MS 290 Farm Boss

I picked up an older model MS 290 at a garage sale for $75 a few months ago. Based on what I knew about the people having the sale I figured it had been used very little(still had original Stihl bar on it) and had set up for quite a while. Brought it home, poured out the old gas, changed fuel filter, new fuel, new spark plug and it cranked right up. I have only used it sparingly since then but had noticed every time I did use it, it had a fuel leak from somewhere around the fuel cap. I had a extra cap laying around so I finally changed it out, but still had a fuel leak. Took a closer look and realized it was missing a vent and tank vent tube and it was spitting fuel out the top of the tank where the vent tube should have been. Picked a vent tube up at local saw shop that looks just like the pic below except it's black instead of clear, but even with the new tube it's still spitting fuel out the top of the tube now. The new tube has what appears to be a small screw in the top end, am I suppose to adjust this screw to stop the fuel leakage?

 

That's called a grub screw and not used for adjusting. It gets pushed into the tube and left alone.



edit: with that properly installed, it shouldn't be spewing gas.

 

Quick question if you don't mind..
Does the grub screw act like a cork in a bottle and stop altogether or do the threads allow for some air to get by to equalize pressure?

 

Threads are to allow air movement.

 

So it leaks when it's running? Some how the tank must be getting pressurized

 

Yes, it spitting fuel when running. Spits more fuel with more RPMs.

 

huskihl and idea what could be pressurizing the tank?

 

I've learned more from this website than 100 hours in youtube ventures.

Probably same amount of time on here though if I'm being honest...

 

I couldn't find anyone that had that one in stock, my local saw shop sold me the grub screw vent and said it would work. I'll take a closer look at it this evening and try to determine where I'm still getting fuel leakage.

 

There is a LOT of saw nonsense on the YouTubes.....

 

It needs a check valve like the one shown. It lets it breath in but not out. Also a filter. That air that gets sucked in when it uses fuel should be filtered. Screw isn’t going to keep sawdust and stuff out.

 

Actually, the grub screw was already in there for him so he’s good on that.
Can’t imagine they let much fuel come out so any idea on how the tank is apparently building pressure?

 

Any chance there is a hole drilled in the screw ( or screws)

 

I am far from a grub screw expert but, no they don't. Air is allowed to wind around the threads to escape or enter. It would significantly slow the flow of liquid should the tank get flipped but,, during running of the saw, gas being pumped from the tank will keep air flowing in.
I'm at a loss as to how this saw would be building pressure while running. I'm guessing it's coming from elsewhere.
I read some threads on a more saw dedicated forum and a few guys there were not fans of these grub screw vents and swapped them out w/ something else. One case was a ported 084 that drank fuel faster than the vent could keep up with and saw went lean during a long cut.

 

Not sure if there’s supposed to be an umbrella check valve underneath that vent inside the tank on the older ones or not.

The tank gets pressurized just by splashing the gas tank around. Running the saw warms and excites the fuel more and makes it want to evaporate, creating more pressure yet

One of these on top of that vent line would be better than just the screws and you wouldn’t have to dik around with the umbrella vent inside the tank.

I didn’t do any price-shopping. I just searched for Echo tank vent and this is what popped up. I’ve used many of them

ECHO Air Vent Check Valve A356000030 chainsaw trimmer fuel gas tank part New | eBay

 

But like mentioned before, being the 290 it probably had the one with the hose and vent on the end of it. If it’s that style, I would get one of those to put back into it.

 

I’m no expert either but a screw in a piece of hose seems like a half azzed way to make a vent since it is nothing more than an air bleed. Will it work? Maybe. But if it worked properly we would not be looking at this thread.

I do agree with huskihl is saying about fuel expansion from heat and saw running.
Put a saw with a check valve and a half tank of fuel in the sun, wait a half hour and loosen the cap chances are there is pressure from expansion.