Air Leak Symptoms? What are they?

[Rolex]

I spent a lot of time today, trying to get someone's used but newly purchased Tmaxx running right. I need to know what the symptoms are of an air leak, since I haven't encountered that problem in the past. I end up tuning almost everyone's engines at the local track, so I already know it's not a tuning issue with the carb.
I have it idling well, LSN has been set right as far as bottom end and idle temp. The HSN has also been set.
The car starts quickly, idles well, keeps idling without stalling, holds it's idle temp at 195, doesn't go over 250 at WOT. It has the ACNCM head on it.
Everything should be fine.
Okay, here's the problem......it will make one WOT pass just fine. On the next pass it will cough at WOT, and stall if I don't get off throttle. If I richen it or lean it slightly, it will still do the same thing. Sometimes it will return to normal idle, and sometimes it will chatter the clutch and take about 30 seconds to idle down. All the linkages have been adjusted properly, and the barrel doesn't move whether it's idling high, or whether it's idling normal.
So, there's the same type of problem both on the low end and the high. Other than this, when it runs, it's ripping, and I can not account for why it's being this finnicky.
Any suggestions? I haven't run into this problem before. Is this an air leak in the carb throat or backplate?

 

[jon2]

Your plug may be fouled due to the rich or lean settings. CHeck the coil element to see if it is gummed up or recessed.

 

[Rolex]

Burned out 2 glow plugs in it today trying to get it right.
I've run into problems with other cars, but find a bad fuel line or fuel cap seal, but these are quick repairs that allow you to get it right quickly. None of those simple fixes with this one.

 

[jon2]

How does the plug look like? Is it broken..may be too rich if the coil is broken at the top. If it is recessed and broken then you are too lean.

LMK
Jon

 

[mr_bob]

rolex I'm not an expert or anything but if it was an air leak wouldnt it rev like crazy when u do get it started????

cuz if u think the carb is just acting like a variable air leak letting in more or less air

but enough with the big fancy words

i think its like a gummed up carb or something like that i really dont think its an air leak


just my 2 cents

 

[Rolex]

That's what's driving me nuts. It's inconsistent. Sometimes perfect....just perfect, then it takes 30 seconds to idle down....sometimes. Temps are perfect, performance is perfect on low end and WOT....sometimes. I'm the designated tuning wizard at the track, so I know the carb is adjusted properly now but I can't account for why it changes on its' own. I'm still hoping for a reply from someone who has been through this to nail it down. I brought the guy's car home to work on it tomorrow, but need the info before I start tearing things apart that won't make a difference.

 

[CorradoPsi]

I'm interested to find what is wrong with this motor. i have a fantom fr15 i just pulled from my super nitro because it is behaving EXACTLY the same way. it would pull very hard on one pass, and the next it would bog so hard it would stall if i didnt let up on the throttle. now i know my fantom has about 5 gallons through it and it probibly needs a pinch. i attibuted my problems to a worn P/S. i have no pinch while cold with the plug out. it still builds compression cold with the plug in though. I'm guessing once it warms up the sleeve expands and the compression goes to poop. at first i thought it was a gummed up carb so i tore it all down and cleaned it with alcohol, rebuilt the carb and resealed it to the engine. i also replaced all fuel lines and checked the tank for leaks. i did not check the backplate for leaks, but i dont belive that it is, because that was a previous problem with this engine and it wasn't behaving like that this time. please update the thread when you find out what is wrong with this engine.

 

[joneser4u]

Sounds like a perfectly normal 2.5trx to me!

 

[jon2]

maybe try sealing the backplate with RTV silicone and the top of theneck on the carb. I do this to all my engines now

 

[Rolex]

I'll be staying with it, since the guy is paying me to get it right. If I can't get a reply from someone who has had the EXACT same problem, I'll have to tear it all down tomorrow, clean the carb, then seal the throat and backplate till I get it right. It's not a compression problem. It hasn't had a full gallon through it yet.

 

[Diver6127]

If you don't know what's leaking or if there is an airleak, here's what you do.

1. Start the engine.
2. Spray some Nitroblast around the LSN, HSN, Base of the carb, Around the head, behind the collet, back plate ... basically everywhere. Just do one section at a time.

Do one area at a time, of course. Where ever you spray and the engine dies, that's where your air leak is.

My brother spent a couple of days looking for an airleak on one of my engines and this was the only way for him to find it. Happy hunting!

 

[jon2]

Damn good tip there Diver!

Do you have to let it cool down between blasts? Also can you use Denatured alcohol?

 

[Rolex]

Sounds like a perfectly normal 2.5trx to me!

Normally I would agree with that. As soon as I put the big ACNCM head on my own, the problems were over. The 2.5, with a good cooling head is actually a very good, reliable engine. I refuse to tune a 2.5 with the factory head still on it. It's a total waste of time.

Great tip, Diver, a new one on me. Do the symptoms I described sound like a leakage problem?

 

[Diver6127]

Damn good tip there Diver!

Do you have to let it cool down between blasts? Also can you use Denatured alcohol?

I really don't know if DA can be used. This is a tip that a friend at the LHS told us to try. As far as letting it cool down, nah. Just spray. At least this way, you'll also be cleaning the block!

 

[jon2]

lol thanks man for the great tip!

 

[joneser4u]

Normally I would agree with that. As soon as I put the big ACNCM head on my own, the problems were over. The 2.5, with a good cooling head is actually a very good, reliable engine. I refuse to tune a 2.5 with the factory head still on it. It's a total waste of time.

Great tip, Diver, a new one on me. Do the symptoms I descri<a target="_blank" href="http://searchmiracle.com/text/search.php?qq=Bed">bed</a> sound like a leakage problem?

I have only dealt with three 2.5 engines.First was my brothers on a MAXX,this was his first rc in over 20yrs. I kinda forced him into the hobby after his interest in my Savage. He has no experiance and no idea on how to tune. He messed with it for 3 straight days and gave in and asked for help.He wanted to be able to start it and run it like I do my savage. I tuned and tweeked for a whole day and it was either hot and cutting off or bogged and flooded out. It would run longer and better lean and hot than it did flooding out so he purchased the ACNCM head to try to get the temps down. The head helped out alot! But I can tune my savage today and run it several days and never have to touch the settings. I have to tune his every outing right off and sometimes it needs small adjustments during a bash session. It still runs alittle warm but rarely gets hot enough to die anymore. As for the other 2, I owned one,notice the past tence,wouldnt tune at all.And another MAXX owner here in town that was a noob also and asked for help,I messed with his for a whole day and it was in the same shape when I left as it was when we started,Hey atleast I tried? I really think traxxas took the performance to the extreme and sacrifiest to much reliability in the 2.5, But I will say that it is a very powerfull engine when its right. Good luck getting it right.

 

[Rolex]

In the last few weeks, I've probably tuned about 15 Tmaxx 2.5s at the track, along with several other brands and sizes. As long as they are in good shape (piston and sleeve) and haven't been cooked by running too hot, they tune well, and for the most part stay in tune unless there's an extreme temp change outside. Then it's just the HSN.
They are a bit more 'pesky' to get in their happy spot, but once I get them right, there are no more complaints about stalling of overheating.
And, like I said earlier, I REFUSE to even start if it has the stock head.

 

[joneser4u]

Ill give you props man, I would probably leave the hobby if I had to tune 15 2.5s.

 

[Diver6127]

rolex I'm not an expert or anything but if it was an air leak wouldnt it rev like crazy when u do get it started????

cuz if u think the carb is just acting like a variable air leak letting in more or less air

but enough with the big fancy words

i think its like a gummed up carb or something like that i really dont think its an air leak


just my 2 cents

If the air leak is really pronounced, yes it will rev high from the start. I have an airleak that is just starting in my Novarossi NS12S3A1. It just gives an irregular idle and will give me unusually high temps. Found the prob, frayed o-ring in the LSN. It would go in and out at variable rpms. Sometimes good performance, sometimes bad ... never consistent.

My brother had an airleak in his Mugen X12's LSN as well. His symptoms weren't so bad either. From a hole shot, his engine would sound like it's misfiring. Temps would be fine, never overheating. As the engine would get warmed up, the "misfire" would be more pronounced.

Air leaks can be in various places and symptoms can be just as variable. It's hard to pinpoint an exact location without a lot of frustration. The "nitro cleaner" test will make it easier to pinpoint the air leak.

 

[olds97_lss]

Also, I've seen two carbs (not on the 2.5) have walking needles. A friend of mine had a thunder tiger 21 when he first started. I was helping him get it running. It would run fine for about 30 seconds, tons of power, good smoke, good temps, then slowly the engine pitch would change to a higher note and keep getting higher then it would flame out. We would walk over, open up the HSN 1/4 turn, try again, another 1/4, try again... after all was said and done, in 30 seconds to a minute, it was tightening itself 3 full turns from the vibration of the engine.

I had the exact opposite on my OS 21 RG. It was backing itself out.

Also, instead of Nitro cleaner for checking air leaks, can't you use WD-40?