Impulse tube?

[dodge man]

This comes under the heading I do not need to know, just curious. What is the impulse tube for on a Stihl chainsaw. Does it pull fuel into the area under the piston to lube the crank? Does it help get fuel into the cylinder?

The truth is I do not know how these engines work. Years ago I broke down an old two stroke lawn mower. The carb fed into the crankcase through a butterfly arrangement. The exhaust port and intake port were offset a lot more than on a chainsaw. I could picture how this setup worked. On the engine I just rebuilt on my Stihl the ports almost straight across from each other. I can not picture how you get enough draft to get fuel and air through the carb. Maybe the impulse tube does this?

 

[Snobdds]

This comes under the heading I do not need to know, just curious. What is the impulse tube for on a Stihl chainsaw. Does it pull fuel into the area under the piston to lube the crank? Does it help get fuel into the cylinder?

The truth is I do not know how these engines work. Years ago I broke down an old two stroke lawn mower. The carb fed into the crankcase through a butterfly arrangement. The exhaust port and intake port were offset a lot more than on a chainsaw. I could picture how this setup worked. On the engine I just rebuilt on my Stihl the ports almost straight across from each other. I can not picture how you get enough draft to get fuel and air through the carb. Maybe the impulse tube does this?

It's basically the fuel pump. The vacuum creates a negative pressure to pull the fuel to the carb. There will always be 3 ports on a saw. Fuel in, fuel return, and impulse line.

 

[ericm979]

This animation shows how a chainsaw carb works, and what the impulse line is for: How a 2-Stroke Carburetor Works 'Unique Animation' - YouTube

Two stroke motorcycle carbs are gravity fed so the pump part is replaced with a float bowl and needle, and the metering parts are usually more sophisticated. A saw really only needs to run right at idle and full throttle while a motorcycle needs to run well at part throttle.

The "butterfly" valve was reed valve. It prevents the fuel/air from being pumped back out past the carb when the piston goes down. Most modern saws are piston port instead of reed valve. The position of the intake port determines when it is closed by the piston skirt. Higher up makes it open later and close earlier. The saw designer (or someone porting the saw) can move that around to get the performance characteristics they want.

When the piston goes up enough to uncover the intake port there's a vacuum that pulls fuel/air through the carb and into the crankcase. When the piston goes down it closes off the intake port and the top of the piston uncovers the tops of the transfer ports which transfer the fuel/air from the crankcase to the cylinder. Then the piston goes up again, closing off the transfers and the exhaust port so it can compress the fuel/air and the plug ignites it.

What's interesting is that all these gas flows have inertia and also have pressure (sound) waves which can effect how they work. The exhaust waves in particular can be very powerful. A properly tuned exhaust pipe along with the right ports and timing can make huge power. Saws have very low power for their displacement but they are designed to be light and compact. You can see big bulky exhaust expansion chambers on race saws but not on work saws.

 

[dodge man]

So how does the crank and rod bearing get oil? I know Stihl use ball bearings there, does it even need any oil?

 

[Snobdds]

So how does the crank and rod bearing get oil? I know Stihl use ball bearings there, does it even need any oil?

When the air is sucked in through the intake, it's below the piston. As the piston goes down, it pushes the air and oil/gas mixed air down into the crank case, which lubricates the crank. As the piston moves back up, the air is sucked through the transfers ports by vacuum and put on top of the piston to be burnt. Then the cycle repeats.

Pretty brilliant and simple, really.

 

[ericm979]

The simplicity, and the science behind the exhaust and intake tuning and porting is why I love two strokes. Not that there isn't as much science behind four strokes, its just different.

The roller element bearings used on modern two strokes need only a tiny amount of oil. And the piston does not need much either. That's how they can get by on 50:1 oil ratios or even less. Not like plain bearings used on many four strokes where the oil is the bearing and a constant flow is needed.

 

[dodge man]

When the air is sucked in through the intake, it's below the piston. As the piston goes down, it pushes the air and oil/gas mixed air down into the crank case, which lubricates the crank. As the piston moves back up, the air is sucked through the transfers ports by vacuum and put on top of the piston to be burnt. Then the cycle repeats.

Pretty brilliant and simple, really.

I think I get it now. I pictured the air/fuel entering above the piston but what you say clears it up. I rebuilt a Stihl MS250 and put a new jug on it. When I look at the old one, I see slots that are fairly wide, 4 of them, which must be the path for the air/fuel to get on top of the piston. I was so stuck on the way the two stroke worked on a lawn mower I broke down years ago I wasn’t getting how the chainsaw worked.

 

[xyz123]

This guy does the best explaining videos for learning. Hope it helps you understand.

 

[newbury]

On the subject of impulse lines -
I've a Stihl 660. The saw won't drop down to idle w/o stalling. Impulse line may need replacing versus a tear down to replace seals.
I've got a new OEM impulse line. Months ago I ran across a youtube vid that made replacing the impulse line on the 660 look tricky and complicated, can't find it now.
How difficult is it? How much teardown is involved? Any good video links?
thanks in advance

 

[xyz123]

660 easy peasy to change over.

Also make sure sure your clutch springs still good or shoes wore and staying engaged. GL

Last 660 I did I just used oem echo black line 6mmx3mm for impulse.