Chainsaw Porting Theory

[drf256]

So what's the impulse line do?

 

[drf256]

So the tank, which isn't under pressure, has a fuel line in it. How does that fuel make it to the carb?

Yeah, I know there's a line it travels in. It gets pulled out of that line hy the vacuum that the engine builds and sends the carb via both the Venturi and impulse lines.

Seems like we are arguing about who's right more than what's right. I don't need to be told that the fuel line delivers fuel to the carb, that helps less than zero. I hear there's this thing called a "piston" that goes up and down inside this thing called a "cylinder" too. Sheesh.

 

[Terry Syd]

The impulse line is providing an AIR pressure change on the side of the pump diaphragm to move it and pump fuel through the fuel valves on the pump diaphragm.

 

[drf256]

The impulse line is providing an AIR pressure change on the side of the pump diaphragm to move it and pump fuel through the fuel valves on the pump diaphragm.

No shït. Just what I was saying before you told me that the fuel line supplies fuel.

Then the pump diaphragm check valves move to and fro and build up some fuel pressure, but less than the metering valve pressure.

Then when fuel is sucked into the Venturi, it pulls up on the meeting diaphragm and opens the needle so more fuel flows into the area above the metering diaphragm. There's vacuum there, so not pressure, that opens the metering valve.

That's all I'm trying to say and understand better. I don't care if I'm right for any reason other than to make sure I understand. I'm not professing to know anything more about carbs than anyone else.

 

[MustangMike]

In answer to the question about what "pulls fuel" it is always the venture effect created by air passing through the carb, that is why they all have that "neck" in there and will not run w/o it. I think for most applications, it is not as important when the valve opens, just that it does ope at WOT.

I don't generally operate a saw at part throttle.

 

[MustangMike]

Al, you and I were posting at the same time.

 

[Terry Syd]

that is why they all have that "neck" in there and will not run w/o it

OK, what happens at idle? There is so little suction at the main fuel nozzle that they have to put a valve in the nozzle to keep the air from being sucked back into the wetside. - (Essentially, both the low speed circuit and the high speed circuit in the carb will be sucking fuel from the wetside once you get past the transition phase, but we have to get to that point.) At idle the amount of fuel that gets sucked out of the wetside is very small and the metering diaphragm needs a bit of help to ensure it has an appropriate amount of fuel for the idle mixture - so you have the pump diaphragm to ensure it gets topped up - stable idle and such.

The low speed circuit has it own fuel curve that is different from the venturi fuel curve. The object of 'blending' those two fuel curves together gives you the overall fuel curve for the air flow through the carb. The low speed circuit is in the straight section and has small fixed orifices that max out fairly quickly. The high speed circuit is located in the venturi and becomes more active as the air flow increases. - Blend the two together with the air flow and you get a more optimum fuel mixture over the powerband.

 

[drf256]

OK, what happens at idle? There is so little suction at the main fuel nozzle that they have to put a valve in the nozzle to keep the air from being sucked back into the wetside. - Essentially, both the low speed circuit and the high speed circuit in the carb will be sucking fuel from the wetside. At idle the amount of fuel that gets sucked out of the wetside is very small and the metering diaphragm needs a bit of help to ensure it has an appropriate amount of fuel for the idle mixture - so you have the pump diaphragm to ensure it gets topped up - stable idle and such.

The low speed circuit has it own fuel curve that is different from the venturi fuel curve. The object of 'blending' those two fuel curves together gives you the overall fuel curve for the air flow through the carb. The low speed circuit is in the straight section and has small fixed orifices that max out fairly quickly. The high speed circuit is located in the venturi and becomes more active as the air flow increases. - Blend the two together with the air flow and you get a more optimum fuel mixture over the powerband.

Exactly

Now Terry,

The idle hole is in front of the throttle blade, and the two holes behind that blade act as air bleeds at idle and supply fuel at mid throttle?

 

[Terry Syd]

The idle hole is in front of the throttle blade, and the two holes behind that blade act as air bleeds at idle and supply fuel at mid throttle?

A bit of semantics, I would say that the idle hole is in BACK of the throttle blade as I generally reference it according to air flow direction, but I know what you mean. Thus, the holes BEFORE the throttle blade act as an air bleed for the idle hole which is BEHIND the throttle blade where the suction is. They then transition from air bleed holes to fuel delivery holes as the throttle blade moves past them.

I mentioned the fuel curve of the low speed circuit, the max flow of the circuit on the Walbro is determined by the position of the low speed needle. On the Zama twin jet the max flow of the low speed circuit is determined by the size of the jets (Poleman as done a fair bit of work on those).

Here's the tricky part - making the transition from idle to full throttle. If the metering spring wasn't there to provide resistance as the suction increases - the engine would flood - thus, it become necessary to 'ramp' the fuel in a bit slower to keep pace with the rising air flow. The 'ramp' is determined by the spring pre-load, the strength of the spring rate and the size of the metering orifice. Those three things (along with the position of the air bleed holes) will give you the fuel mixture as the air flow increases and thus the throttle response and part throttle mixture.

The part throttle mixture is not as important in a saw, but you can see from the Mikuni manual how much effort they put into it. They want the karts and boats to have good part throttle power while in the turns and then coming out.

 

[lwhaples]

A lot of good info being kicked around.Now to just understand it all.

 

[drf256]

That makes sense Terry.

I have an issue at times with the 026 walbro wt194. On some saws, they load up with fuel at idle if set too rich. But leaning them out causes awful throttle response.

I'm also amazed how much the L circuit affects the H circuit mix. I can have a saw tuned perfectly on the H, then play with the L needle and get a totally different high end tune.

 

[Terry Syd]

I can have a saw tuned perfectly on the H, then play with the L needle and get a totally different high end tune.

Exactly, when Poleman and I start screwing around trying to increase the low speed circuit, we have to turn down the high speed circuit. In some cases we have to block out the auxiliary high speed jet in order to be able to lean out the top end enough to tune it.

A few more posts and you'll be modding carbs for guys.

 

[Terry Syd]

On some saws, they load up with fuel at idle if set too rich. But leaning them out causes awful throttle response.

So what you happen if you put a metering spring in with greater pre-load? - The higher pre-load causes a resistance to flow at idle that works just like lowering the float level in an old float bowl carb. Essentially, you lean out the idle, but you keep the same low speed circuit flow that you wanted when cutting wood.

EDIT: The next time you get into that situation, try putting a few shims under the metering spring.

 

[Deets066]

I like mikuni flat slides, much easier to tune the entire powerband. Wish they made a baby flat slide for saw

 

[paragonbuilder]

So what you happen if you put a metering spring in with greater pre-load? - The higher pre-load causes a resistance to flow at idle that works just like lowering the float level in an old float bowl carb. Essentially, you lean out the idle, but you keep the same low speed circuit flow that you wanted when cutting wood.

EDIT: The next time you get into that situation, try putting a few shims under the metering spring.

Terry,
So how do I know the difference between needing to adjust metering lever height, and metering spring strength?


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[Simondo]

So what you happen if you put a metering spring in with greater pre-load? - The higher pre-load causes a resistance to flow at idle that works just like lowering the float level in an old float bowl carb. Essentially, you lean out the idle, but you keep the same low speed circuit flow that you wanted when cutting wood.

EDIT: The next time you get into that situation, try putting a few shims under the metering spring.

The restricted L needle flow you get by shimming or putting in a more resistant spring, thus "Mechanically Leaning " down the mix would require the "Idle" screw to change reducing the increased RPM , and moving the butterfly slightly more closed (I presume ). That is going to reduce even more air flow available through the carb at Idle speed ....this is not a problem as you see it ?
My interest is also with the Stihl 024 026 saws as they can prove a little tricky on the idle with some carbs.

 

[paragonbuilder]

That's the job of that long bit of hose with the filter on the end of it inside the tank, it drops down with the fuel when you change position.

ROFLMAO
That's funny *s-word right there


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[paragonbuilder]

The restricted L needle flow you get by shimming or putting in a more resistant spring, thus "Mechanically Leaning " down the mix would require the "Idle" screw to change reducing the increased RPM , and moving the butterfly slightly more closed (I presume ). That is going to reduce even more air flow available through the carb at Idle speed ....this is not a problem as you see it ?
My interest is also with the Stihl 024 026 saws as they can prove a little tricky on the idle with some carbs.

I think what he means is by mechanically leaning it, you can then richen it back up with the L needle, essentially providing more fuel for transition, better spool up, without it loading up at idle.


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[Simondo]

I think what he means is by mechanically leaning it, you can then richen it back up with the L needle, essentially providing more fuel for transition, better spool up, without it loading up at idle.


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Tricky all this as there are many things to consider . Lower delivery start point volume from the idle hole is what im assuming will be the result , but until you open the throttle from its resting position to get into the transition side you are working on the fuel available . So i assume less fuel meant less air needed to maintain idle speed thus the gap around the butterfly would be tighter and less air movement through the carb.

 

[paragonbuilder]

Tricky all this as there are many things to consider . Lower delivery start point volume from the idle hole is what im assuming will be the result , but until you open the throttle from its resting position to get into the transition side you are working on the fuel available . So i assume less fuel meant less air needed to maintain idle speed thus the gap around the butterfly would be tighter and less air movement through the carb.

I see what you are saying... I need to look at the carb diagrams again while thinking of this.
Hopefully Terry can explain this better.


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