chrisk
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Dellortoman said:
Maybe I've been away from home for too long, but ................... there's something that reminds me of.
Float Needle Valve.
- Thread starter RCGBOB44
- Start date
Maybe I've been away from home for too long, but ................... there's something that reminds me of.Interesting conversation. I'm not going to argue about what word is correct with you native English speakers, I have just used the words I have found on English literature. My native language is more far from English than Chinese is, at least that's what language teachers say here even not joking.
I think the float (needle?) valve (or what the heck it is) still has some metering capabilities but that happens on very limited amount of stroke. But still also the fuel level is also altering just a littlebit when riding and therefore these may be actually quite similar lengths. If the lever ratio is 10:1 for example then 1mm difference on fuel level moves the valve 0.1mm.
I guess the float level may not be necessary that extremely sensitive to how fuel is waving on the bowl as a result of vibration. There is two floats on the same lever, quite small dimensions of the bowl in where the walls are reflecting the fuel waves back. The float has a mass and some vibration frequencies high enough has actually quite small impact on that kind of system. Even if the valve is vibrating fast it may have a very small effect to fuel level if the vibration is fast enough when compared to the inertia of the whole float system. The vibration may also impact many other ways to carburation such as how the fuel flow inside jets.
I think this is some similar conversation than bolt vs. screw. I have heard many different definitions to that and all of them make quite much non-sense. There may be no correct answer:
-Screw is able to make its own threads. --> Then take a file and make a simple notch to the end of the bolt, is it then a screw? If a small screw has M3 thread, is it then a bolt? Are Taptites then screws to be replaced with bolts? (Taptite is a self-threading component to be replaced with similar but non self-threading component.)
-Bolts are used when two peaces to be joined which either has no thread and it needs a nut. --> Then a same item can be a bolt or a screw depending on how it is used.
-Bolts accept a nut --> Small M3 screw is then a bolt.
-Bolts are larger than screws. --> Where is the limit then?
-Holes are required for inserting bolts. --> Many screws also need a predrilled hole in practice. Try the small screws to steel plate. Is also M3 screw then again a bolt? Are small sheet metal screws then allways bolts?
-Bolts are always fixed at a single position and do not need to be turned. --> If the other part (not a nut) has an internal thread is it then a screw again?
I think the float (needle?) valve (or what the heck it is) still has some metering capabilities but that happens on very limited amount of stroke. But still also the fuel level is also altering just a littlebit when riding and therefore these may be actually quite similar lengths. If the lever ratio is 10:1 for example then 1mm difference on fuel level moves the valve 0.1mm.
I guess the float level may not be necessary that extremely sensitive to how fuel is waving on the bowl as a result of vibration. There is two floats on the same lever, quite small dimensions of the bowl in where the walls are reflecting the fuel waves back. The float has a mass and some vibration frequencies high enough has actually quite small impact on that kind of system. Even if the valve is vibrating fast it may have a very small effect to fuel level if the vibration is fast enough when compared to the inertia of the whole float system. The vibration may also impact many other ways to carburation such as how the fuel flow inside jets.
I think this is some similar conversation than bolt vs. screw. I have heard many different definitions to that and all of them make quite much non-sense. There may be no correct answer:
-Screw is able to make its own threads. --> Then take a file and make a simple notch to the end of the bolt, is it then a screw? If a small screw has M3 thread, is it then a bolt? Are Taptites then screws to be replaced with bolts? (Taptite is a self-threading component to be replaced with similar but non self-threading component.)
-Bolts are used when two peaces to be joined which either has no thread and it needs a nut. --> Then a same item can be a bolt or a screw depending on how it is used.
-Bolts accept a nut --> Small M3 screw is then a bolt.
-Bolts are larger than screws. --> Where is the limit then?
-Holes are required for inserting bolts. --> Many screws also need a predrilled hole in practice. Try the small screws to steel plate. Is also M3 screw then again a bolt? Are small sheet metal screws then allways bolts?
-Bolts are always fixed at a single position and do not need to be turned. --> If the other part (not a nut) has an internal thread is it then a screw again?
Measured the parts, I got the old parts from my SFC 50cm away from my computer.
Float:
-Distance from the center of the float cylinder to center of the axle is about 27mm.
-Distance from the center of the valve to center of the axle is about 7mm.
Lever ratio 27mm / 7mm = about 3.9
Valve:
-Distance from the seating location of the viton taper to end of the tip is about 2mm.
Lever ratio * metering valve stroke = 3.9 * 2mm = about 8mm
Then the fuel level can alter about 8mm when the valve is on the metering range.
Float:
-Distance from the center of the float cylinder to center of the axle is about 27mm.
-Distance from the center of the valve to center of the axle is about 7mm.
Lever ratio 27mm / 7mm = about 3.9
Valve:
-Distance from the seating location of the viton taper to end of the tip is about 2mm.
Lever ratio * metering valve stroke = 3.9 * 2mm = about 8mm
Then the fuel level can alter about 8mm when the valve is on the metering range.
Dellortoman
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- Tasmania, Australia
That's an interesting calculation Motojouni. Thanks for taking the time to measure and work it out. But I think it needs a little refining.
To make it model carby performance a little better, we need to work out how far the needle has to drop for the open area of the float valve to be sufficient to feed the main jet at WOT (wide open throttle = maximum fuel flow). So here goes ...
Flow rate depends on fluid pressure across the orifice, viscosity and size of orifice. The float valve is subject to the static head of fuel in the tank while the main jet is subject to manifold vacuum. I won't bore you with all my calculations, but making some assumptions about manifold vacuum at WOT, fuel level in the tank, also allowing that the flow through an orifice is proportional to the square root of the pressure (all other things being equal). I've come up with an estimate that the orifice area of the float valve needs to be about 2.5 times that of the main jet.
I believe Dellorto jets are rated by size rather than flow rate, so a #120 jet (near enough to stock size for a Lav triple) has an orifice diameter of 1.2 mm. So presumably a #250 float valve has an orifice of 2.5 mm diameter, which will have more than 4 times the open area as the main jet.
To make an opening of 2.5 times the area of the main jet, the conical tip of the float needle has to drop 0.68 mm away from its seat (again, I won't bore you with my workings, it's simple geometry).
So, using MotoJouni's 3.9 multiplier between float and needle, a 0.68 mm valve stroke gives a fuel level drop of 2.65 mm.
I suspect it's unlikely that it'll ever drop as much as 8mm under normal operating conditions - perhaps only when you're running out of fuel and have to switch to reserve.
Cheers,
Cam
To make it model carby performance a little better, we need to work out how far the needle has to drop for the open area of the float valve to be sufficient to feed the main jet at WOT (wide open throttle = maximum fuel flow). So here goes ...
Flow rate depends on fluid pressure across the orifice, viscosity and size of orifice. The float valve is subject to the static head of fuel in the tank while the main jet is subject to manifold vacuum. I won't bore you with all my calculations, but making some assumptions about manifold vacuum at WOT, fuel level in the tank, also allowing that the flow through an orifice is proportional to the square root of the pressure (all other things being equal). I've come up with an estimate that the orifice area of the float valve needs to be about 2.5 times that of the main jet.
I believe Dellorto jets are rated by size rather than flow rate, so a #120 jet (near enough to stock size for a Lav triple) has an orifice diameter of 1.2 mm. So presumably a #250 float valve has an orifice of 2.5 mm diameter, which will have more than 4 times the open area as the main jet.
To make an opening of 2.5 times the area of the main jet, the conical tip of the float needle has to drop 0.68 mm away from its seat (again, I won't bore you with my workings, it's simple geometry).
So, using MotoJouni's 3.9 multiplier between float and needle, a 0.68 mm valve stroke gives a fuel level drop of 2.65 mm.
I suspect it's unlikely that it'll ever drop as much as 8mm under normal operating conditions - perhaps only when you're running out of fuel and have to switch to reserve.
Cheers,
Cam
Just Jouni.
Exactly what you said Cam.
Only purpose to my calculation was an approximation when the tip starts to restrict the hole. I also omitted many other geometrical factors, for example if the carbs are tilted 20 degrees forwards (cos 20 degrees) and the lever turning (cos(arctan (8/27))), these drop the value to about 7mm instead of 8mm. And because of the square root law (for circle area), the metering is very nonlinear and I think the last 3mm has a very small influence. Most of the metering can happen approximately on the first 5mm level drop from the closing position.
I presume you used these formulas: area for a circle, flow on a channel which is a square root from a pressure difference multiplied by some constants and formula for hydrostatic pressure. That 2.65mm sounds what would really happen there on normal conditions and would have reasonably small effect to mixtures.
Exactly what you said Cam.
Only purpose to my calculation was an approximation when the tip starts to restrict the hole. I also omitted many other geometrical factors, for example if the carbs are tilted 20 degrees forwards (cos 20 degrees) and the lever turning (cos(arctan (8/27))), these drop the value to about 7mm instead of 8mm. And because of the square root law (for circle area), the metering is very nonlinear and I think the last 3mm has a very small influence. Most of the metering can happen approximately on the first 5mm level drop from the closing position.
I presume you used these formulas: area for a circle, flow on a channel which is a square root from a pressure difference multiplied by some constants and formula for hydrostatic pressure. That 2.65mm sounds what would really happen there on normal conditions and would have reasonably small effect to mixtures.
All looks like jolly good fun, just a thought about release points and 'tip' travel....
Some valve shuttles are spung, between the [float] input end and the conical tips.
So, some change in float bowl level may be taken by the spring and so not fully present iteself at the sealing tip.
Some valve shuttles are spung, between the [float] input end and the conical tips.
So, some change in float bowl level may be taken by the spring and so not fully present iteself at the sealing tip.
Gravelroad
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Cam is right Andy, Dellorto jets are rated by size, so a 120 jet acutally means 1,20mm diameter hole.
According to Dell'Orto manual, the sizes are 0.01mm's.
I was once told those Dell'Orto jets are very accurately made, even much more accurately than many other manufacturers do which actually state the size respect to flow characteristics. I don't know how true this is. It would need a flow bench to test that.
I was once told those Dell'Orto jets are very accurately made, even much more accurately than many other manufacturers do which actually state the size respect to flow characteristics. I don't know how true this is. It would need a flow bench to test that.
and yet their pumper jet sizes are calibrated in terms of flowrate or have I been misled there too?
Gravelroad
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I once measured them on a very accurate microscope and have to agree with Piet, not too bad with the original INC logo imprinted stuff, but even those can be 0,05mm (5 digits) or even more off their rating. At least they usually tend to be bigger than rated. 
I have a gauge and reamer set. The man stated that they flow similarly but the diameters vary, not the opposite way as usual. He said that the original Dell'Ortos are flow bench calibrated and therefore refuse to ream jets for multicylinder engines if a flow bench is not available and use only factory sizes on them. If this is true, Dell'Orto must have a some kind of flow vs. nominal diameter chart when they state the size in diameter.MotoJouni said:
He also stated this is the reason why they are sold in single units and has relative high price. I know some other brands are mostly sold in sets and most of the sizes end to the trash bin.
I still have no clue did he actually ever worked with them on a flow bench.
Dellortoman
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- Tasmania, Australia
AndyW said:
A Gerni should get it off
I think Mikuni jets are calibrated for flow rate. But without looking it up I'm not sure, or what the units of flow are.
These two things doesn't necessary have anything to do with each other how accurately the jets actually flow or how the sizes are stated (such as some kind of nominal or flow diameter, flow rate or something else for example). According to many other conversations this may make some confusion.