Pressure in the fuel tank

[Will Heal]

Today i was cutting logs with my jonsereds 520 sp. when I unscrewed the fuel filler cap there was pressure in the tank and the fuel started bubbling. I fuelled the saw up and cut for ten mins again then unscrewed the cap to see what happened and a fountain of petrol bubbles shot out and nearly hit me in the face! Never seen this before, what could it be and how could I fix it?

Thanks will

[wisecobandit]

fuel tank breather issue

Im not sure on the 520 but normally the breather vent usually on j/reds and partners looks a bit like an inline fuel filter and normally black midway thru a fuel type line if that makes sense.

Edited February 28, 2015 by wisecobandit

[GardenKit]

The tank vent lets air IN to the tank to replace used fuel. It will not let pressure out.

 

If the saw is working hard the fuel will heat up and, along with the vibrations it will vaporise, causing the pressure build up.

It has been mild today, but if you are using fresh fuel it will be winter grade and therefore vaporise much too readily. Even more so if it's a high ethanol content fuel from a supermarket.

 

Another cause can be a weak spring on the inlet needle in the carb. If it's seating below 5 psi it can lead to this problem.

 

Firstly, just change your fuel for some good stuff if you are currently using Tesco, then if that does not work overhaul the carb.

 

Make sure you are on 'summer' setting of the carb heating valve too, and make sure your cooling fins are clean.

[Will Heal]

fuel tank breather issue

 

Im not sure on the 520 but normally the breather vent usually on j/reds and partners looks a bit like an inline fuel filter and normally black midway thru a fuel type line if that makes sense.

 

 

I replaced the tank breather with a cheap one about a month ago cos I couldn't find a similar one to the original. Can you still get the 'inline' ones?

[Will Heal]

The tank vent lets air IN to the tank to replace used fuel. It will not let pressure out.

 

 

 

If the saw is working hard the fuel will heat up and, along with the vibrations it will vaporise, causing the pressure build up.

 

It has been mild today, but if you are using fresh fuel it will be winter grade and therefore vaporise much too readily. Even more so if it's a high ethanol content fuel from a supermarket.

 

 

 

Another cause can be a weak spring on the inlet needle in the carb. If it's seating below 5 psi it can lead to this problem.

 

 

 

Firstly, just change your fuel for some good stuff if you are currently using Tesco, then if that does not work overhaul the carb.

 

 

 

Make sure you are on 'summer' setting of the carb heating valve too, and make sure your cooling fins are clean.

 

 

Fresh fuel this morning from texaco mixed with stihl red oil.

I will check if the carb warming valve is open, it may be missing!

The cooling fins are clean, although half of the top one is missing and it's missing three fins from the fly wheel.

I cleaned the carb a while back but never replaced the insides so I will do that. Thanks for the help guys

[Mat]

Fresh fuel this morning from texaco mixed with stihl red oil.

I will check if the carb warming valve is open, it may be missing!

The cooling fins are clean, although half of the top one is missing and it's missing three fins from the fly wheel.

I cleaned the carb a while back but never replaced the insides so I will do that. Thanks for the help guys

 

Do you give your saws names? If so, I guess this one isn't called lucky!

[GardenKit]

Fresh fuel this morning from texaco mixed with stihl red oil.

I will check if the carb warming valve is open, it may be missing!

The cooling fins are clean, although half of the top one is missing and it's missing three fins from the fly wheel.

I cleaned the carb a while back but never replaced the insides so I will do that. Thanks for the help guys

It may be an idea to get some different fuel anyway, seeing as this problem only started on this mix.

 

It is not unheard of for a filling station to get a duff load of fuel.

[Will Heal]

Do you give your saws names? If so, I guess this one isn't called lucky!

 

 

Ha no not really, I call it the jonny, no one else has one

[Will Heal]

I'll try new fuel in it next week

[spudulike]

A vacuum in the fuel tank will stop the carb pulling fuel out of it and cause bogging in the cut and lack of top revvs. This is typically caused by a bad breather.

 

Pressure in the tank isn't so bad and many breathers do not allow to vent positive pressure, only vacuum. Generally caused by either temperature differences or leaking carb needle valve.

 

The only promlem with positive pressure is when it forces fuel past he needle valve and floods the engine!