Have run about 0.5 galon through my new trx 3.3 engine. Was working fine before untila while ago.

The problem is the idle. When i rev the engine up the revs stay up for about 3-4 seconds before going down to idle. (This is not a servo problem i have ensured the servo returns to neutral position.).
When i rev higher- above half throttle the engine revs stay up longer, maybe about 5-6 seconds. Sometimes it takes over 10 seconds for the engine to return to idle revs.

The engine never used to do this. Until the guy at the local hobby shop rebuilt the carb after some fuel had dried and blocked up in there, but has got worse since then.


Thanks in advance to any replies.

Info: engine trx 3.3
Car: Traxxas Jato 3.3
Engine Age: 0.5 galon
Car age: 2.5 galon

With the engine off make sure the carb itself is returning quickly to a closed position. Even though the servo is moving fast doesn't mean the carb is closing fast. If it is closing slowly I would take the carb apart myself. Make sure everything is clean and the correct ”O”” rings were used.

These traxxas carbs are pretty simple.. But if it was just taken apart, perhaps the carb isnt seated all the way down into the crankcase. You may want to put some RTV silicone around that o-ring just to eliminate a chance for a leak. Otherwise, I suggest inspecting the throttle return spring. A weak servo will move the throttle slowly, causing throttle response to be slow, and a slow return to idle. I had it happen in my Jato - needed a stronger servo and the phenomenon went away. The return spring is pretty stiff, and the stock traxxas throttle servos arent strong enough.

Just my thoughts.

ok i will try that tomorow. But dont you think after 8 seconds the carb would have definetly returned to closed anyway even when the spring wasnt pulling enough.

Thanks for reply

Without seeing and hearing what its doing, I have no idea how to answer that. There's obviously a reason its acting strange. Either there is binding or maladjustment in the throttle linkage or carburetor itself, or there is a problem with the servo. Best thing to do is have it running and look at it and listen to it. sometimes you just have to mess with it as there's a lot of other variables that can come into play if the mechanicals all check out.

Problem still not fixed

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Hanging idle is a sign the idle gap too big & LSN on the rich side.

Use the TRX guide to clean out the engine with wd-40 before your attemp. Its in the manual.

When it does return to low idle, after several seconds as you describe can it remain idling steady for at least 10-15 seconds or more? If so, have you tried pinching the fuel line after its idling for about 10 seconds to get an idea of the LSN setting? Aside from an air leak as others mentioned, it could be the needles are not cleaned completely.
If your not familar with adjusting nitro engines i would suggest returning it to the shop that did the work and get them to make it right.

Hanging idle is a sign the idle gap too big & LSN on the rich side. When the carb closes the gap is too big causing a high idle, but the LSN set rich catches up and slows the idle speed down, known as a "hanging idle"

To tune it out, we lean the LSN until it stays at a high idle, reducing the gap as we go to slow the idle speed until it drops to the set idle speed as soon as the carb closes, but a lil bit on the rich side is not a bad thing.

Or you can adjust the carb gap to .5 to .7mm right away and work on the needles from there.


Just pop off the air filter and look at the idle gap, big gab = rich LSN with a hanging idle.


Pinch test is a good idea and should be done to tune the LSN, but looking at the carb gap will give you an idea before even starting the engine.

Thanks for your replies.
Here is a picture of the carb. I think the gap is ok.

I am going to try to post a video today of the issue.
<a href="http://s1056.photobucket.com/albums/t363/guitar3451/" target="_blank">
http://s1056.photobucket.com/albums/t363/guitar3451/
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That does look fine, but at this point do a pinch test. If you could post a video of the pinch test with an up to temp engine it be really helpful.

The 3.3 carb is touchy, very small window for optimum performance with a wide "running' window. The pinch test at this point can reveal how the idle needs adjusting.

<div>I explained the problem to traxxas support and I reciever this reply. Do you guys agree with what he said?</div><div>
</div><div>Hello </div><div>
</div><div>Start with a fresh plug.  We recommend using the Traxxas glow plugs.  For the TRX 2.5 or the TRX 3.3 engines use the  SD plug (part #3232.....$7) or the .Super Duty Plug (part 3232X...$7).</div><div> </div><div>If you cannot find Traxxas plugs you can substitute the McCoy MC9 (cold) and the K&amp;B 1L (medium).</div><div>Your best source for glow plugs will be your local Traxxas hobby dealer.  You can find your nearest U.S. Traxxas dealer at this link!  http://www.traxxas.com/support/dealerlocator </div><div>
</div><div>Next try resetting both of the mixture needles and the idle stop screw back to factory base settings.  The high-speed needle should be threaded in (clockwise) gently until you feel resistance, and then backed out (counterclockwise) four full (360 degree) turns.  The low-speed needle should be set flush with the slide ring located between the throttle arm and the low-speed adjustment screw.  The idle stop screw should be adjusted to where there is a 0.7mm to 1.0mm gap between the slide valve and the inner side of the inlet tract of the carburetor. </div><div>
</div><div>Be sure to tune the high-speed needle first so that the engine will clear out with good top end power while maintaining a steady stream of smoke from the pipe.  Next, set the idle and low speed adjustments together so that the engine idles down within 5 to 8 seconds after a few high-speed passes.  The low speed should be lean enough for quick response, but it should also be rich enough to allow the engine to idle down to a low and steady idle.  Remember that the idle gap should never be more than 1.0mm.  If the engine is idling very low and sluggish, do not adjust the idle stop screw beyond 1.0mm.  In this case, it would be a rich low-speed needle.  The low-speed mixture would need to be leaned out little by little until the throttle response is crisp and the engine idles smoothly. </div>

Pinch test the idle speed.

""""""""""Low-Speed Fuel Mixture Adjustment
The low-speed mixture is always set after the high-speed needle is
correctly adjusted. The low-speed mixture is set using the “pinch test”:
1. Once the engine is warm, do several high-speed runs to confirm
that the high-speed needle is set correctly.
2. Bring the vehicle in and pinch closed the fuel line going into the
carburetor (use the engine shut-off clamp). The engine should run for
2-3 seconds, speed up, and then shut off.
3. If the engine runs longer than 3 seconds, then lean the low-speed
needle 1/16 turn, make several more high-speed runs, and retest.
4. If the engine shuts off immediately without speeding up, then richen
the low-speed needle 1/8 turn, make several more high-speed runs,
and retest.
When the low-speed needle is set correctly, the engine’s throttle response
should be very quick, even to the point of making it difficult to keep Jato
from doing a wheelie when you accelerate!""""""