Stihl Grub Screw Tank Vents?

[SteveSr]

Hello,

Saw a recent post from Brad @blsnelling to get rid of grub screw tank vents, I suspect, because that can cause lean running conditions and burnt pistons if they plug up.

I am currently working on rebuilding 4 Stihl 1123 series "plastic" saws. 2 of these have the newer MS tanks with the remote regular vent. However, the other two have the grub screw vents and I don't believe that the plastics will allow the newer vents to fit these tanks.

For one of these I have already fashioned a hardware store set screw replacement and the other is an OEM with what looks like shrunken PUR tubing. I have tested both of them with a vacuum tester and they both "vent" i.e. leak a little. Should I use these or does anyone have any other suggestions?

Thanks,

Steve

 

[HarleyT]

Sure, use them.

 

[blsnelling]

I lost an otherwise perfect 084 topend to a grub screw tank vent. Never again.

 

[SteveSr]

I lost an otherwise perfect 084 topend to a grub screw tank vent. Never again.

So what other options are there if the saw was designed to use grub screw vents?

 

[HarleyT]

I lost an otherwise perfect 084 topend to a grub screw tank vent. Never again.

How?

 

[HarleyT]

Just use one of the ms250 vents if you want.

 

[blsnelling]

An 084 burns a lot of fuel. I had a long bar buried in Oak. The saw ran lean because the grub screw caused a vacuum in the tank. The saw was in otherwise perfect condition. I removed the grub screw and installed the orange rubber valve in the tank, as more modern saws use. Problem solved.

 

[HarleyT]

That doesn't sound very conclusive, sounds like a guess.

 

[SEAM]

Never had any problems with grub screws... The only important thing to remember is to PUSH the screw into the hose. Turning the screw will cut a matching thread into the wall of the hose closing the gap needed for the venting action.

 

[blsnelling]

That doesn't sound very conclusive, sounds like a guess.

What happens when a tank vent is blocked? A vacuum is drawn on the tank and prevents the carb from getting fuel. The saw gradually goes lean. When in the middle of a long cut you don't realize this is happening until it's too late. Take a flexible hose and thread a screw into it. Let that set a few years and the hose conforms to the threads, slow allowing less and less air through it. Sure, you can replace the hose. But why use an inferior design when not necessary.

Believe what you want, but this is without question what happened to that 084. It had no air leaks and was not tuned leaned. A new topend and a new tank vent fixed the saw. BTW, this was in a huge Burr Oak that was massive, up to 7' in diameter. Unobstructed fuel flow is absolutely required in big long cuts like this.

 

[HarleyT]

So you threaded the grub screw in?

 

[HarleyT]

What line did you use?

 

[tomdcoker]

What happens when a tank vent is blocked? A vacuum is drawn on the tank and prevents the carb from getting fuel. The saw gradually goes lean. When in the middle of a long cut you don't realize this is happening until it's too late. Take a flexible hose and thread a screw into it. Let that set a few years and the hose conforms to the threads, slow allowing less and less air through it. Sure, you can replace the hose. But why use an inferior design when not necessary.

Believe what you want, but this is without question what happened to that 084. It had no air leaks and was not tuned leaned. A new topend and a new tank vent fixed the saw. BTW, this was in a huge Burr Oak that was massive, up to 7' in diameter. Unobstructed fuel flow is absolutely required in big long cuts like this.

The grooves on the grubbs will eventually become filled with whatever it is that gets in there and not vent. I have removed several of them that are corroded. It may still draw enough air for a small saw used to cut fire where you are not cutting continually, but I agree with Brad that on a big saw, especially if it is used for milling it is a bad choice. Tom

 

[SEAM]

Newer tank vent types will also get clogged, especially when milling .

There is no easier vent type to check/clean/repair than a grub screw with a transparent hose... If there are no bubbles rising in the hose when shaking the saw (or having it idle) with a freshly filled tank the vent is plugged. It is also possible to attach some filter material (sponge, small fuel filter etc.) to the hose end to avoid penetration of contaminants.

 

[hotshot]

Just run a thin Dremel cut off wheel lightly up each side of the grub screw before you push them in, guaranteed not to plug up if the hose later forms into the shallow threads...which they usually do!

 

[dugide]

An 084 burns a lot of fuel. I had a long bar buried in Oak. The saw ran lean because the grub screw caused a vacuum in the tank. The saw was in otherwise perfect condition. I removed the grub screw and installed the orange rubber valve in the tank, as more modern saws use. Problem solved.

I had the same thing happen when milling (084) thought I was running out of gas so I stopped to gas it up but the tank was half full. couldn't find anything wrong until I noticed the tank vent plugged with junk. mine had 2 screws in the line because it would leak with one. cleaned out the line and put both screws back in and never had a problem after that. but I do pressure check that vent line once in awhile.

 

[Mad Professor]

So 084 You negelected and filled with crud?

I've 80s 038/028 that are still OEM and work fine. Keep it simple Brad

 

[HarleyT]

Just run a thin Dremel cut off wheel lightly up each side of the grub screw before you push them in, guaranteed not to plug up if the hose later forms into the shallow threads...which they usually do!

I don't think that they make dremel wheels that thin.

 

[HarleyT]

If I am wrong, I'd love to see a pic of this procedure. The "finished" grub screw, at least.

 

[HarleyT]

I have seen threads with cuts in them, I'd just love to see you do it with a dremel wheel, on the size screw used in these vents......