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so....I've got a little tuning problem. >

so....I've got a little tuning problem.

so....I've got a little tuning problem.

Old 12-02-2010, 04:42 PM
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Default so....I've got a little tuning problem.

I'm a little embarrassed to even ask about this but I'm having a little trouble with an OS .18TM that I have in the T-Maxx I goof around with.

Background info:
OD 30% fuel
OS LC3 plug
RB TM523 pipe
Stock Traxxas air filter
Motor still has quite a bit of compression
Engine starts up and runs just fine, will idle well (when it settles down...I'll get to that in a minute), isn't burning glow plugs, and does not get overly hot (230-ish).

Here's the problem. When up to operating temperature, if I do a high speed run, then come to a stop, the idle will drop down for just under a second, then immediately rev back up quite a bit. It will stay high for 10-15 seconds. then drop down to normal. It will stay at there, nice and stable, for about 10 seconds before doing the normal loading up and dropping down a tick more.

It seems like when the engine comes back down to idle, it's burning the left over fuel in the case, then leaning out. If I richen the HSN, it just gets blubbery and doesn't run for crap. I thought perhaps richening the LSN might be the solution, but doing that makes the pinch test go from a reasonable 4-5 seconds up to 9 or 10 seconds. Heck, even taking the needles back to factory settings and starting fresh doesn't give me any different results.

I've sealed up every part of the motor, the tank has no leaks, & I'm not getting bubbles in my fuel line. Neither the OS Speed, nor O'Donnell SS.21 that I run in my race truggy has ever done this. I'm at a bit of a loss on this one. I can tune my other motors just fine, but this one is giving me fits. The only thing I can think of is that the TM 523 pipe isn't a good choice for this motor (too much/too little back pressure), or the silicone sleeve connecting the header & pipe is creating an air leak. Maybe an internal air leak somewhere in the carb, but I'm grasping for straws on that one.

So, anyone care to chime in on some thoughts on what could be causing the symptoms I described? If nothing else, this little puzzle will be a good learning opportunity in problem diagnosis.

Thanks to all...
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Old 12-02-2010, 05:02 PM
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Air leak, either going to the carb or from pipe to tank. Its a hard call without being there while its running. Thats were I would start though.
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Old 12-02-2010, 05:09 PM
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If you know that your fuel lines and tank are good i would put a new plug in get the motor UP TO TEMP(190-200) set the high speed then go to your low speed after clearing motor out STOP and do a 10 sec count u want a clean crisp take off you can also do the pinch test ( let the motor idle down and pinch fuel line bout an inch from carb it it should rev just a bit and die it should do this around 4-5 sec this works for me. change ur exhaust gaskets too hope this helps you
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Old 12-02-2010, 05:35 PM
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Not sure if it's causing your troubles, but yeah that pipe is not good for the TM. I ran mine when I had it with the traxxas SC resonator and it worked well.
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Old 12-02-2010, 06:08 PM
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The carb boot (the "bellows") utilizes the small groove on the carb slide to allow air to enter/exit it as it stretches/compresses. A tear or leak at the carb boot might cause those symptoms.
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Old 12-02-2010, 06:25 PM
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Front bearing will cause this also, defintly a air leak somewhere. also if you pinch off the gas line and it stays running for 15 seconds or so is also a indication of air leak, set back to factory setting and try pinching fuel line at running temp and see what happens?
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Old 12-02-2010, 06:29 PM
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Originally Posted by motomatt View Post
The carb boot (the "bellows") utilizes the small groove on the carb slide to allow air to enter/exit it as it stretches/compresses. A tear or leak at the carb boot might cause those symptoms.
A tear in the boot will not cause an air leak, boots are simply there to keep debris out of the slide.

OP, Im with boom on that pipe. That pipe is made for a that RB to breathe. The THS full header and pipe is one of the best for that .18 mill in a T-maxx.
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Old 12-02-2010, 06:32 PM
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Originally Posted by got_nitro View Post
A tear in the boot will not cause an air leak, boots are simply there to keep debris out of the slide.

OP, Im with boom on that pipe. That pipe is made for a that RB to breathe. The THS full header and pipe is one of the best for that .18 mill in a T-maxx.

Pull the boot off, remove your slide..............see the groove?
torn/loose boot= air leak.
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Old 12-02-2010, 06:33 PM
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Originally Posted by inferno13 View Post
Front bearing will cause this also, defintly a air leak somewhere. also if you pinch off the gas line and it stays running for 15 seconds or so is also a indication of air leak, set back to factory setting and try pinching fuel line at running temp and see what happens?
I agree that a front bearing will cause the sporadic idle but if you pinch a fuel line and get 15 seconds you have bigger issues than a air leak. Air leak or not, you still need fuel for it to fire, 15 seconds is way off. If you were running stupid rich with no air leak you would not get 15 seconds out of it.
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Old 12-02-2010, 06:35 PM
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Originally Posted by motomatt View Post
torn/loose boot= air leak.
No sir, you are sadly mistaken.
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Old 12-02-2010, 06:37 PM
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Originally Posted by got_nitro View Post
No sir, you are sadly mistaken.
oh yes it will!!! been there done that
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Old 12-02-2010, 06:41 PM
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Originally Posted by inferno13 View Post
oh yes it will!!! been there done that
So have I and no it does not.

Tell me, How does a boot that is not "air tight" to begin with have anything to do with a air leak? The boot slides on and is not "air tight" been there done that too and it's not the reason for a air leak.

Boots are only on a slide to keep debris out, nothing else.
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Old 12-02-2010, 06:43 PM
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Originally Posted by got_nitro View Post
So have I and no it does not.

Tell me, How does a boot that is not "air tight" to begin with have anything to do with a air leak? The boot slides on and is not "air tight" been there done that too and it's not the reason for a air leak.

Boots are only on a slide to keep debris out, nothing else.
if dirt can can get in so can air
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Old 12-02-2010, 06:46 PM
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Originally Posted by inferno13 View Post
if dirt can can get in so can air
Dirt can get on the slide and ruin it/make it bind up, thats why they have boots. Not for air leak reasons.
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Old 12-02-2010, 06:49 PM
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anyway...........there is a airleak somewhere, good luck with your search
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