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Stihl 460 Magnum

Knots

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Very low hours Stihl 460 Magnum won’t take throttle.Will start and idle but trying to advance throttle kills it. Worked fine for a few cuts then quit. No carb adjustments have ever been made.
After saw sat for a day or two I noticed that the fuel tank was highly pressurized when I removed the cap. Startled me. Don’t know if this is normal or not.
Any help/guidance would be greatly appreciated.
Thanks,
Knots
 

Woodwackr

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Very low hours Stihl 460 Magnum won’t take throttle.Will start and idle but trying to advance throttle kills it. Worked fine for a few cuts then quit. No carb adjustments have ever been made.
After saw sat for a day or two I noticed that the fuel tank was highly pressurized when I removed the cap. Startled me. Don’t know if this is normal or not.
Any help/guidance would be greatly appreciated.
Thanks,
Knots
Tank vent…isn’t…venting
 

hacskaroly

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the fuel tank was highly pressurized
Did it the pressure suck in or blow out when you opened the cap. If it sucked in, then like @Woodwackr said, I would look at the vent, any type of blockage would make it so the saw could only suck in enough fuel until the impulse cannot overcome the pressure in the tank and then your saw shuts off from fuel starvation (even though there is fuel in the tank). One way to test this is to turn your saw on its side (with the fuel tank up), get it running and crack the fuel cap to act as a vent, if the saw keeps running, then I would look at cleaning or replacing the vent.
 

Knots

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Did it the pressure suck in or blow out when you opened the cap. If it sucked in, then like @Woodwackr said, I would look at the vent, any type of blockage would make it so the saw could only suck in enough fuel until the impulse cannot overcome the pressure in the tank and then your saw shuts off from fuel starvation (even though there is fuel in the tank). One way to test this is to turn your saw on its side (with the fuel tank up), get it running and crack the fuel cap to act as a vent, if the saw keeps running, then I would look at cleaning or replacing the vent.
Tank pressure was positive. Blew stuff out when opened. Vent seems to function as a one way valve, lets air in but won’t let fuel out. If it worked both ways the tank would leak when turned on its side.
Would a bad coil cause my problem? Enough fire to allow idle but spark getting killed by compression when you throttle up?
Thanks you for your earlier reply. Much appreciated….

Knots
 

hacskaroly

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Tank pressure was positive. Blew stuff out when opened. Vent seems to function as a one way valve, lets air in but won’t let fuel out. If it worked both ways the tank would leak when turned on its side.
Would a bad coil cause my problem? Enough fire to allow idle but spark getting killed by compression when you throttle up?
Thanks you for your earlier reply. Much appreciated….
From what I understand when there is positive pressure it usually happens on hot days when through vibrations the fuel vaporizes in the tank and creates positive pressure.

But...I am working on a 562xp...the carb is starting to piss me off. Purge bulb is sucking air, I pump it up many times and nothing, however when I open the tank it releases pressure (positive), if I pull off the carb, the fuel line starts wizzing everywhere (like a little boy when you change their diaper, pull it off and they turn into a water fountain!!). I bypass the carb with the purge bulb and it cycles fuel like a champ from the fuel line back to the tank with no positive pressure afterwards. I don't think you have this issue since you don't have a purge bulb...but these are the only two instances I know of that cause positive fuel tank pressure (or overpressure).

Does your tank vent look like this?:
1728316834229.png

You might try popping it off and try running your saw, see if anything changes in how it runs. If it does change and the saw runs better, I would replace the vent, if nothing changes, then the vent is fine. My opinion, I don't think the coil is the issue, however, you can verify that the gap is correct and maybe even clean it off if there is any grime and build up on it.

Otherwise, things I would check: 1) fuel filter, 2) fuel line - pinches, holes, 3) take a look at carb diaphragms and inner parts for gunk or buildup. Also, make sure any vent holes on the carb are clear, some times these get clogged or fines get in and sit on the diaphragms causing issues too.
 

Canadian farm boy

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Low hour 461 makes me think it doesn’t get used very often.
How fresh is the fuel?
As mentioned, change the fuel filter and check the micro screen in the carb.
Maybe try running it without the air filter and see if it will take throttle and make a couple cuts.
 
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