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Basic engine tuning.

Discussion in 'Gas Engines' started by Terryscustom, May 6, 2015.

  1. Terryscustom

    Terryscustom 640cc Uber Pimp

    I took the time to write this up once a long time ago and recently posted it on another forum. I wanted to also post it here for those that may be new to gas engines and are having trouble with the local field "engine guy" not really knowing how to tune an engine.

    This was describing a typical DLE tuning but the same applies to DA100 / 120 and pretty much all other 100cc class engines starting from a base point of 1.5H and 1.5L

    Basic tuning:

    1 - Tune the engine when it is warm, and double check the tune after a flight.

    2 - On the DLE there is a little silver screw on the carb that is an "idle set screw". it has a spring on it and it sets how low the low throttle arm will physically go down. Take that screw OUT and deposit it in the trash can. Do not remove the black H and L screws obviously.

    3 - Start the engine and run it for a good few minutes with varied throttle and some full throttle run ups to get it nice and warm.

    4 - If you have not done so, adjust the servo travel for a consistent idle, does not need to be perfect as long as it is low. 1600 is too high to properly tune that low, it should be much closer to 1200.

    5 - Tune low needle first. Now run the engine up to clean it out with a couple full throttle run ups and then go to to low idle. Listen to the engine.........how long does it take to start to "load up" or start to slow down in RPM? If it is rough right away you are likely way to rich. Normal starting point on 100cc engines is 1.5L and 1.5H. I have had to go as low as 1 3/8 on the low needle on the DLE carbs. Adjustments on gas carbs are VERY small, like the width of a screwdriver blade or 1/16th of a turn or less.
    A good "safe" tuning on the low needle you should be able to clean it out with some run ups, return to idle and it should have a nice steady low idle for at least 15-20 seconds before it starts to load up. If it does this it is still just a "touch" rich but will not die on you and it will run nice and smooth.

    6 - How low should react. Now that you have it running, clean out the engine and let it go to idle. Let it sit for 10 seconds or so and then slam the throttle to full, not slow, nail it!! Listen to what it does. If it dies or almost dies, you are too lean. If it stumbles and works it's way up (may see smoke in exhaust) you are rich. As tuned in number 5 above it should run up quickly with very little or no noticeable stumble.

    7 - Now tune the H needle. There are several ways to do this with RPM but the most simple way for a safe needle setting is this. With the engine warm and the low needle tuned run the engine up to full throttle for about 5 seconds. Now chop it in one quick motion to low. Listen to what it does then.
    - If it returns to a steady idle, you are very close.
    - If the idle goes way low and works it's way up to a steady idle you are too rich.
    - If it stays at a high idle and then slows down you are too lean.
    - If it dies you are either way too rich or way too lean. With the H at 1.5 you should not be too lean.

    8 - When H is set return and double check the L and then double check the H. Then it's time for a flight. Listen to the engine carefully for all of the noted symptoms while you are flying. One engine test I do in-flight is for the H needle. Get the engine nice and hot, maybe a high speed pass or something. Now do a full throttle straight up line. Listen! Engine should be smooth and steady until you can't go higher. If it starts to sag at all, land immediately and richen the H needle just a tad and try it again.
    This fade can also be caused from overheating not tuning related so make sure your engine is properly baffled if you now the tuning is correct.

    There you go, pretty simple and really only takes a few minutes once you get it down.

    This is a properly tuned DLE120 that is not broken in yet so I have it set as described above. 7100 top RPM and a consistent 1100-1200 low idle. I have an "idle up" switch for flying mode that sets the idle to about 1600 or so when setting up for landing I idle down. This is highly recommended for all gas engines. You will notice at the low end I am also testing my idle up / down switch while checking the tune of the engine prior to maiden.

  2. Great guide Terry, for anyone to follow, not just someone that is new to two-cycle gas engines. Barring anything unusual the steps above should get any new or broken in engine tuned and running nicely.
    thurmma and 49dimes like this.
  3. Jetpainter

    Jetpainter 640cc Uber Pimp

    Nicely done. :way_to_go:
  4. Thanks for that!
  5. 49dimes

    49dimes Damn I'm hungry

    Good Stuff. I try to explain it like that to members at the field and then just start thinking to myself.......

    "you can lead a horse to water":rolleyes:.

  6. Good job Terry.

    dimes, I know where you're coming from but every once in awhile, there is that one guy who actually cares, listens to what is being said and learns from it. That makes it all worth while!
    thurmma and 49dimes like this.
  7. Knife-Edge

    Knife-Edge 50cc

    Excellent stuff!
  8. Terry,
    This should be posted in the top of the engine section as it solves many of the folks that are new to gas engines and would be very helpful. Hopefully either Bart or Sleepy sees this and makes it a permanent piece there. Thanks a ton for taking the time to post this.

    Just so you know....I reprinted it, laminated it, and posted it at our field for others to read. It will solve hours of tuning others engines.

  9. @SleepyC @Bartman can we get this as a sticky on the engine thread? This is some AWESOME info and simple to follow! Thank you @Terryscustom for taking the time to write it up.
  10. djmoose

    djmoose 70cc twin V2

    Thanks for this. I've had some great tuners help me out...but they never told me what they were listening for...and then which way they were turning the needles. Again, thank you!
    Terryscustom likes this.

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