Losi XXX-NT Question

[jnye6625]

Hello all! I am getting back into the RC hobby after being out of it for about 9 years. I used to race a Losi XX-T electric truck, and now getting back into things I found that they have a XXX-NT out and bought a used one off ebay. My experience with nitro engines has been very limited, and the only ones I've used are the ones in a couple nitro boats I built in high school. The used truck I bought has a O.S. .15CV-X engine in it (blue head) and seems to start up and run really well, but after about 1/2 or 3/4 tank of fuel it just all of a sudden dies and wont restart again until it cools off. Does anyone know why it would do this? Also, I downloaded the OS instruction manual for this engine, but it doesn't give any good starting point adjustments for the low speed needle. Can anyone give me a quick idea on where to set everything to start, and how to adjust according to what the engine is doing?

The other issue with this truck is that the brakes seem to be very poor. I might be used to an electric truck that can lock the wheels up going full speed, but it seems like they barely work on this thing. I have been looking around online trying to find a new brake shoe and drum to try and get it working better, but I can't find one anywhere. The brake as sown in the XXX-NT instruction manual looks like a disc on the left side of the transmission that one pad on each side. The brake on MY truck has a curved plastic shoe that rubs on a steel ring next to the spur gear. I can't find a single xxx-nt anywhere online that shows a brake setup like this. So... I'm wondering if what this guy advertised on ebay as a XXX-NT is really a XX. Anyone have any input or seen anything like this? I'm excited to be getting back into the hobby and I just ordered a XXX-NT AD2!

 

[HeartBreak]

Sounds like you could have a hole in your fuel tank, which would cause the engine to not want to restart when hot. If you have access to a temperature gauge, check the temp next to the glowplug. It should be around 220F, give or take 20 degrees.

Now, as for the brakes, it is possible the disc is worn, but you could also have oil on the brake bars. Make sure that the brake bars are clean and dry when you change the disc.

 

[jnye6625]

Thanks for the reply...
The engine seems to just quit, and will not restart. I am going to get an infrared temperature gun to check that out... I've heard from several people that I should do that. There is a tiny hairline crack in the top of my fuel tank, but it looks like it doesn't go all the way through so its probably not a leak. I know that fuel is still getting to the carbuerator. Is it possible that a worn engine could loose compression when hot and not run?

As far as the brake bars go... where can I get replacements? I can't seem to find the replacement parts anywhere. Is there an easy way to tell if I have a XXX-NT or just a double X?

 

[olds97_lss]

Is it possible that a worn engine could loose compression when hot and not run?

Yes. A sign of a dieing engine is stalling when hot. Erratic tune, flame outs for no reason, loss of power and minimal compression when cold or no compression when warm, all are signs of a dieing engine.

However, a poor tune and hot running engine will also flame out and not restart until cooled if it's running far too hot.

Do you happen to have a photo of the rig? I used to have a xxx-nt RTR 6 or 7 years ago. It was my first into the hobby.

 

[jnye6625]

Thanks for the information. Is there a way to test compression? How do I find out if the engine is bad? Can i take it apart and look at everything without having to use new gaskets to put it back together? I'll get a picture of it and post it for you when I get back into town on Sunday.

 

[olds97_lss]

When cold, with the glow plug in the engine, if you can easily rotate the flywheel with a finger, it's probably worn out.

The only true way to tell if it's dead is to take the engine completely apart and see if the piston slides through the end of the sleeve. It doesn't have to slide all the way through, but if it slides through even a little, the compression is most likely gone. For example, take a look at the bottom two here:

These were some of mine I had re-pinched in March.

These are after the pinch was done. A new or re-pinched one typically only goes about this far:

If you think the engine is worth the hassle, you can get it re-pinched (like the above ones) by guys that do it. I had www.RayARacing.com do the above ones. It lets you get a few more gallons through an engine for $25.

 

[MrClean]

Hey Olds, when you do that test, does the sleeve have to be out of the engine as well or could you disconnect the con-rod and check it with the sleeve in the engine still?

 

[olds97_lss]

You can't disconnect the con-rod without taking the sleeve out. There isn't enough play. Well... I've never had an engine that had enough play without the sleeve out.

 

[MrClean]

Ah, DUH..........I wasn't thinking it through far enough...LOL.

 

[jnye6625]

Thanks for the response.. i'll have to take my engine apart and see what it looks like. So when you get it re-pinched... does it close the top of the sleeve a littl eso the piston can't travel through the top or what?

 

[olds97_lss]

Yep. The way these engines build compression is with the fit between the piston and sleeve wall. The sleeve is shaped like a cone. You can't really "see" it I don't think, but the internal dimensions differ between the exhaust port and the top of the sleeve.

Over time, the piston grinds down (since it's aluminum and the sleeve coating is chrome) and the fit is no longer optimum. As the sleeve heats up, it expands and the fit becomes even more loose and all the compression blows by the piston which makes for a much smaller bang. Which causes erratic tuning, flameouts, hard starts, only starts when cold, runs poorly when hot, etc... When the fit is proper, it's tight when cold and just right when then engine temp gets up over 200F.

What getting it pinched does is mechanically compresses the entire perimeter of the sleeve equally near the top of the sleeve. So much so that it stays at the smaller diameter when done. Again, we're talking micrometers here, so it's not really visible, but you can definitely feel the pinch as you attempt to slide the piston towards the top of the sleeve.

If you go to www.rayaracing.com, he has some documentation on there to show you what he does and what the process is. I believe he compresses differently as he moves up the sleeve, so it isn't just one size. It gradually gets tighter as you go up, which is very similar to how they are when new.

Now this won't give you the life of a new piston and sleeve, but the ones I've had done have held up for many gallons (4+ so far on my oldest) after the re-pinch was done. If you take care of your engines, this can be done a few times before it's pushed too far and you get leakage and issues around the outside of the sleeve and the chrome coating also does wear along with the piston getting too thin. So, in a perfect environment with a well maintained/operated engine, 3+ times adds a lot of life to a $50 part, considering $75 (pinched) is much less than $150 (new). This process has a much higher pay off with higher dollar engines where the piston/sleeves run you $100 or more. Still is only $25 to re-pinch.

Before I send the piston/sleeve in, I mark it so he knows where the piston was in reference to the sleeve... not sure if it matters, but it made me feel better.

Sorry for the rambling...

 

[jnye6625]

Olds... thanks for the rambling! I definately understand the process much better now. I took my engine apart last night and found that i could push the head of the piston about almost a centimeter through the top of the sleeve. I also found that the gasket/ring that goes between the head and sleeve is missing. I'm not sure if this is intentional or if someone just took it off without knowing better. Anyways... I might end up sending the piston and sleeve in to be reconditioned. Is the push through test valid for all nitro engines? I know the ones you show above are from different manufacturers than the O.S. engine I have. Thanks for your help!

 

[olds97_lss]

As far as I know... any engine I've had that I could do that with was either poorly running or not really runnable at all. Orion, OS (3 different models), SH, Dynamite and Traxxas engines are all ones I've had that wore that much. I didn't get all of them pinched as they weren't all worth the hassle or I didn't have a need for the engine anymore.

The washer is a brass shim. It allows the sleeve top and head button or head (depending on your engine) make a good seal. It's soft, so it forms around in abnormalities to make a better seal. It also sets the compression level to a point specific to the engine/fuel type. Some people think running without one is a way to boost power, but all it really does is throws off the combustion timing and causes air leaks. You always need at least 1 shim in there for sealing purposes. You can use various shims depending on your nitro content. Higher nitro requires more shims. Most RTR engines are shimmed to run 15%-25% nitro fuel out of the box. If you run lower or higher, a shim change is a good idea.

 

[jnye6625]

Olds... again, thanks for the good information. Why dont they have an information packet somewhere on the internet to explain all these little intricacies of nitro engines?

 

[olds97_lss]

Yeah... I found the internet forums about 6 months into the hobby... I had destroyed an engine by then though. lol!

The info is out there, you just have to look. It helps to know what your looking for though, which is what you learn at forums like this and with experience.

I have found that most people new to the nitro scene don't want to hear a lot of jaw jacking... THEY JUST WANT IT TO GO!!!

A month, engine and lots of gray hair later, they tend to listen!