?

I think I'm too thick. What benefits do they provide? Are they suitable for any 2 stroke engine??

the main benefit is that more inlet duration can be used, without expelling the fuel and air that is sucked in, this can help to produce more power, helps to reduce stand off from the carbs and can help with economy, they can act as a restriction to the fuel and air entering, and can reduce maximum bhp sometimes, but can increase torque and driveability, size and type of valve can affect performance, can probably be fitted to most 2 strokes, even chainsaws and grasscutters as an example.

Revvers wrote:I think I'm too thick. What benefits do they provide? Are they suitable for any 2 stroke engine??

You can't just buy a reed valve and bolt it on, well you can but its a pointless exercise. You have to convert your engine to run properly with a reed valve, which involves (buying the right reed valve for a start) and then having your cylinder/piston ported on a Lambretta and/or your crank and inlet cutting on a vespa.

It's not a cheap exercise, but the upshot is better bottom end and midrange power.....more so on engines which have been over tuned in this area, less so on engines well setup in this area.

Ok, all 2 strokes have a tendency to spit fuel/air mix back out of the carb to a greater or lesser extent. This is due to the exhaust port being open whilst the inlet is still open and is called 'port overlap'. The more an engine is 'tuned' the more port overlap you will have. Add to that the fact that expansion type exhausts are designed to shove a shock wave back into the cylinder which can also happen whilst the inlet is open, you can see that putting a one way flap (reed valve) in the inlet manifold will allow fuel/air in but will prevent it being spat back out of the carb. This means as already said that highly tuned engines can be 'tamed' at lower revs. They can be bolted on and will run ok but as has been said, the valve itself posses a restriction to the flow of the mixture towards the upper end of the rev range and to get the most out of it, the piston has to have windows cut into it as well as additional transfer ports (finger ports) cut into the barrel.

Hope that helps

I think I get it. Just wondered what they were. My Mugello 185 Lambretta and P2 will be left alone

Just another thing that can go wrong........

I have just fitted one to a Rapido Race with a TS1 piston, no changes to the cylinder except the inlet port and having trouble jetting it up.
Everything I have read suggest that I need to richen the jetting but I seem to be weakening the needle!
Is this correct?

Meds wrote:I have just fitted one to a Rapido Race with a TS1 piston, no changes to the cylinder except the inlet port and having trouble jetting it up.
Everything I have read suggest that I need to richen the jetting but I seem to be weakening the needle!
Is this correct?

have you done a plug chop, what does it run like, what type of carb etc, need a little more detail to help you ?