OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Chainsaws and Concrete Cutters Petrol Chainsaws and Concrete Cutters help understanding chainsaw operation

Hi all..Just joined forum to hopefully get some advise.I have an old Oleo Mac 251 saw which up to now has been ok.I have the following problem...
A vacuum is being formed in the fuel tank and I cannot understand the tank breather arrangement.The tank has a solid cap.On the top of the tank is a diamond shaped bolt on unit which takes connections to (a)underside of it to fuel pipe/filter inside of tank and (b)on top one tube connection to the carberetta and another to the outside.
Has anybody a clue on what should happen.
Thanks in advance
Norm(trying to cut logs before the snows!)
Ps. is it pos to upload pics?

Hi Normy, and a big warm welcome to the forum. It's good to have you onboard. I can't advise on your chainsaw prob, but one of our members will, but I can assist you in uploading pics.

I use 2 free downloads; PhotoPlus6, for resizing pics, and export optimizing; and Photobucket for uploading the pics.
Once the pics have been uploaded into Photobucket, it will appear as a thumbnail; you just copy the image code under the pic, eg. [img.loadedpic12345img/]
and then paste that directly to your new post. It will appear in your post as an img string (as example above) and pics will appear if you do a post preview......Tip...do not delete the pic from Photobucket or it will automatically disappear from your post as well!!
Once again to the forum

Are you sure the fuel tank cap is supposed to be sealed? I don't currently have a chainsaw to look at, but my line-trimmer has the same requirement as a chainsaw to run with the engine in any position, and it has a leakproof fuel cap that is not actually sealed. If you look at the underside of the fuel tank cap you may find a sort of plastic dome with a small hole in the center. If you remove the plastic dome you will find a cunning device, similar to the device on a car air conditioner's Evaporator-Blower Assembly for letting water out. (It is known in automotive circles as a 'bloop tube' for obvious reasons: when enough water accumulates it suddenly opens, drops the water out onto the ground under the car, and re-seals itself). The one on the line-trimmer fuel cap lets air into the tank: when enough vacuum is generated it suddenly burps and lets in a short burst of air, then seals again. If the device is allowed to dry out for a long period it is prone to sticking closed: it ends up with a film of oil in it, and the oil oxidises, leaving a sticky film.

The blooper is quite tiny: about 3 mm diameter, with a squeezed-flat section. If it is working, you can suck air through it but can't blow through it. If you can't suck or blow through it, try squeezing it from the two sides until it pops open, to unstick it. Washing it in petrol would help to remove the sticky oil film.

Grumpy, the fuel cap is sealed , there is a separate fuel tank vent
line which goes to the right had rear side of the crankcase. Normy check to make sure the vent line is not blocked.

gmax

Thanks gmax, what brand of carburetor is that? My line trimmer has a Walbro WA, and the crankcase pulsations are delivered directly to the carburetor's fuel pump compartment through an internal vent from the crankcase that is part of the carburetor body. With the WA the only hose to the tank is just used to suck fuel - there is no need to deliver air to the tank through a hose. I can't say I'm fond of the WA, but the system at least has simple plumbing. It does have a lot of mysterious lean-mixture flameouts, though, when that bloop tube in the tank cap gets stuck shut. It gets mighty hard to start, too.

Grumpy, The "Oleo Mac 251" uses a "Tillotson HS" carby, similar to the Walbro WA, no impulse line, here's some photo's of my "Blitz 501" (rebadged Oleo Mac)



Here's some photo's of a "oleo Mac 946" which has a Walbro carby that uses an impulse line. There is an air vent chamber on this model, the tank vent outlet still goes to the right hand rear of the crankcase,


There is no valve in the tank vent, the lines just have to be clear.

Hi all,
just thought I would threw this in there so please correct me if im wrong but I would assume that chainsaws do not vent thought the cap because if you are operating the saw at a funny angle, say with the filler cap down then the fuel would run out the vent. My stihl vents into a funny white thing- pointing towards the bottom of the saw so it would only leak if it were upside-down!
I know if you turn a brush cutter upside-down it is likely to leak out the cap- that�s why most caps are located at stupid angles to avoid this.
Regards jay

Hi Wayne, how do you edit your pics with drawn lines and labels?....I does make it easy to understand where things go, etc.,Well done, mate

Jay, the bloop tube in the fuel cap will not allow fuel to leak out: it is closed and sealed except when there is a vacuum inside the tank. When the tank pressure is below atmospheric pressure the bloop tube burps briefly to let air in. I think both chainsaw and line trimmer manufacturers try to have the fuel cap above fuel level as much of the time as possible, but I think both machines have submerged fuel caps much of the time when they are running, especially when the tank is full. If your brushcutter leaks fuel when it is upside down it sounds as if there is something wrong with it. I would prefer not to store the thing with the vent submerged, but that applies to chainsaws too. There is always a vent somewhere, and the pressure in the tank varies with the outside temperature: some of the time there will be a higher pressure inside the tank than outside it. If everything is working properly it won't leak even then, but it is better not to find out the hard way that there is a leak.

Hi Darryl, I used "Paint Shop Pro 8" there is an option to add text to a photo, I used the paint brush tool to draw the lines.
Most photo editing programs have these options.

Grumpy, I don't think the valves in the fuel caps on the older chainsaws were very reliable (they always seem to dribble a little fuel). A separate fuel vent line with no valves is a better system.

Here's a photo of my "Stihl 07", the tank vent is a 2" length of tube
with a grub screw in the end, simple but effective.

gmax, from my chainsaw-using days I seem to recall that the total range of angles you operate the saw at, is fairly limited. I basically used them for vertical cuts or horizontal cuts, and the handles were designed so that the only way I could comfortably hold them for horizontal cuts had the fuel and oil tank caps facing upward. Of course the saws also got tilted a bit in the fore-and-aft direction. The advantage of the vents you have shown is that if necessary they can let some fuel out without doing any harm, and without interrupting the operation of the saw. However I'd be very surprised if the tank isn't designed so the vent is above fuel level when you lay the saw down, so whenever it isn't being used, it can let vapour out without ejecting liquid fuel. I'm not sure that line trimmers are always laid down in a consistent way when they aren't being used - mine isn't anyway. The bloop tube system guarantees that there will often be pressure in the tank when it isn't being used, so you are asking a lot of the bloop tube to hold that pressure indefinitely without leaking an occasional drop of fuel. In other words, I think the simple vent-line solution is better than a bloop tube for chainsaws, but I'm not so sure it's better for line trimmers.

Grumpy, My kawasaki trimmer has a large flat diaphragm valve in the fuel cap, it's never leaked, I wonder if Normy has fixed the problem he was having.

Hi Wayne, is paint shop pro 8 a free download or do you have to purchase it?

Hi Darryl, Paint shop pro 8 is not free but "Paint.NET " looks like it would be ok and is free,

http://www.getpaint.net/

Hi Wayne, thanks for the link....all downloaded, all I gotta do now is learn how to use it

Darryl, There is a forum on the Paint.NET website, if you get stuck
have a look there.