Stihl Impulse Line (What Does It Do and Is It Important)

Nobody likes when their chainsaw suddenly starts leaking or stops running properly. Sometimes even after fixing carburetors or fuel lines, the engine doesn’t run properly.

So, before you pay a mechanic, you might want to know what an impulse line is and its importance. 

So, what is a Stihl impulse line what does it do

The impulse line connects the carburetor with the crankcase which is attached to the engine. The main purpose of this line is to create pressure. Then this pressure creates a vacuum inside the carburetor. After that, the attached fuel will start pulling fuel from the fuel tank. So, the overall flow of fuel depends on a healthy impulse line. 

So, read along if you wish to know more about impulse line and how it works. 

What Is a Stihl Impulse Line? 

Unlike most of the Husqvarna chainsaws, Stihl chainsaws have an intake boot. They are also on the outside, so it’s pretty easy to notice them upon inspection.

An impulse line is a connection between a carburetor and a crankcase. A crankcase is a chamber with a piston in it. 

On top of that, a carburetor has a fuel pump area with a fuel intake. So, an impulse line can create a vacuum inside the fuel pump. This vacuum will also be felt by the fuel intake and the connected fuel tank. 

Simply put, an impulse line’s goal is to create a vacuum inside the carburetor. 

What Does a Stihl Impulse Line Do? 

stihl impulse line what does it do

The functionality of a fuel impulse line is built on fluid dynamics. It’s part of Bernoulli’s principle too. 

The process itself is quite lengthy. But that’s not an obstacle since you’re trying to learn about it. However, we can certainly make it easy for you to understand. 

So, we’ve come forward and divided the process into 3 steps- 

Phase 1: Creating a Pressure in the Fuel Pump 

So, when you pull the rope to turn the engine, you’re actually turning the crankshaft. When the crankshaft turns, it lowers the piston.

As the piston is lowering itself, it starts to apply pressure in the lower crankcase. This then pushes the reed valve and shuts it off. 

Since the impulse line is connected, the remaining pressure will go through the line and fill the pump. 

Now, if you didn’t know, there’s a diaphragm inside the fuel pump. Once it fills up, it will push the diaphragm downwards. This will then force the fuel valve to turn off. 

Phase 2: Releasing the Pressure Back to the Crankcase

So far, the piston has gone lower. And the pressure has been distributed thanks to the impulse line. Without it, the pressure had no other way to go. 

But now, the piston will start to go upwards. As it’s going upwards, it’s creating a vacuum. So, this time, the pulse line will start sucking air from the fuel pump.

Remember the diaphragm which got lowered? Now, it’s going to go back to its initial position. Therefore, a vacuum gets created in the lower part of the fuel pump. 

This action causes the fuel valve to open again. To fill up this vacuum, the fuel is going to start entering the area. 

This way, the fuel enters the metering area following the path from the fuel pump. After that, the fuel then enters the lower part of the crankcase.

And with that, the first cycle is complete. 

Phase 3: Compressing the Air and Fuel Beneath the Piston

At the end of the previous phase, the fuel has entered the crankcase. Now, the second cycle will begin. But this time there’s air and fuel inside the crankcase. 

So, again, the piston starts lowering itself. As a result, both air and fuel start getting compressed. 

This forces the reed valve to shut off once again. Again, the pressure will enter the impulse line and fill up the fuel chamber. 

Naturally, the diaphragm will go lower once again. This action will again turn the fuel pump valve off. So, the remaining fuel will go to the metering area. 

So, when the piston goes upwards next time, the diaphragm will reset itself. Since it has already pushed the remaining fuel, there’s another vacuum. 

This lets the next chunk of fuel enter the pump area. This is how the impulse line affects the fuel circulation. 

So, be sure to inspect the impulse line from time to time. It’s usually behind the flywheel area. And if you do notice a crack or any holes, just pull it out and replace it. 

On top of that, be sure to check the fuel line as well. Likewise, it also can get clogged or get cracked. This isn’t good for your Stihl chainsaw. Thus, in situations like those, you’ve to replace the fuel line immediately. 

Now, speaking of impulse lines, here are our recommended picks- 

Now, you can select whichever you like the most and get it fixed!

Frequently Asked Questions

Question: Does a Stihl 025 have an impulse line?

Answer: The impulse line is very small and it can be hard to see. But don’t worry! You only need a flashlight and you’ve to focus on the flywheel. Usually, it’s behind the flywheel. 

Question: What is the fuel mixture for Stihl chainsaws?

Answer: The most used fuel mixture for the Stihl chainsaws is 50:1. You can either use ready-to-use mixed fuel or mix the fuel yourself. 

Question: How does an impulse fuel pump work?

Answer: The impulse fuel pump follows the pressure physics. It simply applies pressure and then creates a vacuum. That vacuum then pulls the fuel inside the fuel pump. 

Wrapping Up 

That was everything we could explain on the Stihl impulse line what does it do. Hopefully, we were able to satisfy your curiosity and make you feel content. 

If you have further inquiries, please let us know in the comment section. And finally, good luck!

Kevin Smith

4 thoughts on “Stihl Impulse Line (What Does It Do and Is It Important)”

  1. Hi, I just have a question regarding a Stihl TS400 con saw, it starts fine and runs well, then I smell fuel and I can see fuel seeping out under the air filter. So I took the air filter off to see what was happening, and as i press the throttle it sprays fuel out even though it will rev up, and when you release the throttle and it ticks over it still spits out fuel. I put in a new carburetor thinking it would solve the problem but it didn’t, so I don’t know what I can do next, so I hope you can help.

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  3. Kevin!
    I appreciate your explanation of how the carburator works. But can’t you say what problems one would be faced with if the impulse line is faulty or ceases to function?
    I have an 028 AV Super i got it going by pouring gs through the throtle and the pump started working at Hi revs. Itried to adjust the revs with all 3adj.screws but had minimal affect. It looked as some fuel was spitting out from throtle oriffice. I shut off the ignition and after a few minutes tried torestart normally, but it will not start this happened with a new carb. I havent tried with the old one but it would not start.

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