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Gas Engine Basics

Gas Engine Basics

When you’re starting your gas engine, get into the habit of using the same procedure all the time so you can easily identify any problems that pop up. It’s important to always have a friend secure the airplane’s tail and make certain the prop bolts are tight. Switch the ignition circuit (kill switch) off, close the choke, and flip the prop a few times until the fuel begins to flow to the carburetor. Open the throttle fully, turn on the ignition switch but keep the choke closed. Flip the prop several times until you hear the engine “cough” or rumble telling you the engine is ready to start. Open the choke, reduce the throttle to a few clicks above idle, and then flip the prop again until the engine starts. The engine should fire up on the third or fourth try. Be sure to wear a thick leather glove for protection.


  • If your engine starts, burns off the prime then quickly dies, this indicates a fuel-draw issue. Check your fuel lines for any kinks, blockages or pin holes, and check your tank for proper internal setup.
  • If the carburetor won’t draw any fuel, check to make sure the needle valves are open. Also, check to make sure the carburetor is tightly fastened in place and that the small hole and passage feeding pulse pressure from the engine case to the carburetor isn’t blocked. Check the fuel intake screen filter and make sure it is clean. The screen is located under the top carburetor cap (the one held in place with a single screw). If it’s dirty, carefully remove it and flush it with fresh gas until it is clean. Finally, check that the engine head/cylinder case is tightly fastened to the engine case and that the gasket is undamaged. Even a small air leak here can prevent the engine from starting.
  • If there is no spark, check to make sure the kill switch is in the correct position. With electronic ignitions, make sure the battery if fully charged and the wiring to the timing sensor is properly connected.
  • If your engine loses compression, check for a stuck or broken piston ring. If this happens suddenly during flight, don’t turn the engine over by hand as this could score or gouge the sleeve. Carefully disassemble the engine and check for internal damage. If you don’t want to do it yourself, send the engine in for inspection and repair.
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Add a Comment
  1. I think you should post needle settings for a break in engine and after break in so people New to gas engines will know.

  2. Have just bought my first petrol engine,NGH GT35 for my Spitfire so am looking for hints regarding this,above will be helpfull I am sure..

  3. Don’t -EVER- crank a gas engine with the throttle wide open while the ignition is on. I’m not sure I’d do it even with the ignition turned off.

    The proper method:

    1. Set throttle to idle or a click or two above idle.
    2. Close the choke.
    3. Turn the ignition on.
    4. Flip the prop until the engine fires.
    5. Open the choke.
    6. Flip the prop until the engine starts.

    If you have to flip the prop more than 6 times in steps 4 or 6, something is wrong.

  4. I have had problems with a fouled plug, probably due to too much oil content in my fuel as well as possibly running the engine slightly rich during break-in. Whenever I have problems starting my engine, I can usually replace the plug and it starts right up. I also purchased a small sparkplug cleaner from Harbor freight ($20) and can clean up my plugs and reuse them. Another problem is the ethanol in in most of the gas available. It seems to clog up some of the fuel passageways in the carb. I can tell that this is the case when I have to open the high speed needle valve to get it to run, then after a flight or two, find the engine running too rich causing me to lean out the high speed needle.

  5. Wow, when you say “basic”, you weren’t kidding… May be helpful for the complete newb I guess, I was hoping to learn something at least a little something. Oh well

    1. Yeah, I mean really. I was looking for metallurgy reports on the sleeves from different manufacturers, Changing compression ratios at the field with minimal tools, A study on different fuel mixtures vs altitude, Prop diameter and pitch vs engine displacement charts, P factor comparisons between gas and glow, and last but not least, a comparison of bore and stroke as it relates to torque, HP, and BHP presented in calculus equations…

  6. Actually this was good
    Have not flown my planes in 8 + years and need
    The basics again to get back into it
    Next will be to replace my batteries, practice
    On simultor etc

  7. Well, I’ll say thank you. I’m not a newbie and I was wisely reminded and learned something from the article. Everybody has a starting point in various aspects of our hobby and this kind of information is really helpful. Gas power is really taking off and that means that there are a lot of newbies and near-newbies out there. I’m thinking that’s a good thing!

  8. Always good to review the basic information. Thanks. Starting procedures may vary slightly with temperature and altitude. We often run our gas airplanes when it is below freezing temperatures. If you find fault with the above article please write your own and submit it.

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