Need some help with a TRX 2.5

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littleblazer

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Good morning everyone. This post is actually the main reason I joined this forum. And mind you, I've been plagued with this issue for close to 15 years now and I am finally ready to get it straightened out because I'm done. I want it to work for dad, so here it goes.

TLDR version: Truck leans out from the second it is running, pressure tested for leaks and found none, new hoses, new engine and new carb made no difference. Continually wants more high speed fueling.

Some 15 years ago I (well mom and I) bought dad a T-MAXX 3.3. He absolutely loved it. So we did the break in and everything and started running it. And that's where the issues started occurring. That 3.3 was next to impossible to keep from leaning out/overheating. So for this sake I will outline what was happening. We broke it in on the street per traxxas recommendation. It was fine. Once we got to tuning I fattened it up a little and went from there. Our properties are about 100 feet wide so I was making maybe 400 foot pulls in the street, it would just about be in second gear for three seconds before braking and turning around and coming back. At these settings it starts easy and leaves a nice smoke trail for about 3 round trips. Then it starts cutting out and has no smoke from the exhaust. bring up the HSN a quarter turn, cools off and runs great for about three more passes then does the same thing. Do the same thing to the HSN and it's fine again. And this cycle keeps repeating. Basically it just keeps asking for more fuel up top to the point the needle is out some 6.5 turns. And this was even worse on dirt. Now on dirt it got to the point where it would be wildly hot and stall constantly even with adding fuel. You'd lean it to restart and then have to fatten it to get it to stay running.

This went on for a year or two then eventually the engine launched a rod through the piston. I rebuilt that 3.3 and ran the rebuild for another year before the same thing happened. Feeling that the block itself could be defective, I did the power up with a 2.5R into it and at the same time added a reverse kit to the truck. Dad was uncomfortable with how fast it was with the 3.3 and the 2.5 slowed it down just enough that we both feel it's perfect now. However, truck still did the exact same thing with the entirety of it's parts replaced. So now thinking I don't know how to tune, I about gave up. This was in 2012 or 2013. I received a nitro sport for my birthday. Expecting the same issues I broke that in and was ready for it to be a nightmare. Nope. I ran probably close to 5 gallons of fuel through that thing over a year and a half with maybe a +/- 1/4 turn adjustment on either needle on it in that period. Thing never gave me an issue. And that's when dad gave up on the T-Maxx and since I no longer had someone to run with, I gave up shortly after as well.

Fast forward to yesterday. I pulled the 2.5 from the t-maxx. This is the first time the truck is being touched since 2014. I clean the engine and pressure test it for air leaks. Bingo, think I have it, I reused the carb from the 3.3 since the linkage arm was adjusted right for the truck. Sure enough several leaks on the carb and slide. Replace it with the new in box 2.5 carb and run it. Same exact issue. Truck keeps leaning out and losing power as I run it. Got maybe half a tank through it before I was out 6 turns. Idling for a minute it cools and is fine again but then one solid pass heats and leans it right out to the point its cutting out. The pro .15 in the nitro sport was perfect and I didn't even touch the jets from last time I ran it. So now what I am asking is... what the hell have I missed. Fuel lines gar good, no leaks, everything is tight. What else is there other than me being incompetent at tuning? The truck is cool but it sucks it is basically unusable in the back yard. Any help or input is greatly appreciated.

Thanks,

-Paul
 

johnnydmd

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Well, I wish I had a better answer than "dump the 2.5 and get a Dynamite .19", but that's what I did when I had a similar issue with my 2.5 that I couldn't resolve.
 

littleblazer

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Well, I wish I had a better answer than "dump the 2.5 and get a Dynamite .19", but that's what I did when I had a similar issue with my 2.5 that I couldn't resolve.
I'm tempted to slap the pro .15 from my nitro sport in it and order another one for the nitro sport. That thing has had the living crap beaten out of it and I really don't take care of it either lol. But when I grenaded the second 3.3 I was thinking exactly that... another "better" brand and call it a day.

Trust me, I feel you and respect that statement... I'm for sure getting to that point. And the fact it's been across 3 different engines now is what really bothers me.
 

Losi215

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Your fuel tank might be faulty man, try replacing it even if it seems to seal fine. Use the exact same fuel tubing brand and style for pressure line and fuel intake line. Make sure you’re idle gap is small(.5 to .7mm) and retune from there. Too big of an idle gap is going to completely mess up your low speed tune, and tune in general. What fuel and glow plug are you using?
 

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Did you try sealing the carb/backplate at all with RTV? I know some say don't bother, but if I have an engine that has a wandering tune when it heats up, I'll tear it down, clean it and RTV the carb neck/cinch bolt/banjo fitting and backplate, then try again.

This covers sealing pretty good: https://www.rctalk.com/forum/threads/nitro-tuning-tips-and-tuning-flow-chart.60832/#post-726317

The one issue that does happen to me even on engines that do hold a tune is time causes the o-rings to harden. Then as the engine vibrates, the HSN will walk in/out on it's own. I don't drive nitro much anymore, but when I do (or when I use an old engine from the drawer), before going out, I take the HSN out, lube the o-ring with green slime, then put it back in. If it doesn't feel like there is much tension from the o-ring as I screw the needle in/out, I replace the o-ring. The LSN usually doesn't have an issue, but I check that too.

One thing that has always seemed to work is replacing the traxxas carb with an OS carb. It's a pricey upgrade, but my 3.3's with an OS 21TM/18TM/18TZ carb will hold a tune all day long. They don't perform any better/worse, but the tune holds solid. I have an old 18TM that I used the carb on my 3.3 from when I ran a 3.3. Ended up replacing the 3.3 with a 21TM. The OS part number is 21982000, carb model 11K. There are 2 of them, but the difference is just the arm on the slider. One has a ball like most carbs, the other is just straight (for the revo).

There was another that fit the 3.3/2.5 from OS... can't remember it though. I had it 20 years ago on an old grey TRX2.5 when I had a t-maxx.

The carb from the losi .34 also fit, but it was also completely composite. I had one on my revo and it would bind very easily and the throttle would get stuck open. It did perform better though for the few minutes it ran before it started binding. It's venturi was larger, so would let more air/fuel in.

Might be more cost effect to just get the dynamite .19.
Your fuel tank might be faulty man, try replacing it even if it seems to seal fine. Use the exact same fuel tubing brand and style for pressure line and fuel intake line. Make sure you’re idle gap is small(.5 to .7mm) and retune from there. Too big of an idle gap is going to completely mess up your low speed tune, and tune in general. What fuel and glow plug are you using?
I've had that too. Fuel tank not sealing properly or had a crack in it.
 

Losi215

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Also, tune your engine at the half tank mark. Do not tune at full level, as by the time you get a good tune, it will probably run lean as the tank goes below half. If tuned correctly it should run rich for awhile until it heats up, run perfect then until it gets below half, and then maybe run slightly lean at low fuel levels( as all nitros do)
 

gandalfnz

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Hi Paul

I had the exact same issue when I was starting with nitros and it turned out to be my tuning.
I now have about 5 TRX 3.3 and they ALL RUN HOT but after years of experience I realized that these engines just like to run hot.
I stopped paying attention to temps long time ago, and will now do only a one off check with a temp gun, but can mostly go by touching the cooling head with my hand.
As long as its around 130C, your are ok, some will say this is high, but for for a TRX its not, and dont worry about "shortening your engines life" by running it hot, just enjoy the moment.

Are you sure engine was broken in properly, you will need to keep it at a certain temp while breaking in. Traxxas manual only gives the break in basics, but does not mention other important details.
I have broken in a 3.3 6 months ago on my rustler, runs like a dream.
In fact, I have 3 3.3's and 1 2.5 as spare engines.

Sorry maybe I missed the info, how old are the engines you are having issues with?
 

littleblazer

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Your fuel tank might be faulty man, try replacing it even if it seems to seal fine. Use the exact same fuel tubing brand and style for pressure line and fuel intake line. Make sure you’re idle gap is small(.5 to .7mm) and retune from there. Too big of an idle gap is going to completely mess up your low speed tune, and tune in general. What fuel and glow plug are you using?
I may check the tank... easy enough to swap it over from the other truck even though it isn't the same. I did replace the o-ring on top after it was having issues. I eyeballed the idle but it's fairly low for what it is and pretty tight but I can get out my jet reamers and really set it.

As far as fuel I normally ran S&W 20% but I actually grabbed a can of VP 20% just to get everything going again. Running a 3232x plug.
Did you try sealing the carb/backplate at all with RTV? I know some say don't bother, but if I have an engine that has a wandering tune when it heats up, I'll tear it down, clean it and RTV the carb neck/cinch bolt/banjo fitting and backplate, then try again.

This covers sealing pretty good: https://www.rctalk.com/forum/threads/nitro-tuning-tips-and-tuning-flow-chart.60832/#post-726317

The one issue that does happen to me even on engines that do hold a tune is time causes the o-rings to harden. Then as the engine vibrates, the HSN will walk in/out on it's own. I don't drive nitro much anymore, but when I do (or when I use an old engine from the drawer), before going out, I take the HSN out, lube the o-ring with green slime, then put it back in. If it doesn't feel like there is much tension from the o-ring as I screw the needle in/out, I replace the o-ring. The LSN usually doesn't have an issue, but I check that too.

One thing that has always seemed to work is replacing the traxxas carb with an OS carb. It's a pricey upgrade, but my 3.3's with an OS 21TM/18TM/18TZ carb will hold a tune all day long. They don't perform any better/worse, but the tune holds solid. I have an old 18TM that I used the carb on my 3.3 from when I ran a 3.3. Ended up replacing the 3.3 with a 21TM. The OS part number is 21982000, carb model 11K. There are 2 of them, but the difference is just the arm on the slider. One has a ball like most carbs, the other is just straight (for the revo).

There was another that fit the 3.3/2.5 from OS... can't remember it though. I had it 20 years ago on an old grey TRX2.5 when I had a t-maxx.

The carb from the losi .34 also fit, but it was also completely composite. I had one on my revo and it would bind very easily and the throttle would get stuck open. It did perform better though for the few minutes it ran before it started binding. It's venturi was larger, so would let more air/fuel in.

Might be more cost effect to just get the dynamite .19.

I've had that too. Fuel tank not sealing properly or had a crack in it.
The RTV was the next step if I can't figure it out. All valid points and noted as well. New engine is looking more and more favorable lol.
Hi Paul

I had the exact same issue when I was starting with nitros and it turned out to be my tuning.
I now have about 5 TRX 3.3 and they ALL RUN HOT but after years of experience I realized that these engines just like to run hot.
I stopped paying attention to temps long time ago, and will now do only a one off check with a temp gun, but can mostly go by touching the cooling head with my hand.
As long as its around 130C, your are ok, some will say this is high, but for for a TRX its not, and dont worry about "shortening your engines life" by running it hot, just enjoy the moment.

Are you sure engine was broken in properly, you will need to keep it at a certain temp while breaking in. Traxxas manual only gives the break in basics, but does not mention other important details.
I have broken in a 3.3 6 months ago on my rustler, runs like a dream.
In fact, I have 3 3.3's and 1 2.5 as spare engines.

Sorry maybe I missed the info, how old are the engines you are having issues with?
The temperature wouldn't concern me if it wasn't also exhibiting cutting out, sputtering and hard restarts. If it cools a little it restarts fine but otherwise is a bear like an old BBC a friend had with way too much compression lol. The last time I hit it with a temp gun I saw something like 360 F (180 ish C). That was a run before the first one launched the rod.

As far as break in, I did it on a 90 degree day and always checked temp periodically. It was in the 270 range (low 130 c)

When it originally happened, the engine was new, now its probably close to 10 years old but coincidentally runs much better than it ever did. Appreciate the input.
 

gandalfnz

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The temperature wouldn't concern me if it wasn't also exhibiting cutting out, sputtering and hard restarts. If it cools a little it restarts fine but otherwise is a bear like an old BBC a friend had with way too much compression lol. The last time I hit it with a temp gun I saw something like 360 F (180 ish C). That was a run before the first one launched the rod.

As far as break in, I did it on a 90 degree day and always checked temp periodically. It was in the 270 range (low 130 c)

When it originally happened, the engine was new, now its probably close to 10 years old but coincidentally runs much better than it ever did. Appreciate the input.
So the engine cuts out when its hot and will not restart until it cools down? This is a typical sign of a worn engine (piston & sleeve).
If the temps were around 180C or thereabouts on your previous runs, that's what killed it.
If it were me, as the guys said I would check for air leaks around the motor first, such as fuel tanks and fuel lines.
The TRX itself is sealed pretty well with o-rings, but they also may have hardened over time and need replacing.
Getting a new engine would be ideal!

If you think the engine still has life in it (pinch), you could buy a traxxas seal kit, and reseal that whole engine.
 
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littleblazer

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So the engine cuts out when its hot and will not restart until it cools down? This is a typical sign of a worn engine (piston & sleeve).
If the temps were around 180C or thereabouts on your previous runs, that's what killed it.
If it were me, as the guys said I would check for air leaks around the motor first, such as fuel tanks and fuel lines.
The TRX itself is sealed pretty well with o-rings, but they also may have hardened over time and need replacing.
Getting a new engine would be ideal!

If you think the engine still has life in it (pinch), you could buy a traxxas seal kit, and reseal that whole engine.
I did the old check piston and sleeve at TDC and it still feels pretty tight, unlike the pro .15 lol. But anyway, on a gamble, I wrapped the fuel tank fill cap with some teflon and ran it. And then ran another tank. I'll be damned but I think that may have been the issue all along. If the tank wasn't recommended to check again I never would have even thought to try that. So I am going to order a new tank, and then tune and go from there but this was the most I was able to run it ever. :)
 

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I did the old check piston and sleeve at TDC and it still feels pretty tight, unlike the pro .15 lol. But anyway, on a gamble, I wrapped the fuel tank fill cap with some teflon and ran it. And then ran another tank. I'll be damned but I think that may have been the issue all along. If the tank wasn't recommended to check again I never would have even thought to try that. So I am going to order a new tank, and then tune and go from there but this was the most I was able to run it ever. :)
Did you do the piston and sleeve check with both removed from the engine or by just removing the cooling head?
 

littleblazer

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Did you do the piston and sleeve check with both removed from the engine or by just removing the cooling head?
I just popped the head off, didn't want to break the thing down that far. I also checked it glow plug out and spinning the crank. Has a nice pop/snap as you roll right on past which I believe is correct. The last quarter inch or so of stroke/piston travel.

And a clarification, this 2.5 never was allowed to overheat, should've been clearer about that. It gets hot, but never 180c hot. maybe 145c was the hottest it got. I can smell the exhaust change and shut it down when it started getting to that point and allowed it to cool. I was alot more vigilant than I was with the 3.3... mainly I was 16 by the time I was messing with it vs 10-12 when I blew up the first 3.3. And on the hard starting, I further messed with the truck, its only when the tank was low. I topped it back up (prior to resealing the cap) and it was fine? I'm wondering if the extra head helped gravity feed fuel, and seems to further confirm that it is something with the tank not pressurizing...
 

littleblazer

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Try it a few more times, see if the tank is the issue.

Also when checking for pinch, piston and sleeve need to be out.

Cheers
Will do, and yes, I can be very lazy sometimes lol.
 

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I'm tempted to slap the pro .15 from my nitro sport in it and order another one for the nitro sport. That thing has had the living crap beaten out of it and I really don't take care of it either lol. But when I grenaded the second 3.3 I was thinking exactly that... another "better" brand and call it a day.

Trust me, I feel you and respect that statement... I'm for sure getting to that point. And the fact it's been across 3 different engines now is what really bothers me.
What fuel are you running in the 2.5? I've had the 2.5 since it first came out and still run it to this day with no problems. Sorry to hear your haven such issues
 

littleblazer

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W

What fuel are you running in the 2.5? I've had the 2.5 since it first came out and still run it to this day with no problems. Sorry to hear your haven such issues
I ran S&W for a long time, now I am running VP 20%. That being said I ran three full tanks through it today with absolutely no issues refueling on the fly. I am going to say the replacement tank was the issue. I started stalling on deceleration today but it seems like the clutch bell is locking to the crank and locking to the spur at idle. So that needs adjustment/checking out but I'll be damned. I think the tank was the issue all along. :)
 

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Glad you got it fixed! Check your clutchbell bearings next, probably toast.
Glad you got it fixed! Check your clutchbell bearings next, probably toast
 

littleblazer

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It's for sure one or the other. I'm working the next three days so that is a project for monday.
 

littleblazer

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Update: the pin that the clutch shoes ride on walked out and was wedged up against the bell. I peened the hole so it was a tight fit again. Then I started running it. Runs great with about half a tank now, pretty dialed. But now, the only issue seems to be the idle end. nailing the throttle it bogs until it hits the highspeed side. It seems tweaking the low speed is okay until the level drops a little more, then you richen it again and its fine. There is a really fine line between good and stalling. I have the idle gap set to .8mm, any lower it doesn't like it and any higher it idles too high. I'm guessing I need to get the bottom end a little more adjusted but I ran out of nitro. Tomorrow is another day but any tips are appreciated. (Yes I am going to re-read everything above again.) :cheers:
 

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