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  1. #1
    RC poster
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    Mar 2013
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    14

    Lightbulb Weird engine issue

    Hi,
    Not sure what to search for so I'll describe the problem:

    My 2.5 engine seems to run fine, blue smoke, decent acceleration, etc but when I brake, the engine will seem to idle down and then suddenly rev up like I'm hitting the gas with 2 or 3 quick pulls on the throttle. As the brake is applied, it spins the spur gear.

    Thing is I'm not and the throttle is not moving (pulled the cover off and observed).

    Any Ideas? I'm hoping I just need to tune it better.

    For reference, it's on a T-Maxx 2.5

    Thanks,
    Stefan

  2. #2
    RC Turnbuckle Jr. Nitronaught's Avatar
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    Oct 2005
    Location
    South Florida
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    18,537
    1st Always tune with 1/2 a tank of fuel. Once the 1/2 way point is met the tank likes to lean up a little. This when when you are full it's slightly rich and as you run it leans up but not too much... Also helps in maintaining a reference point.

    2nd When tuning, do a full wide open throttle pass in between adjustments. Especially when tuning your LSN.

    3rd adjust in this order HSN, LSN, and then the HSN for fine tuning, and finally set your idle...

    The spur gear is spinning because you have reverse and the idle is high enough to engage the clutch....

    Here's a link to tuning, also check for loose header bolts, check your fuel line connections for any splits, your exhaust coupler,...

    Clean your Truck really well and run it on a clean, dry surface... Bring it back in and see if you notice and fuel anywhere on the chassis, motor mount, the body... If it's not burnt fuel it will not be brownish in color, else it would be the same color as if in the fuel tank,,, clean...
    If it's burnt fuel, you know this is from the exhaust, or the cooling head.
    I fit is clean fuel you will know it is coming from a fuel tank, split fuel line, leaking fuel filter, leaking front bearing of the engine (usually see it on the flywheel in this case).

    Here's a tuning helper and I hope this helps.

    http://traxxas.com/forums/showthread...ul-Tuning-tips.


    To me your issue sound like the tune,,, if this is an older engine check all the above.
    Robin Williams
    He will be greatly missed...

  3. #3
    RC poster
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    Mar 2013
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    14
    Thanks for the detailed response I'll follow these steps and let you know.

  4. #4
    RC Turnbuckle Jr. Wolfslash16's Avatar
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    Mar 2012
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    Riverton Utah
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    4,361
    HA HA HA! This is the EXACT same problem I had, and I never could find out what it is! Good luck!
    Sl2sh Slayer Summit Sl4sh Teton/Rally T-Maxx Pnde

  5. #5
    RC Turnbuckle Jr. Nitronaught's Avatar
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    Oct 2005
    Location
    South Florida
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    18,537
    Quote Originally Posted by Wolfslash16 View Post
    HA HA HA! This is the EXACT same problem I had, and I never could find out what it is! Good luck!
    Symptom, engine runs hot, poor performance.

    That is the same symptom for 90% of Nitro RC issues.... So It's a logical process of elimination.

    I Love it when it is the last thing on my checklist... So often I just go over it ALL (to the best of my knowledge)...

    Still makes troubleshooting these things so much fun... Not anything like 1:1 engine troubleshooting in many cases IMHO....
    Robin Williams
    He will be greatly missed...

  6. #6
    RC poster
    Join Date
    Mar 2013
    Posts
    14
    Nitronaught, just wanted to update you: it *was* the idle and I adjusted the low speed needle too.

    I wasn't able to tune before now due to repairs (a arm *and* then rear bulkheads!)
    I followed the instructions you provided and it's now nearly there.

    Thanks!

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