Chainsaw: venting the tank (1 Viewer)

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Gotta get outta here...
Sep 20, 2003
Had to replace the fuel line on my Stihl 025. It has served me faithfully for the last 25 years or so. This is the first time I had to replace any line on it. Damn good if you ask me. Moving to all orange and white saws... In large part due to expert info generously provided here by Dan.

Anyway, I removed the carb to put the new fuel line in, which proved uneventful. Well, until I touched an odd affair sticking out of the fuel tank. The plastic line broke on contact, no doubt embrittled due to gas and old age. Again, no complaint, it would have still been there had I not touched it.

At first glance it looked like a vent of sort. On closer examination, it looks rather odd. It was a piece of tubing about 1.75" long with 2 screw-in inserts, one at the very end, the other one about 1/2" in. I thought they may be orifices but I looked with a magnifier and I don't see any holes. So it looks like a piece of tubing plugged at both ends and inserted in a nipple communicating with the tank ( I can see a small hole in the roof of the gas tank). Any idea what that is for? Vents with a microscopic hole? Some sort of resonator? A bug and tracking antenna from some secret agency?

I made a new one trying to reproduce what was there by using a piece of 1/8" ID chinese pseudo Tygon, and put it on. But thought I should ask here first before firing it up, curious about its purpose and wanting to make sure that what I made will do.

Here is what I'm talking about. Sorry about big pics.

The repro piece I made:


The repro piece in place:


Know what this is and what it is for?
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Dammit, it's indeed a vent? How come I see no orifice in those inserts? It's not relying on the fit in the tube, I hope...

Had a biggish Husky too but sold it when I decided to go all Stihl. Down to 3 saws.
It seems to be the fit in the tube that does it, no hole :meh:

I see some advice to replace with newer type:
Tank Vent for Stihl MS201, MS201C - 0000 350 5802
Part No. 0000 350 5802

Tank Vent for Stihl MS201, MS201C - 0000 350 5802 | Stihl 021/MS210 Chainsaw Spares

Oke I got it, when they needed to get the screw into the tube they talked to an very old sailor and his dad reveled a technique that you now will have to learn to fix the vent tube:

thanks for all the info. Including the bottle stuff, may come in handy indeed.

Shoot, then what I made may not work. Is there a slot maybe on the side of the inserts? didn't look for that. Either way, I imagine the inner diameter of the line must be critical.

So I went back and measured the ID of my tube and the OEM one and mine is slightly bigger if anything. That would be good but I tried to blow through the tube and nothing... Not good...

Then again, if I try it and the engine doesn't bog down after a while it might be OK.

What an odd way to do a vent...
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crud! I was wrong. I thought that the vent I made would not allow air in based on my blowing check, but if anything it looks like it's too loose. Even gas is fairly easily coming out of it. I see drips at the end if the tank is very full and the saw inclined. The saw is working but I'd rather not have wet gas under there if I run the saw vertically, although the fumes would get sucked in the air filter admittedly.

A few more tests to try and understand what's going on and then I may just buy an OEM or commercial vent gizmo. Maybe the alternative one that our vriend up there mentioned.

Anybody knows if a running saw in the sun might develop positive pressure in the tank or is it always a slight vacuum? If the latter, I might be OK.
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ah, interesting, I was playing with the saw with the DIY vent. As seen before, it did indeed spill gas when laid sideways with a full tank. However, even in that position, when running it was clearly sucking air in with no spilling. Seems like the tank is under vacuum when running. So that means that the saw could be in normal upright position and also sideways and it would not leak when running, at least when coldish as I tried it. That would be most of the time when used I would think. So when stopped, just lay it on the ground horizontal and upright and it should be fine. And much of this is only for a full tank anyway. That leaves the issue of the saw dangling vertically from a harness or a rope, though. That is worse if the gas dribbles towards the front if the muffler is there.
I bought some generic bulb-type vents, I'll try those.
Damn, this thing is much more delicate to set up than it appears. I wonder if the OEM design may not be such that it leaks and they just don't care if it's very little... Since it's all buried under a cover, one may not see any of it. Which is also why I never paid any attention before to this vent.
got some cheap commercial vents for a quick trial. Generic ebay type. See pic below.
From what I can vaguely see, they look like some rubber duckbill affairs inside.
I can suck air in (hard) but not blow out.
So that should take care of the gas spills and vacuum avoidance but it could pressurize the tank if left in the sun without running.

I called Stihl and the tech I talked to (very nice and knowledgeable guy) did not think that pressurization could damage the tank. And at worst would force fuel through the carb if the pressure were to exceed 10psi which is unlikely even in the sun (but would flood the engine I imagine). So a true one-way valve should be OK to use. He also confirmed that the original tube and insert vent would let gas out, and that is normal.
When I asked him, he confirmed that the new vent used in the replacement MS250 is a one-way diaphragm valve and would not let gas out. It is an improved replacement for the original one and can be used in my 025 (and probably many others, looks like I have one on my MS290). The p/n is 00 350 5802. There is a pic in one of the links above.

- OK to use the original tube and insert vent but may have some slight gas drips. This is a crude design IMO. My DIY is probably fine to use.
- Better to use a true one-way vent like the superseding one on the 250 etc or the generic ebay ones if you would rather not have gas dripping under some conditions. I will test one of those to see if it's true that no gas comes out.


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