Opinions on how long a container of pre-mixed 2 stroke fuel will last for varies significantly, some say it is only good for 3 – 4 weeks and others up to 2 – 3 months. In my experience it depends on the motor, but all motors will perform best with a full tank of freshly mixed fuel.
“Fresh is best!”
Generally the higher the performance of the motor, then the more noticeable it will be if the fuel is stale.
I had an old suped up outboard race boat that hated stale fuel, it wouldn’t idle and would choke up when you gave it a fist full of throttle when the fuel was going bad. I found it best to run it down quite low and top it up with a fresh jerrycan or two of fuel just before heading out.
There were a couple of times where I had to drain the tank where I’d left half a tank or more for over 2 months, there wasn’t enough volume left in the tank to dilute the stale fuel. Your only option is to drain or syphon it out and use it for cleaning in the shed.
For your two stroke garden equipment you can get away with leaving it a bit longer, but if you’re having trouble with pull starting, you’re best to throw out the old fuel and start with a fresh batch.
BP have an interesting fact sheet on fuel storage, interestingly it states that:
“When two stroke fuel is mixed correctly it has the same storage requirements as petrol. Problems arise when excessive two stroke oil is added. Under this condition the oil may separate from the fuel.”
But they also state that fuel store in a fuel tank only lasts about a month:
At the end of five weeks the fuel is 5% heavier and the fuel air mix will contain more fuel. This means your engine will run richer possibly resulting in fouled spark plugs and higher fuel consumption.
As for sealed containers it states:
The storage life of petrol is one year when stored under shelter in a sealed container. Once a seal is broken the fuel has a storage life of six months at 20°C or three months at 30°C.
As stated earlier, “fresh is best” so be sure to mix smaller batches more frequently. Head over to the front page and you’ll find a handy calculator to help you get your mix right.