Stihl screw style tank vent?

[Pioneer]

If you have a Stihl saw with the older style tank vent that has screws in a tube, you may want to consider continuing to use it after a rebuild instead of going to the newer style duck bill vent. The new style will not release pressure from the fuel tank, and it may build up enough pressure to overcome the popoff setting on the carb and flood the saw. This is what happened to me on an 025, flooded the saw badly when it went from cold to warm conditions. Put the old vent tube back in, no more problems. This may be carb dependent, it is happening on a Walbro, whereas the ms250 I have that uses a Zama does not have any issues with the new style vent.
This could be solved by going to a stiffer needle valve spring on the Walbro, but then it may adversely affect carb tuning, so the simple solution is probably the best.

 

[SteveSr]

If you have a Stihl saw with the older style tank vent that has screws in a tube, you may want to consider continuing to use it after a rebuild instead of going to the newer style duck bill vent. The new style will not release pressure from the fuel tank, and it may build up enough pressure to overcome the popoff setting on the carb and flood the saw.

This is unlikely unless the carb is defective. Get out your MityVac and try to get one to pop off under 15psi.

This is what happened to me on an 025, flooded the saw badly when it went from cold to warm conditions. Put the old vent tube back in, no more problems. This may be carb dependent, it is happening on a Walbro, whereas the ms250 I have that uses a Zama does not have any issues with the new style vent.
This could be solved by going to a stiffer needle valve spring on the Walbro, but then it may adversely affect carb tuning, so the simple solution is probably the best.

Did you actually measure the pop-off pressure on the Walbro? It is much more likely that you have a needle and seat leak which caused your flooding. These older carbs used welch plug sealant that was not impervious to ETH fuels and would chip off and get stuck in various places, including the needle.

The RVP (Reid Vapor Pressure) of gasoline is only in the 7.8 - 9.0 psi.
https://www.epa.gov/gasoline-standards/gasoline-reid-vapor-pressureSo the carb should hold 10psi indefinitely.

BTW, I have retrofitted at least one 025 with a Walbro Wt-215 with an echo duckbill without having any kind of flooding issue.

 

[Pioneer]

Quite possible there is a tiny flaw in the seat, close inspection under magnification shows nothing however. There is a new needle in place, so no problem there. I noticed that the spring is very light in pressure, I'm using the old one so age may have taken its toll.
The saw holds fuel in the carb overnight, starts easy and runs perfect with the old vent in place, so in this case I choose not to mess with it.

 

[stihltech]

So if you think the new style vent may cause problems, why use it. Clean the two screw inserts, replace the vent hose. PUSH the inserts in the hose, do not screw them in. Worked for 30 years. Still does.

 

[RedneckChainsawRepair]

I've done old 025 and others with them and never a issue. How I started replacing the grub style here.

Both work. I even make my own grub. Push in.

Sounds like you have seat issues to me.

 

[Okie]

.as stihltech says:
PUSH the inserts in the hose, do not screw them in.

I've tested with a vac/pressure mity/vac screwng the grub screw into place in the vent tube vs pushing them in.
If if you screw one into tygon it will not vent and may stop venting anyway after awhile due to the tygon being soft.
I use grub screws for the vents but I use a harder type vent hose.

 

[SteveSr]

.as stihltech says:
PUSH the inserts in the hose, do not screw them in.

I've tested weith a vac/pressure mity/vac screwng the grub screw into place in the vet vs pushing them
If if you screw one into tygon it will not vent and may stop venting anyway after awhile due to the tygon being soft.
I use grub screws for the vents but I use a harder type vent hose.

This is another good reason to use the Echo duck bill... Unlikely to clog up and lead to fuel starvation.

 

[Mikesawed]

.as stihltech says:
PUSH the inserts in the hose, do not screw them in.

I've tested with a vac/pressure mity/vac screwng the grub screw into place in the vent tube vs pushing them in.
If if you screw one into tygon it will not vent and may stop venting anyway after awhile due to the tygon being soft.
I use grub screws for the vents but I use a harder type vent hose.

I didn't notice the date on your reply to OP, my bad there. I'm trying to be sure I'm getting this right before I consider attempting it. So let me get this right, your using a grub screw as a tank vent on a Stihl chainsaw? And you have to push the grub screw into the vent hose and not screw it in, correct? And you're using a more rigid vent hose and not standard Tygon fuel line? These "grub screws", they are the ones with the spring and ball bearing in the end, right? If I'm understanding all this correctly is there a standard spring tension on the bearing in these grub screws that will allow the release of pressure? Any idea at what pressure the vent should release at? I believe a previous post on this thread said that a carb should be able to hold 10psi indefinitely, is that correct? And the pressure in the tank assists the impulse pressure working together doing the job of fuel pump, correct? So if the carb can hold 10PSI indefinitely, and the combination of the impulse and the tank pressure pressurize the void in the tank pushing on the fuel causing it to be pushed into the fuel line and into the carb the top much pressure will cause it to flood so the tank has to release pressure before it reaches the pressure which will cause it to flood, but if it releases to early or to much then you will loose the fuel being delivered to the carb. So what is the optimal tank pressure? And the springs in these grub screws are strong enough to maintain tank pressure so as to continue supplying fuel to the carb and soft enough to release tank pressure to prevent flooding? Ok, I'm definitely curious! What size grub screw and what size vent tube? I'm rebuilding a Stihl 046 originally sold in 2000.

 

[Dennisthemenace]

I didn't notice the date on your reply to OP, my bad there. I'm trying to be sure I'm getting this right before I consider attempting it. So let me get this right, your using a grub screw as a tank vent on a Stihl chainsaw? And you have to push the grub screw into the vent hose and not screw it in, correct? And you're using a more rigid vent hose and not standard Tygon fuel line? These "grub screws", they are the ones with the spring and ball bearing in the end, right? If I'm understanding all this correctly is there a standard spring tension on the bearing in these grub screws that will allow the release of pressure? Any idea at what pressure the vent should release at? I believe a previous post on this thread said that a carb should be able to hold 10psi indefinitely, is that correct? And the pressure in the tank assists the impulse pressure working together doing the job of fuel pump, correct? So if the carb can hold 10PSI indefinitely, and the combination of the impulse and the tank pressure pressurize the void in the tank pushing on the fuel causing it to be pushed into the fuel line and into the carb the top much pressure will cause it to flood so the tank has to release pressure before it reaches the pressure which will cause it to flood, but if it releases to early or to much then you will loose the fuel being delivered to the carb. So what is the optimal tank pressure? And the springs in these grub screws are strong enough to maintain tank pressure so as to continue supplying fuel to the carb and soft enough to release tank pressure to prevent flooding? Ok, I'm definitely curious! What size grub screw and what size vent tube? I'm rebuilding a Stihl 046 originally sold in 2000.

I didn't notice the date on your reply to OP, my bad there. I'm trying to be sure I'm getting this right before I consider attempting it. So let me get this right, your using a grub screw as a tank vent on a Stihl chainsaw? And you have to push the grub screw into the vent hose and not screw it in, correct? And you're using a more rigid vent hose and not standard Tygon fuel line? These "grub screws", they are the ones with the spring and ball bearing in the end, right? If I'm understanding all this correctly is there a standard spring tension on the bearing in these grub screws that will allow the release of pressure? Any idea at what pressure the vent should release at? I believe a previous post on this thread said that a carb should be able to hold 10psi indefinitely, is that correct? And the pressure in the tank assists the impulse pressure working together doing the job of fuel pump, correct? So if the carb can hold 10PSI indefinitely, and the combination of the impulse and the tank pressure pressurize the void in the tank pushing on the fuel causing it to be pushed into the fuel line and into the carb the top much pressure will cause it to flood so the tank has to release pressure before it reaches the pressure which will cause it to flood, but if it releases to early or to much then you will loose the fuel being delivered to the carb. So what is the optimal tank pressure? And the springs in these grub screws are strong enough to maintain tank pressure so as to continue supplying fuel to the carb and soft enough to release tank pressure to prevent flooding? Ok, I'm definitely curious! What size grub screw and what size vent tube? I'm rebuilding a Stihl 046 originally sold in 2000.

No, the screws are just small pieces of fairly coursely threaded brass that push in and screw out if necessary. They are built to leak, or vent, slightly but enough in each direction. Simple and trouble free for decades until about '05 in the case of the 660s anyway. Then some sort of EPA madness must have caused a change

 

[stihltech]

The grub screws and/or vent assembly are available from Stihl. Again, way overthinking this.
All new vents are one way, vacuum only. Good old EPA got us again. Never had an issue with a good carburetor. You want a little pressure in the tank to help feed fuel.
Winter to summer fuel is where I see the most issue. And old fuel.
And the ECHO duck bill is one way also.
Just use a vent. Preferably the one that came on the saw.

 

[Okie]

I've done old 025 and others with them and never a issue. How I started replacing the grub style here.

Both work. I even make my own grub. Push in.

Sounds like you have seat issues to me.


View attachment 1131427
View attachment 1131428

WHAT IS THAT WHITE thing on the hose that you refer to as a homemade vent/

 

[RedneckChainsawRepair]

WHAT IS THAT WHITE thing on the hose that you refer to as a homemade vent/

ECHO TANK VENT is the white part on top.

 

[Okie]

What I've noticed:
You can make a grub screw vent, just use a machine screw and PUSH IT INTO THE proper size/type VENT HOSE. The grub screw vents both ways if pushed into a correct size/type (usually small black rubber vacuum hose (not soft tygon tubing) but won't leak pressure enough to cause a noticeable/bothersome gas leak USUALLY.
Also if you suspect that you have a grub screw vent problem ALWAYS check for TWO grub screws in the hose on some old Stihl chainsaws. It's also common for some of the old Stihl saws vent tube to get hard and break off and it laying in the case somewhere. When you locate it is may have the two slotted head grub screws still inside the little old brittle hose and they may have a slotted on the end, but do not use the slot for screwing them into the hose, just use small screwdriver in the slot and PUSH THEM IN. Usually one is enough to prevent gas leaking out if the proper size/ype hose is used.

I think the EPA Al Gore type vent is suppose to let air into the gas tank (VACUUM) and no pressure out (no fumes to the environment)

What is little strange to me is I see some chainsaws using duckbills in the gas caps and the duckbill is installed to let out pressure and not vent vacuum and the duckbill is OEM from factory. (and can only be installed/replaced one direction)?????
The old Poulan S25's are one example. It appears that the duckbill will let a little bit of vacuum reverse flow.

 

[Old2stroke]

I've always told people to make sure the pop-off pressure is at least 10psi but I've been told by some operating down south that a saw full of fuel left in the hot sun can well exceed this pressure. Vapor pressure plus thermal expansion.

 

[ZeroJunk]

I have had close to a dozen 2 cycle Stihl blowers come in hydrolocked this spring. Mostly SR450 sprayers.
If I don't change the carb it may hydrolock again in the warehouse before they even pick up. Happened last week.

So, may look at what would be involved in changing the vent to a hose and grub screw.

 

[stihltech]

As long as the EPA police to stop!