kpm

Hello,

I am fairly new to Monster Trucks and have attempted some troubleshooting. I have a problem with my T-Maxx engine shutting off when giving throttle and accelerating. The problem happened as the truck was being driven. There were no major crashes but the truck did roll a couple of times when jumping a small mound. Right after that the engine started sounding like it was bogged down and the problem started. Maybe the shock of the landing stripped something in the transmission? when the truck is sitting still the engine idles fine. Also the wheels turn freely when giving throttle if the truck is on a stand. However, the truck stalls when it's on the ground and a load is there when accelerating.

Here is a video that I made that shows the problem:
https://www.youtube.com/watch?v=nGA5...ature=youtu.be

Troubleshooting I did:
- I attempted to adjust the slipper clutch with no luck
- Did basics like check air filter, glow plug and needle valve settings
- Fuel line is free of clogs or leaks
-All wheels move freely and no odd sounds when turning manually.
-No mechanical arms, servos etc appear to be stuck or out of place.
-The brakes work fine and are not locked or jammed.

I am going to dive into this repair, but do not know quite where to start. Is it a transmission or clutch issue etc..?Where do I start?

Much appreciated for any ideas.


Thanks,
KPM

Maxximize

Hmmm that's an interesting one. Did you check the carb mounting at all? It could have come loose from the hard landing and may now have an air leak at it's base. You could try re seating it and see if that does anything. It sounds like it's idling fine though, which makes it seem less likely that the carb has an air leak, but it's worth a look. It is also possible that the fitting on the pipe for the pressure line may have come loose and isn't supplying enough pressure. The pipe and the fuel tank could have a cracked from a hard landing, too. The stock tank is quite prone to cracking and isn't always noticable but will cause issues with pressure.

kpm

Oh great. Thanks for your tips. I checked those things and also all carb/idle settings. As you saw in the video it runs fine in the air. As soon as I put it on the ground and it gets a load on it, it stops running. Could it be in the clutch or transmission? I tried adjusting that slipper clutch as well.
Thanks

Maxximize

I just watched it again, and I'm still thinking air leak. It does sound like a smooth idle, but it is idling a bit low I think. For good measure you could check the drive line. It could be that something is catching or binding to make it stop that quickly. Check for any sort of binding in every spinning drive line part. Off hand I can't really think of any one thing mechanically that could cause that, but you could take the tranny apart and take a look at it. It's pretty easy to take apart and simple enough in how it operates so you should be able to notice any broken parts pretty easily. There are 3 clutches in there for the forward, reverse, and second gear. One issue that is very rare but can arise is that the clutch shoe for second gear is engaging due to a broken spring. I've only ever broken 2 parts in a T Maxx tranny in 14 years of running them. One was a shredded primary gear due to running Ofna Monster Pirate tires in a reverse equipped tranny, and one was a broken spring in the second gear clutch shoe due to over tightening the adjustment screw. I have one tranny in my parts bin that is 15 years old and works like new yet. I've never had a tranny break from crashing, and that includes hitting trees and fence posts and the like. The clutch is another possibility, too I suppose though I've only burned those out from either too much power and weight, or too heavy of tires. When the spring goes it tends to go from excessive heat or just age most times. Traxxas has exploded views of the truck in .PDF format on their site in case you get lost or need some visuals like I do when you take the truck apart

I'm still leaning toward an air leak in all honesty, though. I hope I'm wrong. You stated that it seemed boggy after the crash, which leads me to broken plastic. Any other time if a crash wasn't involved I'd say the tune is off. I've had issues with the TRX 2.5R carbs leaking and I have cracked several tanks as well over the years. I went through 2 carb bodies on mine while I had it until I finally got one that didn't develop a leak. The first one leaked near the fuel inlet nipple right away because the high speed needle stripped it out when I repositioned it (My fault), and the second one leaked from the base after a few runs and wouldn't seal up with the liquid gasket stuff even, I think it was warped. The carb bodies are plastic on all TRX engines now days and plastic carbs are prone to leaking. The fuel tank is still suspect for me especially, the way it mounts to the chassis is just asking for a crack from a hard landing. Did you blow into the pressure line while plugging the fuel line in the tank for a pressure test? I had one that cracked right at the one of the mounting feet that I couldn't see until my buddies dad blew into it with fuel in it and it spewed everywhere out of the bottom. You can fill it with denatured alchohol for that test too so no fuel is wasted. Some people I've heard submerge them underwater and blow into them to check for leaks. There are 2 ways to prevent it from cracking. One is the Ofna tank mod. It involves using an Ofna buggy tank which mounts with 2 posts to keep it isolated from chassis flex. The other is to mount the stock tank on some small O rings or some kind of thin rubber or cork or something to try and isolate it. I only ever mounted my stock tanks on 2 of the 4 screws at opposite corners with O rings between them and the chassis. They broke less often that way.

Carb leaks are equally tricky of course, and will not be visible most times. I don't know if you've heard of the WD 40 trick, if so ignore this part. Spraying WD 40 around the carb while the truck is running will show a leak quickly if it exists. It will either bubble up or the engine will die quickly when it sucks in the WD. You can also plug the venturi and the exhaust stinger and pressure nipple and blow into the fuel line while brushing soapy water onto the carb where it meets the engine block, or spray it with WD 40 while blowing into it. I'd exhaust all methods of finding an ir leak if no mechanical issues are found.

If all else fails and nothing is broken then it's definitely the tune. I know you said you checked the settings, but I'd try and re tune it after setting it to factory settings. Factory is only a go by and is a very rich setting meant for break in. Raise the idle up to see if it will take throttle after it's at factory specs. If the idle gap is larger than about 1-2mm and the truck won't idle or it idles really high it's off at the low end. The pinch test is perfect for tuning your low end. To do that, get it running and let it warm up. Once warm, pinch the fuel line right by the carb and listen. If it increases RPM slightly and dies it's set good. If it revs up high and then dies it's too rich. If it just dies with no RPM change, it's too lean. There should be a very small amount of smoke if any visible at idle, and little to no oil spitting out of the pipe. Traxxas states to set the low end flush, which is not a very precise setting and is a very rich one as well. The high end is also rich from the factory but will have less of an impact on low speed acceleration.

kpm

Maxximize! Thank you very much for your detailed and thorough list of things to check. From your post I drew the inspiration to jump into this project with both feet! This was the push I needed. I have removed the engine, fuel tank, slipper clutch etc.... I am cleaning as I go and have found some interesting things. I did find a fuel line that was very suspicious. It slid off wayyyy to easy and could have been a source of leaking like you said. I also found that spring wrapped around the clutch. It was a little warped and I think it fell right out. I am under the impression that it should have been connected. I might buy a new one cause one end hook was missing. I twisted a piece of the coil and made a new hook for the time being. As you can tell from the pic, I have broken the engine down. I did remove the carb and cleaned it too. I did not see any visible cracks on anything. At this point I will put it back together and test. I will make special not of the fuel tank test and carb tests you mentioned. I have not found the problem, but I am having a rewarding experience learning about the truck. by the way the exploded pdf's you mentioned work great as are some you tube videos.
Thanks!

Maxximize

Excellent! I'm glad it was helpful and you're making some good progress. Yea I forgot about the fuel tubing, that is always a possibility. That stuff is cheap so it's worth having a bunch extra laying around and replacing it as preventative maintenance. Your clutch spring was definitely shot, it should be nice and tight around the shoes. The quick fix you applied should hold for awhile, though. I've done that a few times myself as I was blowing them constantly with my Pirate tires I have since switched to a 3 shoe clutch set up from an HPI Savage now that the truck is a big block, but they also make those style clutches for stock Maxx engines too. They suck to put in, but are a lot stronger than the Traxxas set up is. If you get it running and driving you might be able to hear if the second gear is stuck. If it won't shift and is really strained on take off it might be stuck.

You can also check to make sure your exhaust is sealed up. If it's leaking by the header it will spray fuel there so it's pretty easy to spot. Same with the pipe itself.

Good luck and hopefully you find it's issue quickly! I too love to work on these things, taking them apart and modding and rebuilding them is a really rewarding experience for sure Once it's all fixed up you feel such a huge sense of accomplishment! Heck I even tear my trucks down every winter when it gets too cold to run and clean them up and do preventative maintenance to them.

jkdv8

My Feedback: (1)

Sounds to me like it was the clutch. If it is constantly engaged it will bog the engine down then stall it all together when the throttle is pressed. I would replace the spring before reassembling. They are usually tempered metal. The hook you made probably won't last very long once it heats up or with repeated heating and cooling cycles. If the spring fails completely it will want to take off as soon as you start it.

I would check for air leaks as well like previously stated. Usually with air leaks however, you end up with an uncontrollable idle.

kpm

Sorry for just getting back with a report....
The above advice helped me fix the problem. I put everything back together and the truck worked perfectly. So checking that fuel line and fixing the clutch spring worked.
jkdv8. good advice because me temporary fix lasted for only a couple of weeks. I am replacing that spring with a new one.

Maxximize

Well good! Glad to hear ya got her up and going and it wasn't too bad of a job