MS310 fueltank pressure...

[mrcaptainbob]

The engine is running pretty good now. New engine (actually a 390..) new carb, filter, etc. Had a difficult time for it to run in the wood. I may have found an issue where the carb base snugs up against the boot and it's two rings. Did a vacuum check and there was hissing from that area. So...I put in an 'O' ring and  it now starts super great! But...while running, the fuel starts spitting from the fuel cap. I thought the fuel tank was supposed to be a negative pressure. New  fuel cap 'O' ring? New fuel cap?
I need to learn some stuff. Positive or negative tank pressure?

[sharp edge]

Captain

Any closed container will go under +and- pressure with temperature change. If you are leaking gas out of the cap, get it fixed. If you get gas on your clothes , and go by a fire, you will have to roll on the ground to get it out.

SE

[mrcaptainbob]

'Fixing' the leak is a given, Edge. IF that's the issue. There is no fuel leak at idle. When the saw throttles up is when fuel is being shoved past the cap. Yes, it may just need a new 'o' ring, maybe a whole new cap. Any idea on why there's so much positive pressure in the fuel tank at throttle up?

[sharp edge]

A bad 0-ring with more vibration at high r.p.m.s might do it. If it is pressure doing it, that would be scary. It takes a lot of pressure to make a o-ring  leak.

SE

[Real1shepherd]

Normally with the heat of the engine and depending on how close the P&C is to the gas tank....you can expect the gas to heat up and expand. That's what the tank vent is for....along with atmospheric air coming back in to replace the gas that's being used up.

As the saw sits after a hot run, you would eventually expect negative pressure in the tank because the gas is cooling down.

It's all about temps and your venting design. Some saw designs vent through the cap and a separate tank vent system.....some just through the vent system. On hot days loggin' daze, I saw gas 'boiling' when I removed the cap. Old Jonsereds are known for this issue and if you don't park them in the shade when not using, you're going to have issues restarting.

I would figure out how the tank is vented and examine for failure of some parts. Squeals were spewing gas at fire line fighters with those quick release caps. For a couple of yrs, there wasn't a fire line fighter in the US buying Squeals. I'm sure that's been sorted by now.  

Kevin

[mrcaptainbob]

I have experienced a high pressure fuel tank and the boiling fuel in it after a long run in high temps. This situation occurred after some engine work. Engine down time was due to doing a vacuum leak down test. The engine came out of the shade of the barn, fresh off the bench. Started it up, it ran great. Was  not running enough to overheat the fuel tank. Not fuel leak at idle, only at higher revs.
I'll take a closer look at the tank vent....
Thanks all...

[mrcaptainbob]

I 'fixed' the fuel tank pressure leak.
And you all can JUST SHUT UP!!! about it.
I called me every name in the book.
I do not need (nor want) this to go public!!
I was thinking last night about what I did...oh yeah...running a vacuum check. To do that I put a thin gasket between the muffler and the exhaust port director plate. It was that gasket that I forgot to remove. (SHUT UP!!!). Saw a shard of it hanging out of the muffler opening. Removed the remains.. the saw starts and runs great.
Added benefit is: no more fuel cap leak! Who knew???!!

[Old Greenhorn]

I am certain your secret is safe here. Nobody here would use this bit of info in anyway to remind you further down the road of the incident. ;D
 Besides, we all know we could have done the same thing. Frustration can be a real progress killer and we all fall victim from time to time.
 Glad you found it.