Welcome Guest. Sign in or Signup

3 Answers

primer not locked in

Asked by: 6381 views Aircraft Systems

How does a primer operate mechanically? Why is a primer in the unlocked position bad for flight?

Ace Any FAA Written Test!
Actual FAA Questions / Free Lifetime Updates
The best explanations in the business
Fast, efficient study.
Pass Your Checkride With Confidence!
FAA Practical Test prep that reflects actual checkrides.
Any checkride: Airplane, Helicopter, Glider, etc.
Written and maintained by actual pilot examiners and master CFIs.
The World's Most Trusted eLogbook
Be Organized, Current, Professional, and Safe.
Highly customizable - for student pilots through pros.
Free Transition Service for users of other eLogs.
Our sincere thanks to pilots such as yourself who support AskACFI while helping themselves by using the awesome PC, Mac, iPhone/iPad, and Android aviation apps of our sponsors.

3 Answers



  1. QOL on Feb 16, 2017

    Also, will the primer work when the throttle is all the way out or when the mixture is in the idle cut-off position?

    +1 Votes Thumb up 1 Votes Thumb down 0 Votes

  2. Best Answer


    Skyfox on Feb 22, 2017

    The primer is a little mechanical pump kind of like a syringe that sucks in raw fuel when you pull it out, and then injects that directly into the cylinders or intake manifold when you push it back in. As far as I know it works independently from the throttle and mixture. However, the fuel shutoff valve would cut fuel to it when in the off position. It’s supposed to be in and locked during flight because in the unlocked position, it could potentially vibrate its way out and fill with fuel, and then if the pilot intentionally or accidentally pushes it back in that could cause the engine to flood out and stall, or foul the plugs.

    If you’re flying and find the primer has worked its way out, you’d want to throttle back a bit and very slowly push the primer in so the engine can burn up the excess fuel without flooding. Once it’s in and locked, put the throttle back to where it was.

    +1 Votes Thumb up 1 Votes Thumb down 0 Votes



  3. John Scarry on Mar 12, 2017

    If the primer is not locked, it allows fuel into the cylinders by the vacuum sucking fuel through the primer. It can cause the engine to run richer than you intend or to not shut off when the mixture is pulled.

    The engine on my Cherokee is hard to stop by leaning it out. At the last annual we decided to fix it. The IA noticed that the the primer was not sealing. He explained that it could be letting fuel into the engine when when the mixture was off. Upon further investigation, two primer injection lines were plugged and one was broken, so that wasn’t the problem. It also explained why we couldn’t start it on a cold day by priming.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes


The following terms have been auto-detected the question above and any answers or discussion provided. Click on a term to see its definition from the Dauntless Aviation JargonBuster Glossary.

Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.