Just another tuning question.

[Spikes]

I bought a used 2.5 Maxx that I have been tweaking on daily for a while now. It's a powerful machine, a real wheelie king on both grass and tarmac. I keep it tuned a bit rich as I'm not a top performance junkie yet. The truck was only broken in then put in storage for a year before I bought it. The whole truck looked BRAND new. The engine was gummed solid when I got it. I took the engine out and freed it up with WD-40 and hand power. It's got good pinch and psycho power, but I can't find a happy idle.

Once the engine decides to calm down it will die if the idle is too low or rich.
Take off power is awesome, and I have a good stream of smoke above idle. If I richen the low needle more the idle drops and dies after a bit, if I lean it more I get a hesitation on take off.

The thing is, I know the 2.5 is finiky, but I can never get it to idle right. It absolutely screams on the high end, but either idles so high the tranny lurches every few seconds or drowns out rich and dies. (I know it's rich because I have to have the glow stick on for a bit to clear the excess fuel upon restart and it smokes).
When I stop it (hit something) it idles very high for a random 3 to 15 seconds and then drops down to a lower idle quickly when it's ready. It 'feels' like it's got an air leak and is leaning out, but why would it run so perfectly on the high end, and idle GREAT once it decides to? It usually starts up with a high idle and doesn't calm down much until it's warmed up. The strange part is there are two distinctly different idles. The high one, with the truck lurching every few seconds every time I stop driving and the low one a few seconds later when all is well. The transition takes only a second.

I never thought much of it until I bought a Savage with the Picco .26 and it's the most mild mannered engine I could ever ask for. Set it once for the weather that day and it's good for several tanks of fuel (I run it a bit rich too, but it wheelies like CRAZY!!!).

I know the standard advice will be change O-Rings and seal the engine, but for the engine to go from high idle to great idle manners and run super on the high end. How could an air leak cause that?

Thanks!

***EDIT*** I should add, sometimes the high idle will not drop down. Sometimes I could let it idle high for 3 minutes and it won't come down, but if I blip the throttle it might stay high, then come down - and it might not.

Also, it isn't a servo setting issue. I always check the servo and make sure the slide has returned all the way to idle (it has about a 1mm gap).

 

[Scrogg]

Linkage issues are why the idle does not come down sometimes.
If the engine was gummed up, it needs the bearings scrubbed clean or replaced. I'd bet they will not spin freely. I had the same exact situation as you. I had Revo given to me, that had sat with fuel in it for a year in a Hot garage. I had to completely tear it apart and soak everything in Denatured alcohol for 3 days. I had to scrub it every day too. Finally got everything spinning freely. Put it back together and it fired up and idled perfect first pull. I was shocked.

 

[Spikes]

I always value your experience and advice, Scrogg!
The throttle rod is always slack when the servo is at idle and I always hit the brake a bit and watch to make sure that's not the issue.

I had thought about a bearing sticking up a bit, but I have run well over a gallon of nitro through the engine and always soak the engine inside and out with WD-40 after a run (I use the EZ start and squirt it through the carb, then spray down the outside) and the engine should be clean as a whistle.
Also, the differing idle RPMs are very consistent, one a certain high RPM, and the other a set low RPM. A failing bearing would produce an inconsistent result, I would think.

Sorry if I don't explain things so well, I'm an engineer and thoughts often don't come out the way they look in my head..... If that makes any sense.....

 

[Scrogg]

Put a small rubber band on the carb to act as a throttle return spring.
I have seen the same issue your describing also caused by a worn carb slide, or a tear or leak in the slide dust boot.
Other things to check, make sure the linkage is 100% sliding freely. Traxxas carbs need a fair amount of tension to keep them closed.
If all that is straight then you definitely have a small leak somewhere, happy hunting!
The easiest way I have found to check a carb for leaks is to remove it from the engine.
Then attach a piece of fuel line to the HSN inlet. Hold your figers over the carb's throat and inlet and blow into the fuel line. Spray a soapy mixture all over the carb and watch for bubbles.
Let the carb slide extend all the way out, this is important. Sometimes the boot leaks at the ball link at WOT but not at any other position. If it does this, degrease the carb boot and make sure the ball link is sealing against the boot firmly.

 

[tmxx_boy]

idk man.. all my tmaxx's are pickey,, i just tune them every so often during a run.... but i do kno that you always must let the engine run a good 5 min or so b4 tuning it.. it will sometimes crank up different but once it runs it will return to previous tuning.. hope this makes since.. still traxxas makes a mean r/c. wouldnt change anything

 

[Spikes]

I'll try the rubber band return Sunday. I'm going out of town for the weekend and have to leave my toys at home

One thought Scrogg.... On car carbs or even fuel injection units, air leaks are found by spraying carb cleaner around while the engine is running and listening for RPM changes. I wonder if this would work on nitro engines too? I would probably apply it as liquid drops from a syringe, since spray would be hard to pin point the exact spot.


Happy Thanksgiving all!

 

[Justagamer76]

Dont use any type of cleaner when checking for air leaks. You asking for trouble. I would start in the trouble spots first like tank seal or maybe a clogged exaust outlet. But when you engine is running and if there is a leak what your spraying to check may very well be burned and pow ...No worries youll be buying a new engine. I may be wrong buts this is my opinion. Happy running.

 

[Scrogg]

I'll try the rubber band return Sunday. I'm going out of town for the weekend and have to leave my toys at home

One thought Scrogg.... On car carbs or even fuel injection units, air leaks are found by spraying carb cleaner around while the engine is running and listening for RPM changes. I wonder if this would work on nitro engines too? I would probably apply it as liquid drops from a syringe, since spray would be hard to pin point the exact spot.


Happy Thanksgiving all!

It does work but spraying anything on the engine is not good IMO. Most of that stuff is corrosive to the silicone parts.
Spraying the engine down with WD-40 is a bad idea too, if it gets on the clutch shoes they are ruined.

 

[olds97_lss]

Just a thought, I'd read about guys using canned air to find air leaks while running RC's. It will jack the idle up a lot if there's a leak where your spraying.

 

[Metalhead Matt]

Wd will ruin your brake disc if you get some up there, too. Or the pads. Make a 120oz servo feel like a 40 oz'er.

 

[Spikes]

That's very helpful info.... glad I didn't spray carb cleaner or WD-40. canned air should be safe though..... I'll try that.

Thanks for the help, guys!