the all aussie dribble thread!

[Terry Syd]

Yeah, maybe, just maybe, you put on a 'green cap' to vent the tank - then reverse the existing vent so that the tank now acts like a fully vented tank - why then you could set the POP to what the saw really needs.

Just conjecture at this point....

 

[Terry Syd]

Decided to take another loop off the end of the spring (raining outside, nothing else to do). Each loop is .2mm in wire thickness, so I've now shortened the pre-load by .4mm.

I removed the pump diaphragm to keep from stretching it. Plugged up the two holes in the lid (one is from the impulse line) using a bit of sylastic), greased the gasket for the lid to keep it from leaking air (it will bleed down if you don't) and checked the POP. I ran the gauge up past the 20 psi mark to around 30 psi - and it still wouldn't pop off. It would if you hit the metering lever (another shot in the face, but I wore glasses this time!).

This is a Zama carb, so there may be a significant difference in the POPs compared to other carbs. I've noted from various webpages that there are different ranges of POP between manufacturers.

I may end up purchasing another spring before this is over.

I don't think the tank vent on my saw would be a contributing factor to high POP. I took a look at the manual and the vent is a two-way valve that allows excess tank pressure to vent around 45 kpa.

 

[masculator]

Thanks Porky, the dots are starting to connect. Anybody else remember vented tanks?

Not 100 percent sure but I think my 034 has a tank that vents both ways. oldest saw I own and I have run it hard in 50c plus all day and never noticed a "Phoosh" either way when opening the tank. and as far as I know it vnts both ways through the black plastic tower that sits atop the fuel tank and to the left of the carby!

 

[Terry Syd]

Those threads from 2008 indicated that the guys were miffed about something that had happened with the new saws. Whether it was extra POP to reduce fuel tank emissions or as some sort of hidden EPA 'limiter' to the carb. They wanted to find out what the POPs were on the older saws.

I'll give the change I've made a go, but I don't expect much. From what I read, when they dropped the POP they immediately had to lean out the idle circuit to keep the saw from dying. That didn't happen with my saw, so I doubt there was much change.

If I can't pick a significant difference, I'll order a selection of springs to work with.

 

[husq2100]

terry, I think if your cutting the spring down, it is actually getting a higher spring rate??? so may be harder to release....even though its shorter???

 

[masculator]

terry, I think if your cutting the spring down, it is actually getting a higher spring rate??? so may be harder to release....even though its shorter???

Correct! just like taking the angle grinder to the springs in your VL commodore! less travel but stiffer!

 

[tdi-rick]

terry, I think if your cutting the spring down, it is actually getting a higher spring rate??? so may be harder to release....even though its shorter???

In theory yes, but Terry's taking winds off the coil bound part of the spring so it will drop the measured rate as he's reducing the pre-load.

 

[Terry Syd]

I understand what you guys are saying about shortening a spring, it works like shortening a torsion bar does. However, Rick is right, these springs have 4 coils that are collapsed at each end, all they do is add to the pre-load height when loaded. Each time I nip off a coil it is like removing a shim from the loaded position - the actual spring rate never changes.

I may just keep nipping away (I've got 6 more coils to go), however at over 30 psi POP I don't know if another 1.2mm of pre-load will even get me under 30 psi.

At least as I work my way down in POP I can check to see how the saw runs after each change. It may be that at some point I run into a problem with the saw loading up while idling on its side. That wouldn't be a problem for a cookie cutting saw, but for a work saw in the real world it might.

 

[masculator]

I understand what you guys are saying about shortening a spring, it works like shortening a torsion bar does. However, Rick is right, these springs have 4 coils that are collapsed at each end, all they do is add to the pre-load height when loaded. Each time I nip off a coil it is like removing a shim from the loaded position - the actual spring rate never changes.

I may just keep nipping away (I've got 6 more coils to go), however at over 30 psi POP I don't know if another 1.2mm of pre-load will even get me under 30 psi.

At least as I work my way down in POP I can check to see how the saw runs after each change. It may be that at some point I run into a problem with the saw loading up while idling on its side. That wouldn't be a problem for a cookie cutting saw, but for a work saw in the real world it might.

30 PSI is a lot! put your thumb over the end of a pump with a pressure guage and I bet you cant hold more than about 35 psi!

 

[Terry Syd]

Yep, I can vouch for that, when I hit the metering lever and it sprays fuel all over me I have an indication that there is some energy behind it.

 

[masculator]

correct me if I am wrong but wouldn't taking out the preload (which allows things to start to move more consistently and from a lower pressure) just make things come on more suddenly rather than actually reducing the overall pressure required to compress the spring?

 

[Terry Syd]

You have to remember how the metering lever/metering valve is constructed. The spring pre-load is acting on the lever to hold the metering needle closed. An increase in that spring pre-load will hold the metering needle tighter on the seat - thus, it needs more POP to move the needle off the seat.

I'm trying to drop the POP so I need to decrease the pressure of the needle on the seat.

 

[tdi-rick]

Some basic spring theory (and sorry if I'm teaching some to suck eggs)

If a linear rate spring is rated at 100lb/in, this means it takes 100lb force to compress the spring 1", another 100lb to compress the spring another 1", etc. (disregarding any closed, or closed and ground ends, etc that reduce the rate)
In other words, it takes 100lb to compress the spring 1", 200lb to compress 2", 300lb to compress 3", etc.

If we have a 100lb/in spring with a 10" free length and pre-load the spring by fixing it's length between two points to 7", it will take more than 300lb's of force to get the spring moving initially, but only an extra 100lb force to move it a full 1" (400lb total), another 100lb to move the next 1", etc.

if we reduce the pre-load to 2" (8"total length) it will take just over 200lb to get it moving, etc.



In effect Terry is lengthening the working coil length and hence reducing the pre-load, which reduces the load it takes to get it to move initially, or POP.

 

[masculator]

Some basic spring theory (and sorry if I'm teaching some to suck eggs)

If a linear rate spring is rated at 100lb/in, this means it takes 100lb force to compress the spring 1", another 100lb to compress the spring another 1", etc. (disregarding any closed, or closed and ground ends, etc that reduce the rate)
In other words, it takes 100lb to compress the spring 1", 200lb to compress 2", 300lb to compress 3", etc.

If we have a 100lb/in spring with a 10" free length and pre-load the spring by fixing it's length between two points to 7", it will take more than 300lb's of force to get the spring moving initially, but only an extra 100lb force to move it a full 1" (400lb total), another 100lb to move the next 1", etc.

if we reduce the pre-load to 2" (8"total length) it will take just over 200lb to get it moving, etc.



In effect Terry is lengthening the working coil length and hence reducing the pre-load, which reduces the load it takes to get it to move initially, or POP.

yes but there would come a point where the preload no longer becomes relevant and you would be faced with the same required pressure to compress the coils, regardless of the length of the spring and despite the fact that it takes less to get it to start moving, the MOMENT (like a bending moment not a measurement of time, like in timoshenkos strength of materials) where it compresses enough to release the needle from the seat is still the same so shortening the spring by reducing the preload coils is just that, shortening the spring, the amount of travel required remains the same thus the amount of pressure required to release the needle from the seat would also remain the same or greater because the spring is under higher stress, wouldn't it?

 

[masculator]

I guess what I am trying to say comes back to what rick was saying somewhat.

say it takes 20 psi to move each coil of a spring 2 mm it then takes 30 psi to move each coil to move the spring another 1mm

5 coils moving 2 mm each takes 20 psi for an overall travel of 10 mm

it then takes 4 coils 30 psi to travel the same overall distance which is required to release the needle from the seat.

I may be wrong and am more than willing to be proved otherwise. but that is my take on it.

 

[tdi-rick]

If you take all the coil bound coils off and start reducing working coils you are increasing the spring rate and making the spring stiffer, so you need to then go for a softer rate spring, one with more winds/turns or a thinner gauge wire.

 

[masculator]

If you take all the coil bound coils off and start reducing working coils you are increasing the spring rate and making the spring stiffer, so you need to then go for a softer rate spring, one with more winds/turns or a thinner gauge wire.

Thankyou. that is what I was trying to say.

 

[masculator]

In effect Terry is lengthening the working coil length and hence reducing the pre-load, which reduces the load it takes to get it to move initially, or POP.

Ok so yes have looked back at this one and it is clearer in my head now. So instead of the coils being preloaded to the point where it takes 30 psi to move them they are headed to the point where they are requiring less. If I could find the thumbs up emoticon i would click it.

 

[MCW]

What if you take all the coils out? Is it POPless or just POPfree?

 

[tdi-rick]

What if you take all the coils out? Is it POPless or just POPfree?

Matt, I don't think you'll ever be Pop-Free and I reckon we'll all hear evidence at the GTG