075 stihl building pressure in fuel tank

[Kfd518]

Posted this in milling forum no help as of yet. First time out milling with this saw… cut great for first and second cut, sputtered a bit and died. Hissing from fuel cap vent hmmmm, attempt to remove cap and sprays fuel at me. Release slowly… fuel inside tank is bubbling! Saw cranks promptly dies. Rinse repeat, x2 or 3 times had enough quit for the day. Ethanol free fuel 91 octane 35:1 stihl hp ultra. both air and fuel filters fresh out of packages. Got home cranked back up and runs fine again. Any ideas?

 

[SEAM]

The saw might be set too lean and running hot after a while...

 

[William Balaska]

Your cap is the vent on the tank of that saw it is probably clogged. You can attempt to blow it out wth compressed air or replace it.

 

[NewToStihl]

Your cap is the vent on the tank of that saw it is probably clogged. You can attempt to blow it out wth compressed air or replace it.

That's a good thought, but it's been my experience that a clogged vent will typically cause a vacuum to be pulled in a fuel tank, not pressure built. And normally what he would see then was that after opening the cap (allowing the tank pressure to equalize) the saw would fire up & run well for a short while until vacuum built again and then the saw would die once again.

It's an interesting problem, but heat certainly sounds like a viable cause to me - especially considering after the saw sat for the ride home it fired up and ran fine.

OP, do you believe the saw to be tuned correctly? Is it clean with the cylinder finds free of debris/buildup?

 

[Ozhoo]

I agree with SEAM. I've never seen it personally but I'd assume that for some reason the saw was running too hot. Make sure that it's tuned slightly fat, cooling fins are clear, oiling well and a nice sharp chain.

 

[blsnelling]

Tank vents are designed to prevent a vacuum, but to prevent the build up of pressure. Is the saw clean? Some how you're putting a lot of heat into the fuel taking and causing it to boil the fuel. This often happens on the older saws with metal fuel tanks on a dirty saw.

 

[z71mike]

Have you popped the covers? Cylinder fins may be caked with grime, killing your heat sink breathability.

 

[SEAM]

Just had another idea (actually happened to a friend's saw)... Is the fanwheel intact? If a solid object (bolt, nut etc.) happens to enter the fan circle it will shave off some (or all) of the blades causing the saw to overheat badly after a few minutes of use.

 

[bullseye13]

Ditto on the heat. It happened to mine one time when i first got it. If you havent already, pull the covers off and wash the cylinder and rest of saw down with solvent to clean all the grime off of the cylinder. If you still have problems fatten it up a little bit. If your carb was tuned to 50:1 mix and your running 35:1, the oil displaces the fuel and causes it to be a leaner mix, requiring adjustment.

 

[William Balaska]

Put a carb kit in it as well and if that does not fix the problem the ignition module could be the culprit as it died out when it warmed up. Under vacum fuel could force its way out of the tank when opening it.

 

[Kfd518]

New filters fuel and air, flywheel in tact, fins as clean as I can get them did that after posting. Weren't to dirty to begin with surprisingly. Retuned from scratch, 1 turn out from seated high needle 1.25 out from low side needle. High side has been fattened up 1/8-3/16 turn slight burble at full throttle haven't had chance to get back in wood yet. Cap allows pressure in and out for some reason. Carb is freshly rebuilt and did well cutting a dead oak with plenty of solid wood left still after rebuild new impulse and tank to carb line waiting on new pickup line to come in and may go on ahead and order new fuel cap as well .

 

[bullseye13]

You cant tune these carbs based on what it sounds like at full throttle because of the integral governor in the carb. You need to richen it up till it just cleans up and 2 strokes in the wood. Hard to explain with a keyboard but carbs with governors are a weird bird.

 

[Kfd518]

Not trying to be smart, how do I go about tuning in the cut on a mill saw being on its side vibrating like crazy with carb screws on bottom? Not much room to put a piece of hose on it to use as a guide to the screw head…
I'm new to big and or old saws

 

[z71mike]

Don't tune it in the mill rig. Do a bucking cut for tuning.

 

[Kfd518]

Don't tune it in the mill rig. Do a bucking cut for tuning.

True heh heh I do need to trim some of these mill logs to length anyways... Thank you sir

 

[Kfd518]

Anyone able to track SN on these old saws? Just curious.… here it is if anyone is able.
4354471

 

[AKDoug]

That serial number is too short, and the saw is too old to track.

 

[Kfd518]

Alright, cutting great vertical horizontal is a diff animal. It seems like it bogs down real bad on its sprocket side but not quite as bad on the starter side. Guess I need to drop the money and go get press/vac test equipment. Go from there…

That serial number is too short, and the saw is too old to track.

per service manual 050AV,051 AV, 051AV electronic, 075 AV electronic, 086 AV electronic. Which references tech info bulletin # 27.80 on first page. "Page 17 section 3.14 Disassembly of cylinder and piston… From machine No. 4755 571 onward the 0.5 mm thick gasket… only mention this quote from the manual because of the serial number length. I do appreciate letting me know that it's too darned old for stihl to have any record on it though. I was afraid that would be the problem on one this old.

 

[Nate66n1]

Sounds like crank seals are poop

 

[AKDoug]

Alright, cutting great vertical horizontal is a diff animal. It seems like it bogs down real bad on its sprocket side but not quite as bad on the starter side. Guess I need to drop the money and go get press/vac test equipment. Go from there…

per service manual 050AV,051 AV, 051AV electronic, 075 AV electronic, 086 AV electronic. Which references tech info bulletin # 27.80 on first page. "Page 17 section 3.14 Disassembly of cylinder and piston… From machine No. 4755 571 onward the 0.5 mm thick gasket… only mention this quote from the manual because of the serial number length. I do appreciate letting me know that it's too darned old for stihl to have any record on it though. I was afraid that would be the problem on one this old.

I was kind of an inclusive statement... knowing that it was so short, it wasn't going to be trackable.