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4 Answers

Primer vs Throttling

Asked by: 5415 views , ,
General Aviation

Guys, at the first flight of the day, what's the difference between giving Primer or fulling throttle twice or three times before go? Besides, what's the utility of fuel vent?

Best regards,

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4 Answers

  1. Steve Pomroy on Feb 26, 2013

    Hi Cadu.


    Since you have an independent primer, I’ll assume here that your’e working with a carburetted engine (not fuel injected). The primer injects raw fuel into the cylinders. The idea is that if the engine is cold, the extra fuel will assist with starting.

    Advancing the throttle activates the acceleration pump. This pump injects raw fuel into the intake manifold (not the cylinders). The purpose is to prevent a temporary over-lean condition when advancing the throttle – especially if it’s advanced rapidly.

    Some people advocate using the acceleration pump as a primer. Some of the technical arguments in favor of this are convincing. However, it usually contradicts the advice given in the flight manual. And my personal experience (operating in Canadian winters, for what it’s worth) is that the primer works better when used properly.


    The fuel vent serves two purposes, both ultimately boiling down to pressure equalization.

    First: When you fill the fuel tanks with cold fuel – from an outdoor fuel reservoir, for example – and then park the aircraft in a warm hangar, the fuel expands. An unvented fuel tank in this situation runs the risk of rupturing and dumping all of it’s fuel. A vented fuel tank will first dump any air in the tank, and then dump a small amount of fuel due to the expansion.

    Second: As your engine consumes fuel in flight, the fuel leaning your tanks must be replaced with something or the pressure in the tanks will drop. Depending on the design of your fuel system, this will result either in fuel starvation due to reduced/stopped fuel flow, or to the structural collapse of your fuel tanks due to pressure imbalance. The problem is solved by venting the tanks to the atmosphere.


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  2. Chris Carlson on Feb 27, 2013

    Priming spray fuel into the cylinders..usually only the front left cylinder I think on cessnas 4 cylinders. This makes it for when you turn over the engine, it has some ‘bang’ already in there, and doesn’t have to wait for the fuel system to catch up with the ‘sucking’ of the engine. Prime a cold engine, it needs the boost.

    Pulling the throttle engages what is called an accelerator pump. This is used when the engine is on or off. It mechanically pumps a little extra fuel when increasing the power, so that it can keep its richness while the airflow through the carburater catches up with the newly established manifold pressure. You can use this method on a warm engine to just reestablish the fuel quantity in the carburetor since some has evaporated since last engine shutdown.

    Fuel vents are there in order to prevent a vacuum in the fuel tanks. If there wasn’t a hole to let air in to replace the volume of gas consumed, we would have a suction and it would become increasingly difficult for gas to be pulled out of the tanks.

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  3. David Brown on Mar 04, 2013

    Over the years folk without primers seem to have a far better life in terms of happiness of starting.

    I have seen primer lines crack, and nothing about that is good.

    I would have them all removed and use the pump the throttle in and out while cranking technique. Works a treat hot or cold.

    Probably no best answer to the question, except to say less primers in the world is a good thing.

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  4. Cadu on Mar 04, 2013

    Thank you guys for all explanations…



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