stihl 075 revs by itself

[tanked]

For starters a little history on the saw, Its a 075av that was used here in oregon for falling old growth years ago, I got it from a logger that had it sitting in his shed and he said the ignition was junk but that he thought that was the only thing wrong. In one way he was right the ignition was bad so I replaced that and took it out side to try and fire it up, after much pulling on the starter I resorted to dumping some fuel in the carb to get it to fire, when she did fire it went straight to high REV like she was going to scatter.. when I tried to shut it down it wouldn't, due to me forgetting to put the kill wire back in the switch, and choking it wouldn't work either so now panicking I took one for the team and jerked the plug wire off!!! can I ever say ouch!!!! well after tinkering with the carb(complete rebuild, adjusted HL screws and various other things) i think I have a vacuum leak? any help would be great. By the way this is my 2nd of 2 075's I picked up the other one the same way and it runs great. I have compared settings on the carbs and other things between the 2 of them trying to figure this out. any help would be great thanks, Waylon

 

[joe25DA]

my homelite 330 did the same thing, reved so high the switch didnt shut it down, but choking worked. in my case, was the intake boot, was sucking in and out. dont know if you have an intake boot on that saw but it sounds like an air leak. if its been sittin that long the fuel lines are prob junk, one probabaly has a crack.

 

[rmh3481]

Well at least you know the ignition is good!

Air leaks come from a couple different places on that saw. First place to check is the intake boot that connects the carb to the cylinder. Take it off and roll it around in your fingers looking for cracks. Next check the cylinder bolts that hold the cylinder to the crankcase. Sometimes these bolts will loosen up from vibration. Finally check the crank seals. If you have this big of a leak it should be easy to see. If not, you need to pressurize the case and then again vacuum test it too.

Here is a thread that may help, http://www.arboristsite.com/showthread.php?t=42768&highlight=poor+mans

Best wishes,
Bob

 

[Roy M]

There is no flexible boot on that saw, the carb is mounted solidly to the cylinder, but it does have a bakelite insulator that may be cracked. The gaskets, if I remember correctly there are three below the carb, also tend to leak if the nuts holding the air cleaner base are not frequently tightened. Is the throttle butterfly properly centred on the shaft and tight? The throttle link may also be bent.

 

[FATGUY]

For starters a little history on the saw, Its a 075av that was used here in oregon for falling old growth years ago, I got it from a logger that had it sitting in his shed and he said the ignition was junk but that he thought that was the only thing wrong. In one way he was right the ignition was bad so I replaced that and took it out side to try and fire it up, after much pulling on the starter I resorted to dumping some fuel in the carb to get it to fire, when she did fire it went straight to high REV like she was going to scatter.. when I tried to shut it down it wouldn't, due to me forgetting to put the kill wire back in the switch, and choking it wouldn't work either so now panicking I took one for the team and jerked the plug wire off!!! can I ever say ouch!!!! well after tinkering with the carb(complete rebuild, adjusted HL screws and various other things) i think I have a vacuum leak? any help would be great. By the way this is my 2nd of 2 075's I picked up the other one the same way and it runs great. I have compared settings on the carbs and other things between the 2 of them trying to figure this out. any help would be great thanks, Waylon

+1 way to take on on the chin. Welcome to the site!!

 

[ironman_gq]

did you try putting your thumb over the carb to shut it down

 

[Nicholas]

Definitely sounds like a air leak issue. Pressure test to +0.8 bar for 30 sec, and Vacuum test to -0.4 bar for 30 sec (Stihl method). Spray soap water solution all your connections, and seals. If you still can't find/isolate the leakage throw it in a bucket of water and pressurize to +0.8 bar.

The best is when a customer doesn't believe their saw is air leaking. Even after they watch the gauges on the test equipment. They still think you're out to rip them off. The look on their face when you throw their saw in a bucket of water, put it under pressure to give them a visual prospective of the leakage. Looks like a jacuzzi hot tub running. LOL!

Nick

 

[tanked]

Thanks for all your responses. I did allot of what you all mentioned(put my fingers over carb, checked gaskets, had the carb off so I know the bolts that hold it on are tight, put all new fuel lines on it, throttle butterfly is tight and properly adjusted, throtle linkage is straight). I went thru all the basics, Ive owned dirt bikes, harleys, quads, HO motors that i do all my own mechanicing on so I know most of the basics. ROY M what is this bakelite insulator that you talk of? is this the spacer(kind of a hard plastic) / throttle body between the carb and cylinder? NICHOLAS should I tear down the sproket side of the saw and look at that seal or just pressure test it? Do you know if I can inspect that seal from the outside or do I have to split the cases? I have heard that this seal is generaly the source of a leak. also nicholas do you have the pressure specs in psi I dont do bar but I can google it if nothing else. thanks to all of you, Waylon

 

[cbfarmall]

He's talking about the spacer between the carb and cylinder. These saws are notorious for shaking bolts loose. Had to loctite the intake to the cylinder to keep it from falling off on an 050. My 075 has half its bolts helicoil or drilled to the next size up. Your saw has an impulse and fuel hose. Pretty easy to get to.

I'd suggest checking the oiler boot, but your saw would be near impossible to turn over and smoke like a chimney if that boot was bad.

Chris B.

BTW, this spacer does not use the carb studs to hold it down. It has 2 separate allen headed screws.

 

[cmarti]

did you try putting your thumb over the carb to shut it down

Where's the fun in that

 

[tanked]

cbfarmall, I know what you speak of with the bolts rattling lose my other 075 wont keep the exhaust bolts tight, they both striped out and now have thread repair inserts in them. And Ironman gq believe me that was the first thing that I tried and it seemed to make it rev higher. at that point all I could think of was the top fuel and nitro bikes that blow pieces out of the motor and kill riders, so gently I took my hand of the carb and grabed the plug wire and let me tell you at FULL throtle WOW! WOW! WOW! but to the point anybody have info on the crank seal on the chain side do you have to split the cases to replace?

 

[cbfarmall]

Changing the PTO seals (yes, plural) isn't hard. Pull the clutch and drum and all that. Wind out the oil pump drive--it will either have a pin or 2 tangs mated to the clutch drum. Pop the big seal out of the crankcase. There is a second seal in that oil pump drive. Not sure how important it is, though. My Homelite 750 has a nearly identical setup with no 2nd seal.

Chris B.