2016/10/25

Starting a 2-stroke (chainsaw) engine after running out of fuel

Starting a 2-stroke (chainsaw) engine** after running out of fuel

“Troubleshooting” your chainsaw

Everyone should and probably have heard that engines should never run out of fuel completely for many reasons. One of the reasons is that the last drops of fuel can bring in unwanted contaminants but other, more sensible reason is that air gets into the combustion circuit. (Even if you think about human organs, air bubble is an unwanted and can be fatal if it is present in the veins. Similarly, it can be "fatal" for engines too.) Imagine a running engine that just runs out of fuel and for a few seconds that engine will be running without a mixture. This is serious to the metal elements.

Now that I have mentioned some of the "don't-s", I can tell a solution on, how to resolve the problem if it happened. It can even take place after a long time of not being used, when you're trying to start your 2-stroke engine.
Main components under the cover (Note: not all parts are listed.)
Eventually the steps for a cold engine start are:
  1. Engage the chain brake when the chainsaw is started.
  2. Press the air purge repeatedly until fuel begins to fill the bulb. (The bulb need not be completely filled.)
  3. Pull the choke lever to full extent.
  4. Start throttling.
  5. Push the choke control to “half choke” as soon as the engine fires.
  6. Pull the starting chord until the engine starts.
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What you should never do in case, it’s run out of fuel:
  • Never press the purge! By doing this, air will be pumped from the empty tank to the carburetor, that will be completely full of air and it will be difficult to get it to work.
  • Start pulling in the engine: This will move the pistons up and down without any lubrication and mixture. This can result in huge damage again!
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Steps to resolve the problem:
  • Add fuel :)
  • Pull out the choke to full extent (fast idle),
  • Start pulling the starter handle 3-6x,
  • Press the air purge ~6x, 
  • Air purge: Press the air purge repeatedly until fuel begins to fill the bulb. The bulb need not be completely filled.
  • Start pulling the starter again.
  • Press air purge again to release air.
  • Do you see fuel in the purge?
    • Yes. Great, keep pulling in, the engine will start soon.
    • No. Try the following steps:
Try 1.:
  • Start pressing the purge.
  • Open the fuel tank, and while open, start pressing again. This will actually help the air to flow out, but not all. If there's too much of air, it'll not resolve the problem. But in some cases it may help. (Pay attention to have enough fuel while doing this and don’t end up sucking in air.)
Fuel lines
Try 2.:
  • Remove the cover which holds the purge button.
  • Look for the two fuel lines connected to the purge. (Picture above) One is coming from the carburetor, the other one is going to the fuel tank.
Fuel flow diagram
It may seem that the fuel flows to the purge (if you hold upside-down the engine and press a hundred times) but it'll never go beyond it, therefore this line will not be useful.
  • Try to remove the other line (the one coming from the carburetor). This will not flow, since it's the part, which is filed with air.
  • Now try to add manually fuel into this line. The best way is to use a syringe, because it can fit in the 2-3 mm inner diameter pipe.
  • Once you've added enough fuel (20-30 ml), you'll hear the air leaving the carburetor, and now you're ready to put back the pipe.
  • Start pressing the purge a few times.
  • (Now you should see fuel flowing through the purge.)
  • Assemble back the cover and do your normal starting procedure.
  • Success?

    • Yes.
    • No success? I'm afraid you need to contact a mechanic.
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Engine characteristics:

Manufacturer: McCulloch
Type: CS 340

Engine
Cylinder displacement
38
cm3
Stroke
32
mm
Idle speed
3000
rpm
Power
1,3/9000
kW
Ignition system
Spark plug
Champion RCJ 7Y

Fuel tank capacity
0,25
liter
Oil pump capacity at 8,500 rpm
7
ml/min
Oil tank capacity
0,20
liter

** About 2-stroke engines:

The Ideal Otto Cycle is used in all internal combustion engines. This is why it is important to know well the p-V (pressure-volume) diagram for Otto cycle. The one who is interested in the topic, is invited to read at:
http://www.grc.nasa.gov/WWW/K-12/airplane/otto.html

Used reference: 
  • McCulloch - CS 340 Chainsaw User manual

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