Reed Valves Gap

[bob]

Hi,

I was just tinkering with the bike today and thought I'd pop the reeds out to see how they were looking. I'm using the standard reeds and now the block is out it is apparent that there is a small gap underneath two of the reed tips (the two upper ones), maybe only half a millimeter or so, but there is definitely a gap.

So my questions are;

1. What gap (if any) is acceptable, as obviously one the bike is running they might be pushed closed anyway?

2. Can I turn the reeds over so that if they are bent then they are bent in a more favorable direction? I do notice that they aren;t entirely symmetrical, one has a rounded corner and the other a flat on the corner.

3. Would a strip down, clean and rebuild do anything to improve it do you think?

4. Should I just put boyseen reeds in, given I am a student with slightly less money than I'd like?

Thanks in anticipation of your replies.

Bob

[bob]

While I'm here I'd also like to ask about leak past the piston, I was looking in through the inlet to the crank and slowly turning the engine over by hand, and if I apply a constant pressure to the kick start then I can hear air seeping past the piston, is this right?? How good a fit should my piston rings be?

Thanks, perhaps I'm looking into this a bit much, but I'm at a point where doing a top end rebuild would be easywith other bits of the bike in pieces.

Ta.

[dan williams]

The reed are held closed by the back pressure from the downstroke. It's not that significant if there is a little space at the tip at rest. If you put light pressure on it and it sits flat against the seat it should be fine.

Also the air leaking past the rings, if you are turning it by hand is normal. If you pull the cylinder and see significant blowby on the piston then it's time for set of rings.

[bob]

Ok, Thanks,

I reckon all will be well then, the reeds do close if you try to suck through them, and leakage on the piston didn't seem to be massive, and the bike hasn't been run in about six weeks, so I guess once there is a splash of fresh oil on the lower rings they will seal slightly better anyway.

Thanks,

Bob

[dan williams]

It's not really the oil that provides the seal. It helps certainly but mainly it's the pressure above the ring that does the magic. The high pressure above and behind the ring force it into the cylinder wall and seat of the ring groove. When you're spinning the engine by hand there's a lot less pressure so the seal isn't as good plus there's a lot less time for air to leak at running speed.

Edited April 2, 2009 by Dan Williams