Placement of gasket killer bee backpiate

rex f. Sun Aug 04, 2019 7:23 am

Nowone seems to reallse the value of placing the kb backplate
Gasket between mount & retainer instead of between retainer
& crankcase as shown in parts pix from cox & ex the reason for the correct
Placemen you get a further .005 or the thickness of the gasket in reed
Clearance a bonus of some 200 rpm try it specially when using
A s/s reed no more sticking & so mutch more reliable
Up to you but would like to hear your result

I know mine
Rex f.

Levent Suberk Sun Aug 04, 2019 10:50 am

Thanks for this idea. I didn't try it yet, but opening and closing of the reed take more time in this way. Engine may flood or starting problems?

balogh Sun Aug 04, 2019 12:38 pm

Hello Rex

I actually wrote about it somewhere here - e.g. https://www.coxengineforum.com/t8552-mylar-stainless-steel-reeds-with-circlip-type-reed-retainer#108630 - and my impression is that with excessive clearance for the reed to travel to and from the seat, part of the air/fuel charge may even be shot out through the venturi because the reed will close too late after the piston starts to move down and compress the crankcase gases. I cannot substantiate this theory of mine because have not tached the various configurations, but the distance between the retainer and seat cannot be increased without causing the said delay in reed closure.

One known limiting factor of COX reedy performance is also related to the finite speed of the reed movement, which is the reed floating where, beyond a certain rpm, the reed floats and cannot follow the opening/closing timing requirement of the engine at those rpm-s

aspeed Sun Aug 04, 2019 2:52 pm

I wonder if the gasket behind the retainer works better because the reed is closer to the crankcase and not spacing the reed. I only have one of those type reed holders, and did not know there was supposed to even be a gasket. I thought that is what the spacer was for. It seemed like it was a good idea. Most of my Cox stuff was from the 1970s, but I gathered a few newer ones mixed in with other engine deals. Black Widows were a new thing not available when I played with them. Kind of dating myself. As for fluttering reeds, we used to inhale reeds, and sometime doubled them up at the time with the mouse racers. You know that we had to beat Paul at the Nats. Can't remember if we ever did. Probably not.

Oldenginerod Sun Aug 04, 2019 5:26 pm

aspeed wrote:I wonder if the gasket behind the retainer works better because the reed is closer to the crankcase and not spacing the reed. I only have one of those type reed holders, and did not know there was supposed to even be a gasket. I thought that is what the spacer was for. It seemed like it was a good idea. Most of my Cox stuff was from the 1970s, but I gathered a few newer ones mixed in with other engine deals. Black Widows were a new thing not available when I played with them. Kind of dating myself. As for fluttering reeds, we used to inhale reeds, and sometime doubled them up at the time with the mouse racers. You know that we had to beat Paul at the Nats. Can't remember if we ever did. Probably not.

Having the reed retainer closer to the crank may also reduce the volume of the crankcase, allowing for more positive charge transfer. Just a thought.

rex f. Sun Aug 04, 2019 7:44 pm

Tne opening & closing of tne reed is totally controlled bt srankcase compression & decompression &
Yes placement as i have indicated does increase crankcase compression&
Performance.
Rexf

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Placement of gasket killer bee backpiate

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Placement of gasket killer bee backpiate

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