Makita DSC 6401 fuel tank pressure

[Gravedigger]

When taking apart a my new to me CL seized 6401 chainsaw I pulled the fuel line off the carburetor and sprayed fuel until the tank was empty. Any ideas or is this normal?

intake side of piston scored, exhaust ok

Cylinder looks ok but I'm going to install big block kit.


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[XSKIER]

It probably seized because of too high of fuel pressure.

 

[Gravedigger]

How do I resolve? Rebuild carberator? It does mean fuel line is good along with impulse line right? The person I bought from bought it new in 2009.


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[XSKIER]

If it truly was a one owner saw, then you may want to check the fuel tank vent and open it up a little bit.

 

[cobey]

They will do that because of epa check valve. Most saws do that, if the valve is stuck gas WON'T come out

 

[makguy]

That's normal.
The carb creates vacuum pressure in the tank to siphon gas from the tank. When the pressure increases, the tank vent opens allowing air in. If the gas line is pulled off the carb, fuel should shoot out of the tank (path of least resistance) The problem is if it doesn't shoot out, then you could have a tank vent, fuel line or carb issue.

 

[Gravedigger]

That's normal.
The carb creates vacuum pressure in the tank to siphon gas from the tank. When the pressure increases, the tank vent opens allowing air in. If the gas line is pulled off the carb, fuel should shoot out of the tank (path of least resistance) The problem is if it doesn't shoot out, then you could have a tank vent, fuel line or carb issue.

Thank You that is what I conclude after searching this site. Have carb kit, and a big bore kit coming. I could not find what caused the piston scoring, only on intake side. Will vacuum check when it's back together, the po had taken it apart before I got it.

 

[makguy]

It looks like you have damage on both sides of the piston which could indicate a lean mix, but you should pressure test it to be sure. In my experience a one owner 2009 saw should not have oil seal issues (as opposed to a 2009 high hour HD rental saw, like the one on my bench right now) but you never know.
Just be sure you richer the carb up when you install the big bore kit.

 

[MS460WOODCHUCK]

Where is the tank vent on these saws? I was searching for the one on my 6400 and could not see it. I have not tore into it to look. Is it a pain to get to?

 

[makguy]

The tank vent is under the starter, obscured by the rear crankcase mount. There is a white plug over the rubber velve

 

[MS460WOODCHUCK]

How do you know if it is bad? I know when I took the fuel line off of the carb there was a good amount of pressure.

 

[makguy]

There are three ways it fails:

it doesn't seal, and leaks gas out of the valve.

It is plugged or stuck together and does not allow air into the tank, causing a vacuum lock and no gas to get to the carb until the pressure is equalized (by opening the gas cap.)

The third way is that it gets covered with debris (same lack of gas to the carb problem)

 

[Andyshine77]

How do you know if it is bad? I know when I took the fuel line off of the carb there was a good amount of pressure.

Pressure is a good thing. To check the tank vent simply empty the tank, screw in the fuel cap and hook a vacuum tester up to the fuel line and start pumping. You shouldn't be able to create any vacuum. The early tank vents on these saws had a foam filter held in by press in retainer. The later models have a press in white porous plastic disk. Underneath both is a green duckbill valve.

 

[Arrowhead]

Rebuild the carb and new fuel line while your at it.

 

[MCW]

Scoring on the intake side could be a sign of sucking in debris through the very average stock filter. The HD filter kit will stop that.
Andyshine is correct about the fuel tank vent. If you have an older saw with the black foam insert instead of the later white porous insert replace it as soon as you can.
Andy would remember but I had a fuel tank vent on an early model 7900 block solid with wood dust a number of years ago and the pressure build up when I left my saw in the sun on the back of my vehicle (temperature was over 100°f) split the tank along one of the joins. I had to get it plastic welded up and it's been fine since.
As also mentioned though with a bit of warmth it is normal for pressure to build in a tank as volatiles are gassed off from the fuel. Because your fuel line will be at the bottom of the fuel tank it will pump fuel out like there is no tomorrow. 98RON high octane fuel in the eyes DOES NOT tickle I clamp fuel lines off with a pair of haemostats now before removing the hose to avoid being blinded or engulfed in a mushroom cloud...

 

[MS460WOODCHUCK]

Sounds like my saw is completely normal which it should be being almost new.