nitro engine overheating and general tuning questions


Active Member
Hey all,

I'm looking for some nitro tuning advice. I've got a Mad Force Kruiser with the KE-25 engine (.25 cu in or 4 cc). It's my first nitro truck and I love it! Unfortunately, I was never quite able to get it to run right. It always runs hot. I also have two HPI RS4 nitro cars. These run fine without the overheating issue.

I bought it while living overseas. While there I ran Tornado Fuel at 25%. I just bought my first bottle of nitro here. I picked up Traxxas 20%. I've been running LRP glow plugs. I've tried the R3, R4 and R5 heat ranges. The hobby shop I frequented overseas recommended the R4 plug for most situations and they said run the R3 in the winter. I've tried running the R5 in hopes of bringing the temp down, but I don't remember if there was a difference. Although, when I look at LRP's website, I should probably be running the R5 or R6 plugs (mine is .25 and 20% nitro). R5 is medium-cold for .18-.36 engines running 16-36% nitro and R6 is cold for .18-.36 engines running 16-36% nitro. The R4 is medium for .12-.18 engines running 10-36% nitro and the R3 is medium-hot for .12-.18 engines running 10-16% nitro.

I race real cars as a hobby, so when I got the Mad Force, the first thing I did was count the turns out of each of the needles so I would know where they are if I needed to reset. Unfortunately, the low speed needle was tough to turn in right from the start (the o-ring was really tight) and I screwed it in too far (it never felt like it positively bottomed) - I know, ham fisted mistake. I ran the truck like that for a while and it would run good for a little while, but the temp always creeped way up, so I knew I had an air leak. I bought an o-ring kit and rebuilt the carb. It sort of helped. I then bought a new carb for the engine. The truck ran great for a couple tanks and I went to clean the air filter at this time and oiled it. I washed it with dish detergent, let it dry overnight and then oiled it. A couple drops of oil in a bag and then dropped the filter in the bag and worked it in. Once it was oiled, I put it in a paper towel to get rid of the excess oil. Then it started running hot again. I used air filter oil specific to rc cars. I also thought maybe it was because I had a different body on the truck (the 56 Nomad Chevy Station wagon), so I cut a bunch of vent holes, no change. Running with the body off, it does cool down some, but still runs hotter than it should. Now that I'm back in the US, I bought a pickup body so it's getting the same air flow as the stock body. It still runs hot.

I've replaced all of the fuel lines. I also just recently took the entire engine apart and reassembled to ensure it was all tight.

My first time running it in the US was this past Sunday (since installing the pickup body, rebuilding the engine and being in the US with Traxxas fuel). It was in the upper 40s outside, the truck started up great. I started it as soon as I took it outside so it wouldn't get cold. I used to have a **** of a time starting it, I think because the low speed was set too rich from when I screwed up the LS needle before replacing the carb. I started driving it around and it was too rich at first (because it needed to warm up), so I kept driving it to build up heat and then started to lean the HS needle. I was always getting lots of blue smoke. I leaned it until I could get a good clean run. I started driving it like that and the temp held fairly steady, although it was up around 240 F which is a good temp, but considering it was only in the 40s outside, it shouldn't get this hot. I should have to put a sock or tape on the heat sink to keep the heat in. Otherwise, it was running great. Lots of power, it would idle for a long time (over 30 seconds without an issue). When I'd bring it in to check temp, it would idle high for about 7-10 sec and then idle down some and hold that for 30-40 secs before I was ready to drive off again. It would take off cleanly. I would check the temp while idling every so often and see the temp coming down a couple degrees every 5-10 secs, so I assumed the low speed was a little rich and would lean it 12th. When I would take off, it would be crisp and blow some smoke. However when it's idling, you could barely see any smoke at all. At mid throttle, you didn't see much smoke, but at full throttle there was smoke. Anyway, the temp stayed fairly constant and didn't creep like it used to before I replaced the carb. Towards the end of the run as the fuel was running out, the temp started to creep up, but I'm told this is normal. Just before the tank was finished, I snapped a screw for the steering and had to stop driving.

I fixed the truck overnight and took it to work to play with at lunch yesterday. The first tank, the truck started great again and drove really well. It was in the 30s outside. I temped the engine and again, it was in the 240 F range. It ran great and lots of smoke at full throttle. Not much while idling. It would idle for long periods and the temp would drop while idling. I still thought the low speed must be a little too rich as the temp dropped fairly quickly at idle, but after idling for 20 or 30 secs and then giving it full throttle, it usually would take off pretty good (with a cloud of smoke), so I thought it was close to where it needs to be. I've had it a few times where it was too rich and when I would pull the throttle, it would start to move and then die (I assume flood out). I've also had it a few times where the LS was too lean and it would be really high pitched and loud. I ran through a full tank and then let it cool some and refueled. It started up again no problem and ran good for a little while and then started running funny. It sounded different under throttle and would sometimes run on a little after letting off the throttle. It was also getting hotter than 240, almost up to 280. So I tried richening the HS needle a 1/4 turn, hoping to get back to a "too rich" point so I could slowly lean it into the sweet spot again. However, it never got to the "blubbery" sound of being too rich. It continued to pull ok. Although a couple times when sitting idling and then pulling the throttle, it would jump and then die (flood out/flame out I assume). It would be a pain to start (almost start and then die). I assume because there was too much fuel in the crankcase that had to be burnt off. I'd get it running and try to lean it out. At this point I was getting frustrated and not sure what to mess with. (Does it not get blubbery sounding when it's too warmed up even though it's too rich?) One time, I brought it in to check on it and it temped high over 300 F (remember, it's in the 30s outside and I don't have any way of keeping heat in the engine) - it still had good smoke under throttle though, a noticeable plume of blue smoke. As I was temping it and getting my screw driver to start richening it (remember, I had just leaned it to get it running), the engine started to pick up speed and run away. So I quickly ******d the fuel line to kill the engine. I reset the HS needle back to roughly 3 turns out, where it seems to run good, although hot. I drove a little more and then had to stop. It seems the truck always runs good for the first tank. And once it's warmed up and I try to run a second tank, I always start running too hot and having issues. I am starting to think maybe the seal on the fuel tank isn't working? Or the o-ring on the HS needle isn't sealing when I move the needle too much. When it sits overnight, it runs good for the first tank. Maybe moving the needle is causing the o-ring to develop an air leak? And then when it sits overnight, it seals up again?

I'd appreciate some advice on what to look for to set the HS and LS needles. I know the normal stuff. Don't tune until it's warmed up. Don't focus too much on temp. Make sure there's good smoke. Always tune from Rich to lean. Starting when it's rich, lean the HS needle and it should pick up performance. When performance starts to drop off, richen it up a little and you should be good. Always make an adjustment and then run the vehicle to burn off any excess fuel in the crank case before the effect of the adjustment will show up. Once the HS is set, you can set the LS. It should be able to idle for a little while and then take off crisply. If it stumbles and then dies on take off (floods out aka flames out), then the LS is too rich. The pinch test - pinch the fuel line near the high speed needle, it should run for about 2-3 seconds and the rpm will pick up and die. Also, while just idling (no pinch test), it should idle for 7-10 secs and then slow down the rpm a little (the low speed is starting to richen up a little) and then idle like this for at least 20-30 secs or more (mine will usually idle over a minute). The temp should also slowly drop a little while idling (this is the cooling effect of being a little rich on the LS). If the temp rises while idling, the LS is too lean.

So, some questions:

* Is there only smoke while on the throttle or also while idling? I barely see smoke when the truck idles - which seems to make sense since it's not spinning fast (I'm a 4 stroke guy and don't fully understand 2 strokes). When the LS is too rich, I do see oil sputter out the pipe and still barely see smoke, so I assume I don't need to see smoke at idle. At mid throttle, I see some smoke and full throttle is a nice plume.

* How do you know what glow plugs to run? Should I continue with the R4s or go to R3 or R5? What does using a cooler or hotter plug do? I realize that a hotter plug is similar to advancing the timing on a 4 cycle engine and a colder plug is like retarding the timing on an engine. But how does the plug effect temp and performance on a nitro engine? I assume running in colder weather you want a hotter plug, right? Does the plug effect how hot an engine runs too?

*Why does my truck hold temp for one tank and then, if I'm lucky enough to have time to run a second tank, the temp starts to creep right away? If the truck is parked overnight, the first tank is usually stable. (could it be the o-ring on the needle not sealing from me playing with it too much or the seal on the fuel tank?)

* What are symptoms of too rich and too lean HS needle?

* What are symptoms of too rich and too lean LS needle?

* How often do you change glow plugs? How do you know when it's time to change glow plugs?

Appreciate any advice. Even with the problems I'm having, I still like the nitro experience better than my electric trucks!


Well-Known Member
The biggest problem you have is different brands of fuels and plugs. Use Fuel with 18% oil lube to cool the engine down. Use a os #8 plug and I suspect you damaged the mix needle by turning in to far. As for trying a new carb The damage was already done from the improper tuning and wrong fuel used. Once you loose compression you can't get it back unless you buy a new piston sleeve for your engine. Use a temp gun to watch the temps as you run. you will see by adjusting the high speed needle the temperature will change. Always use a fuel filter. This is always overlooked and junk can block fuel flow and ruin a engine fast. A compression gauge will tell you the condition of your engine. I suggest you start over with a new engine and all the advice given.


Active Member
Thx for the reply! I forgot I had posted that novel! Anyway, I finally found the culprit. It was the o-ring on the back plate where the pull starter shaft goes through. I should have realized it when the pull starter string was starting to get soaked with oil that the o-ring was leaking. After replacing the back plate, the engine is a dream to tune! I can very easily get the temps where I want them. I checked everything else for air leaks, even the seal between the back plate and crank case, but I never thought to check where the shaft goes through the back plate.


Well-Known Member

A well tuned engine should be below 250 degrees. More oil lube will prevent overheating and make you engine last longer.