Cv carbs

[Propellor]

A mate was tinkering around with his er5 last night, experimenting with pod filters. The bike won't run properly in the mid range with them on. So we sat down armed with cup o tea n a biscuit to mull over what's happening.

With a basic understanding of how the slide lifts due to pressure differences we reasoned that, since the underside of the diaphragm is vented into the airbox, by removing the airbox we were changing the pressure on the underside of the diaphragm. At first we thought it was the pressure drop across the standard filter which was creating the pressure the diaphragm feels (it must contribute) but we then realised that the airbox inlet is in the form of a letterbox slot and the surface area looks substantially less than both carb throat diameters added together. Sure enough, removing the standard air filter but placing the airbox intake slot into place had the bike running well again.

Our next step will be to fit the pod filters and cover some surface area with baco foil, corresponding to the same surface area as the slot.

But if we want to run the bike with pod filters at full surface area we'll presumably have to counter the greater upwards pressure on the diaphragm? Can we shim the diaphragm spring? Creating a greater preload? We'll have to watch for the spring going coilbound. Obviously the spring rate won't change, but this is presumably a good thing?

There seems to be little else we can do to tune a cv carb except try different needles and perhaps needle jets (emulsion tubes). Maybe different fuel heights might help?

It may need a larger main jet for wot, but that's not really the main dilemma.

If the thing had been vented to atmosphere presumably we wouldn't have this problem?

Cheers

[Steve H]

http://www.intobikes.co.uk/products/002054/004371/Dynojet-Jet-Kits-suitable-for-Kawasaki-ER-5-2004

[Propellor]

http://www.intobikes.co.uk/products/002054/004371/Dynojet-Jet-Kits-suitable-for-Kawasaki-ER-5-2004

Thanks for reply steve. It looks as tho maybe they only do stage 1 kit, whereas stage 3 is more what he's looking for. But thanks for link. We'll peruse it a bit closer.

We're thinking that by removing the airbox, especially the deliberately restrictive intake "snout", it may be actually causing richness anyway. I can feel a manometer coming on. Oo er missus. All good fun and definitely better than watching telly.

Cheers.

[Propellor]

.............But if we want to run the bike with pod filters at full surface area we'll presumably have to counter the greater upwards pressure on the diaphragm? Can we shim the diaphragm spring? Creating a greater preload? We'll have to watch for the spring going coilbound. Obviously the spring rate won't change, but this is presumably a good thing?..........

Thinking about it, I'm not sure that keeping with the same spring rate would be the right thing. My reasoning being that in standard form, underside of diaphragm vented to airbox, the pressure will vary with flow. But venting underside of diaphragm to atmosphere the pressure will be more constant. So a different spring rate would be needed.

Actually we wouldn't be venting directly to atmosphere but into the inside of each pod filter, although those things are generally very free flowing.

Pure guesswork!

[Propellor]

Although the pressure under the piston must surely rise by removing the airbox restriction, it seems that the pressure in the Venturi will also rise. This Venturi pressure rise seems to be the dominating factor. A net rise in pressure difference biased to the top will weaken the mixture and the answer would seem to be a lighter rate spring.  At wot a larger main jet would be expected, I guess. As rhinoman has pointed out elsewhere, the air bleed must be affected too.

As a side issue, I noticed that the carb slides oscillate quite a bit at lowish rpm, but we obviously can't see if they do the same with the airbox and restrictor inlet fitted.