See also All About Fuel.
Most paramotor pilots use pump gas as opposed to avgas. In all
likelihood, that means you're getting alcohol, specifically ethanol,
in your fuel. How much of a problem that is depends on your equipment
and how much alcohol is in the fuel. Apparently, the float bowl carbs
and inverted engines are particularly averse to alcohol. How much
alcohol is in the gas? Here is a way to find out.
You'll add 8 oz (or 8 of whatever units you prefer) of gasoline and 2
oz of water together in a graduated container. The 2 oz of water will
sit at the bottom with the gas on top. Since water attracts alcohol,
after shaking the container vigorously for a half-minute or so, the
water will absorb nearly all the alcohol. Then you set the container
down for 10 minutes so the water can come out of solution, complete with
its suspended alcohol, and sit on the bottom again. If the fuel had
alcohol init, there will appear to be more water than before. How much
depends on how much more water there appears to be.
You can calculate the percent alcohol content of the fuel using the
following method. F stands for the 2 oz of fuel, W is the 8 oz of water
that you started with, and T is the total of both, or 10 in this case.
WA is the amount of water that appears to remain after shaking and
waiting. That water will now be holding most of the alcohol that was
previously in the fuel. More apparent water, less apparent fuel. DIFF is
the difference in water from before to after.
The percentage of alcohol in the sample fuel (%FA) is determined
using the formula: %FA = DIFF/F x 100. So if you ended up with 3 oz of
water and 7 oz of fuel then: %FA = 1/7 * 100 or 14% alcohol in the
It's not perfect since not all of the alcohol will come out of
solution but it's pretty close.
According to the EAA's documents, up to 5% alcohol is enough to cause
problems. They recommend fuel not even remain in tanks or fuel system
for more than 24 hours (this is for aircraft, mind you). Vapor lock may
be a problem.
Over 5% can cause serious problems. They say not to fly airplanes
with this amount. In fact, they recommend draining the fuel
system, flushing all parts, then running with clean alcohol-free fuel long enough to exchange fuel in carburetor bowl.
Here is the rest of the article regarding known problems with
Alcohol attacks some seal materials and varnishes on cork floats of
fuel level indicators. This could cause leakage of seals and release
particles of varnish from floats, causing blocked screens in fuel
lines or blocked carburetor jets. Excessive entrained water carried
by alcohol could lead to fuel line blockage or blockage at screens or
values when operating at low ambient temperatures at ground level or
at high altitude.
Fuel volatility is also increased with the addition
of alcohol in a manner that is not detected by the Reid Vapor
Pressure test, which is used to determine if a fuel meets the
automotive specification. For example, a gasoline with alcohol will
meet the Reid Vapor Pressure limit of 13.5 psi but it will behave as
though it has a volatility of roughly 20 psi.
Gasolines with alcohol
will also phase separate. Phase separation occurs as the
gasoline/alcohol blend cools, such as when a plane climbs to a higher
altitude. When water that is absorbed in the fuel by alcohol comes
out of solution, it takes most of the alcohol with it. The quantity
that comes out of solution cannot be handled by the sediment bowl and
tank sumps. Furthermore, if the alcohol is used to raise the octane
of the base gasoline, the gasoline that remains will not have
sufficient octane to prevent detonation.
A good reference for this
phase separation problem is: Paul Corp., Laboratory Investigations
into the Effects of Adding Alcohol to Turbine Fuel, DOT/FAA/CT-TN88/25 July 1988, FAA Technical Center, Atlantic City International
Airport, NJ 08405.
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