3120xp dies when I throttle

[internet.user.human]

I have a stock 3120xp, maybe a year old. I was cutting some red oak yesterday and unfortunately the bar got pinched as I was making a relief cut in the log I was bucking. I was able to get the bar out of the cut but after that, the saw just dies when I try to go full throttle for cuts.

Saw starts fine and idles ok, but anytime I try to use the saw it just shuts off. It was a cool 30 degrees F here in NC yesterday when I was cutting, and I don’t think I was being hard on the saw.

I read some other threads and became concerned I had smoked it so I removed the muffler and took some photos of the piston. In you guys opinion am I toast? I see scoring but nothing that screams death to me. I’ve yet to take out the carburetor and give it a cleaning/inspection along with the fuel lines. But wanted to rule out the worst if I could.

Thanks in advance.

J

 

[Jacob J.]

The piston definitely has scoring/transfer from possibly bad fuel - was your fuel fresh or had the saw been sitting a while?

 

[internet.user.human]

The piston definitely has scoring/transfer from possibly bad fuel - was your fuel fresh or had the saw been sitting a while?

It had been sitting in the garage for less than a month and it was empty from my last use. So this was actually a fresh batch of fuel, purchased maybe 2-3 weeks ago.

 

[SteveSr]

I have a stock 3120xp, maybe a year old. I was cutting some red oak yesterday and unfortunately the bar got pinched as I was making a relief cut in the log I was bucking. I was able to get the bar out of the cut but after that, the saw just dies when I try to go full throttle for cuts.

Saw starts fine and idles ok, but anytime I try to use the saw it just shuts off. It was a cool 30 degrees F here in NC yesterday when I was cutting, and I don’t think I was being hard on the saw.

I read some other threads and became concerned I had smoked it so I removed the muffler and took some photos of the piston. In you guys opinion am I toast? I see scoring but nothing that screams death to me. I’ve yet to take out the carburetor and give it a cleaning/inspection along with the fuel lines. But wanted to rule out the worst if I could.

Thanks in advance.

J

Saw is dead. Likely caused by an air leak. Do a leak test BEFORE you pull it apart to confirm. That way you won't be guessing at the root cause of a blown saw.

 

[John Lyngdal]

Given the circumstances it sounds like the issue could be caused by a vacuum leak.
Enough vacuum to pump enough fuel to idle, but the fuel/air mix is out of balance with increased throttle.

 

[Jacob J.]

It had been sitting in the garage for less than a month and it was empty from my last use. So this was actually a fresh batch of fuel, purchased maybe 2-3 weeks ago.

What was it mixed at (ratio) and what with?
The fact that the saw is about a year old makes me think an air/vacuum leak is unlikely.

 

[ballisticdoughnut]

You could do a compression test but I suspect you won’t like the result. Either the high speed was set too lean or the saw has an air leak.

 

[internet.user.human]

Can you guys enlighten me on what causes a vacuum leak? I’m bummed that this thing is most likely dead.

Is it something that can be repaired? I’ve replaced fuel lines and carbs and various parts on my smaller poulan pr5020 and I haven’t encountered an issue that I couldn’t do myself yet but I also don’t have a ton of specialist tools. For example I’m going to have to research what I’ll need for a vac test.

@Jacob J. The gas was regular unleaded, 2 gallons mixed with 50:1 Husqvarna oil, for two gallons of gas.

Thanks for the replies.

 

[internet.user.human]

You could do a compression test but I suspect you won’t like the result. Either the high speed was set too lean or the saw has an air leak.

The compression feels fine. I usually start it without the decomp button pressed and it’s a bear, as usual. I don’t notice a huge difference there.

 

[internet.user.human]

I think I’ll get a mityvac vac/pressure tester, comp test kit, and a tachometer to see if I can diagnose this issue.

I’m just really hoping that the saw is salvageable. I got it for a really great deal from a guy who had bid on some dealer lots and can’t really afford to buy a new comparable saw.

 

[ballisticdoughnut]

Based on what I could see in your pictures I’d bet that cylinder can be saved pretty easily. New piston, rings, and cylinder gasket will get it back to functional condition. Just need to determine what caused the saw to run lean.

 

[cscltd]

When you say a year old and you bid on it from Some dealer lots-
do you mean a year old since You got it as a used or demo saw and thus not knowing it’s history?
that could be your problem in that damage may have been done before you got and now it finally acting up?.

 

[internet.user.human]

When you say a year old and you bid on it from Some dealer lots-
do you mean a year old since You got it as a used or demo saw and thus not knowing it’s history?
that could be your problem in that damage may have been done before you got and now it finally acting up?.

No it was brand new in the box when I got it.

 

[internet.user.human]

Based on what I could see in your pictures I’d bet that cylinder can be saved pretty easily. New piston, rings, and cylinder gasket will get it back to functional condition. Just need to determine what caused the saw to run lean.

Running lean means not enough fuel/oil mixture with the air correct?

 

[ballisticdoughnut]

Running lean means not enough fuel/oil mixture with the air correct?

Yes. Could be caused by an air leak or from incorrect high speed mixture setting.

 

[Jacob J.]

I think I’ll get a mityvac vac/pressure tester, comp test kit, and a tachometer to see if I can diagnose this issue.

I’m just really hoping that the saw is salvageable. I got it for a really great deal from a guy who had bid on some dealer lots and can’t really afford to buy a new comparable saw.

As Doughnut said, you'll be able to fix it, no problem there. But you want to know the root cause as Steve pointed out. It's pretty easy to do
a pressure/vacuum test. I'd pressure the fuel tank as well to see if the fuel tank vent is working. There's tons of videos on Youtube about doing
a pressure/vac test and testing the fuel tank.

That one should have the semi fixed-jet carburetor so the high-speed fuel delivery is not tunable. It should also have a rev-limited ignition
unless it came from Asia or South America.

 

[internet.user.human]

So I understand the locations that I could plausibly have a leak, but I want to understand why the saw running lean would cause failure to operate. Is it a pressure problem where the saw has a fault (gasket blows at the base of the cylinder, oil pump side, ignition side of the crankshaft, etc.) at one of the common locations? And then without seal you’ll get no compression etc. ?

 

[Yukon Stihl]

No if you get a vacuum leak then the saw runs to lean and to hot as there is not enough fuel to cool the piston. Then the piston expands and contacts the cylinder and then smears it's self to the cylinder because there is no oil either.
I am putting a 3120 together i got in a bucket.
My first vacuum test failed on it,i heard they have a factory leak under the oiler.

 

[jenko]

If it’s a nearly new saw and the fuel was good it could be a warranty issue, or worth trying for anyway

 

[Maintenance supervisor]

It had been sitting in the garage for less than a month and it was empty from my last use. So this was actually a fresh batch of fuel, purchased maybe 2-3 weeks ago.

What part of NC are you in ? I'm not terribly far from Charlotte.