JLTrent..MS290 spews gas out the fuel cap at high revs..

[greybeard]

I have two MS290s, one about 10 years old that I have used a lot and abused a good bit..the other about 4 yrs old but probably less than 5 total hrs on it. At different times, I've had the same problem with both. At high rpms, gas will spray out the fuel cap..not just leak, I mean spray! Just every once in a while. Changed that funky Stihl cap and the newer one still does it occasionally.. What's the cause...are those tanks supposed to be pressurized?

 

[jltrent]

I hate those flippy caps as from time to time I have dumped a tank of gas were I thought I had the cap on right and wasn't. Very common problem.... I have a Stihl Super 029 which is the same saw with the screw on caps I like a lot better. The tanks have a vent and it is pretty easy to change, but I bet the problem may have been the cap wasn't on good.

I have been temped to try these as they look like they would work a lot better. They have sold a boat load of them. I believe they would be easier to put on right, but I have no experience with them.


https://www.ebay.com/itm/Twist-039-n-fu ... 2035731612

 

[jltrent]

I have owned probably bout 10 saws in that line through the years and currently have 4 (029 Super, 2 MS290's and a MS390). The little 029 Super is my favorite. I have rebuilt/repaired several of those and some say they are hard to work on, but on a good day I can have them completely laid out in parts in 30 minutes and back together in 45.

 

[greybeard]

I don't think it's just the crappy cap. When it happens, fuel will spray out a foot from the cap like it's under pressure, not just leak down the side of it in a dribble, tho I have had that happen before when it didn't seal off good. This generally happens within a few seconds of starting the saw, like when I cleaned them both up yesterday, and was checking to make sure the bar was getting oil. I was holding one of them down low, with the end of the bar next to a barrel watching for oil droplets, and it sprayed gas all the way up on my shirt pocket, right out around the cap.

Where is the tank vent and how does the tank get pressurized to begin with?

Your saws always look so good..mine are pretty rough looking, even after a good cleaning.

 

[jltrent]

They have three different tank vents depending. One is located under the carb cover to the right side of the carb bout half way down. The other vent type sticks out of the tank right above the tank rear left side. They are very easy to test as I always test the fuel line and tank vent when working on one with my pressure/vacuum tester. It will hold pressure on pressure added and leak down on vacuum if working correctly. They build pressure from the gas expansion. Here is a video look at the 4:18 mark on...


https://www.youtube.com/watch?v=R3zU8HLs2OM

This pressure/vacuum tester is a must working on 2-cycles. I have one like this one.

https://www.ebay.com/itm/Mityvac-MV8500 ... SwXeJYIJOu

I have a tach like this one to set the carb air/fuel mixture. This one is the fastest refresh rate you can get as I have had others and they are junk compared to this one.

https://www.ebay.com/itm/DTI-20K-fast-t ... 2239942595


Here are pictures of the three different tanks vents.........

 

[greybeard]

Thanks JL!! I'll look for the vents later today.
You are the Fenceman/FarmFenceSolutions/TexasBred type resource of the 2 cycle world!

 

[jltrent]

On the later ones this is were it is at and what it will look like....

On some of the older ones trace this line and see what it has on the end. As my 029 Super has one like the third one in the post above.

 

[ohiosteve]

Thanks JL!! I'll look for the vents later today.
You are the Fenceman/FarmFenceSolutions/TexasBred type resource of the 2 cycle world!

I agree. I have learned a lot from reading JL's posts. I might have to get those adapter caps he posted that go from the flip style to a screw on. Stihl definitely over engineered those flip caps.

 

[greybeard]

I finded it...and probably the cause of the problem too. :hide:
Seen it before, had no idea what it was so I never messed around with it.

 

[wbvs58]

Some handy info on the pressure tester and tacho thanks JL.

Ken

 

[jltrent]

I finded it...and probably the cause of the problem too. :hide:
Seen it before, had no idea what it was so I never messed around with it.

It does look clogged. You might could clean with air pressure or replace. Part # is 0000 350 5802

https://www.ebay.com/itm/STIHL-FUEL-TAN ... 1106954606

 

[greybeard]

I (finally) got the little hose loose from it and vacuumed it from the side in the picture. I'll find out tomorrow when I do some limb cutting down on a fence that went underwater.