MS271 - Siphoning Fuel?

[steved]

Here is a new one on my new saw...

I bought a MS271 (new) about a month ago, I used it about five hours for that job. Put it in a saw box, put it in the truck, and hauled it 1,700 miles home. Got home, put it on the shelf in the garage. I'm fairly certain that I started it at least once after I got home.

Fast forward to yesterday, I put the saw in the truck, haul it 100 miles to a job. Get the saw out, and it pulls funny/hard? I pretty quickly diagnosed it as a hydro locked engine. So I pulled the plug, RIP it a couple times, and it blows fuel all over. It has good spark, put the plug back in and it pops on the first pull. Got it started and it smokes for a minute then clears up. Let it warm up and used it for about an hour without a hiccup.

So explain to me how this saw managed to siphon fuel up out of the tank and into the cylinder sitting on a shelf? It was never in any position but upright.

Overall, I like this saw...but this is a quirk I have never seen, nor can I understand how fuel siphoned uphill.

 

[huskihl]

It doesn’t siphon. The tank gets pressurized from fuel splashing around and evaporating. When the tank pressure exceeds what the metering needle in the carb can hold, it flows through into the engine.

 

[steved]

It doesn’t siphon. The tank gets pressurized from fuel splashing around and evaporating. When the tank pressure exceeds what the metering needle in the carb can hold, it flows through into the engine.

Ok, but why have I never seen this before? I've had a 391 for nearly 15 years and never anything like this...and it's made similar trips, stored the same way.

Isn't there a vent on the tank?

 

[huskihl]

Ok, but why have I never seen this before? I've had a 391 for nearly 15 years and never anything like this...and it's made similar trips, stored the same way.

Isn't there a vent on the tank?

Don’t know why one does it and not the other. Guessing the needle and seat fit together better in the 391.

Tanks are vented but only to let air in, for the last 10-15 years or so. Before that they were allowed to vent both ways

 

[steved]

Don’t know why one does it and not the other. Guessing the needle and seat fit together better in the 391.

Tanks are vented but only to let air in, for the last 10-15 years or so. Before that they were allowed to vent both ways

Thanks, I'll keep an eye on it...

 

[mbrick]

It sounds like the carb needle may not be sealing as well as it could.

 

[HarleyT]

The needle is likely slightly leaking. Not enough to affect the performance, but enough to flood the engine after it has been sitting. I would stop by the local dealer.

 

[Old2stroke]

Pressure in the tank is just exceeding the pop-off pressure of the carb's inlet valve.

 

[Okie]

Since it's new and UNDER WARRANTY do as Harley says.

Do not do anything to void the warranty.

If you really need to use the saw for a job right now you might consider dumping the fuel out of the tank before storing like overnight, etc and leave the gas cap slightly loose.

 

[huskihl]

Pressure in the tank is just exceeding the pop-off pressure of the carb's inlet valve.

Pop off on most newer saws is in excess of 35 psi. I’m pretty sure it’s just seeping by the needle and not building up that much pressure inside the tank

 

[Dennisthemenace]

Needle probably not quite right, it might seat in with use but just put the saw away empty

 

[Old2stroke]

Pop off on most newer saws is in excess of 35 psi. I’m pretty sure it’s just seeping by the needle and not building up that much pressure inside the tank

You are probably right but with current quality controls, how does one know without testing the carb in question.

 

[huskihl]

You are probably right but with current quality controls, how does one know without testing the carb in question.

What I meant was that since the EPA mandate of the one-way tank vent, pop off pressures have been up around 30 psi and there’s no way that the inside of tank is exceeding that. So it’s much more likely a poor seal between the needle and seat allowing fuel to flow through whenever it wants. 7900’s with the Zama carb had the same problem.

Might be able to polish the needle seat with a Q-tip and toothpaste or valve grinding compound and get the needle to seal up a little better

 

[Okie]

What I meant was that since the EPA mandate of the one-way tank vent, pop off pressures have been up around 30 psi and there’s no way that the inside of tank is exceeding that. So it’s much more likely a poor seal between the needle and seat allowing fuel to flow through whenever it wants. 7900’s with the Zama carb had the same problem.

Might be able to polish the needle seat with a Q-tip and toothpaste or valve grinding compound and get the needle to seal up a little better

Probably, MAYBE, but would this void the warranty on a NEW Stihl chainsaw?

 

[mbrick]

Probably, MAYBE, but would this void the warranty on a NEW Stihl chainsaw?

The dealer probably would never notice.

However the question is whether or not it's easier to take apart and fix at home, or take it in and deal with a warranty hassle.

 

[Okie]

or take it in and deal with a warranty hassle.

The dealer probably would never notice.

However the question is whether or not it's easier to take apart and fix at home, or take it in and deal with a warranty hassle.

or take it in and deal with a warranty hassle.

and have to leave it and then make a return trip to get the warranty repair and then find it's still doing the same thing after the EXPERTS have touched and blessed the saw.

Main thing is keep your money in your pocket. (especially if warranty repairs are expected) I've noticed once the money leaves my pocket it usually does not come back easily.

 

[huskihl]

Probably, MAYBE, but would this void the warranty on a NEW Stihl chainsaw?

It shouldn’t. They have no way of proving you did anything anyway. If it’s that new, Stihl should put a new carburetor on it. I’m sure they wouldn’t want the EPA finding out that fuel is getting blown out onto the ground every time you try and start it after it sits for a week

 

[HarleyT]

Stihl dealers just pop on a new carb, and file the warranty. Unless the tank is full of seaweed.
Time is money.

 

[stihltech]

Test the carb to 10 bar on pressure gauge. If holds, remove metering cover and diaphragm. Inspect lever height and set to spec. Open inlet valve under pressure and make sure it reseals. Test the tank vent under vacuum to make sure it opens. In my shop, I would replace the carburetor.
This could also be a small leak on the pump side.

 

[Dennisthemenace]

10 Bar?