Tuning Diagnostics: What’s Up With My RC Nitro Engine and How Do I Fix It?

We get asked heaps about the problems associated with a nitro engine not starting correctly, or not holding idle, and so we’ve create the following guide to help you sort out the major problems that might be affecting your model.

There are so many engine problems that can be caused by poor tuning, it’s always a good go-to when your engine’s playing up. Whether it’s bogging out, refusing to idle, fading out at random times or not braking properly, the chances are your tuning is to blame. So, what should you do about it? Follow these instructions, that’s what.

The first thing to check is if your engine can successfully idle for a minute or more. If not, I’m afraid you’ve got some serious issues! But don’t worry, we’ll sort it all out soon enough

Make certain the glow plug heater is fully charged and that it is making the glow plug turn a bright orange colour. You can test this by removing the glow plug from the model, place it in the heater and pressing to visually check if it glows.

Check that the main needle valve is level with the outer housing as a starting point on most models. If it is closed too far, fuel will not reach the carburetor. Prime fuel to the engine sufficiently and check that the fuel line to the carburetor is full of fuel. I usually blow down the end of the exhaust pipe when the engine is cold to get it primed for the first time (do not try that when it is hot!). This forces fuel through the feed line and into the carburetor.

Connect the glow plug heater again to the engine and hopefully it will fire straight away on the first few pulls of the starter cord. CAUTION: Don’t pull the starter cord too far, it will break… and you won’t be happy!

Hopefully you’re now able to start. If so run a couple of laps to get it warmed up, and then leave it to idle with no help from you or the throttle or anything. If the engine dies, what you want to be paying attention to is not just the fact that it dies, but exactly how it dies. Revving up right before it stops is a clear sign that your fuel mix is too lean. In that case, you want to richen up your low speed needle tuning a little bit, and see if that helps. This really doesn’t need to be much – as little as one eighth of a turn counter-clockwise should do it for a first test! Once you’ve done that you can send it on a couple of more laps and then leave it to idle again. More on that in a minute.

If, on the other hand, your engine doesn’t rev before it stops, but simply slows suddenly or sputters out, that’s a sure sign that your mix is too rich. The solution? Do the opposite! Push the low speed needle screw in a bit by turning it 1/8 clockwise to lean out your mix, and then go for that idle test again. If the same thing happens, turn it a little bit more. If it switches to the other problem, revving and then dying, you’ve gone too far, go back! Hopefully after a bit of fiddling you should have fixed all of your idling problems – or at least all of them that are to do with the engine’s tuning.

Dying when idling not the problem? How about cutting or bogging out at speed? If that’s the case, you’re got a high speed needle issue, logically enough… So you need to be richening or leaning out your fuel mix at the high speed needle, and again, which one you need to do depends on how your engine is dying. Running a couple of laps at wide open throttle (or trying to) should make it clear enough. Suddenly dying or loosing power for no apparent reason is the sign of a leaner state than you really want, so try 1/8 of a turn of the HSN counter-clockwise to see if that helps. Then off you go on your laps again! If the engine dies with a sputter or ‘bogging’ noise, however, you’ve got the opposite problem, and need to lean out your fuel mix a bit. You got it; a teeny clockwise adjustment to push the screw in a bit should sort it out. Keep experimenting and running laps until your engine isn’t dying either of these ways. Problem solved? Awesome!

The last set of engine problems that can be fixed by adjusting your tuning are to do with braking, and these ones you address using the idle screw. A slightly misleading name in this case, but nonetheless a very helpful adjustment tool! If you’re not quite sure how the idle screw works, it basically sets the limit on how far back your throttle can go when you hit the brakes; further in and the throttle stays more open, but too far back and it will cut of all the fuel getting through, and kill your engine dead! Which is, by the way, one of the main symptoms of a poorly tuned idle screw. If your engine dies when you brake, this is exactly what’s wrong, so push that idle screw in a bit more! 1/8 of a turn clockwise is recommended, then do a quick, hot lap and try braking again. Problem not solved? In a little further. Problem solved? Great! It’s not dying, but the kit keeps rolling even after you’ve braked? Too far, take it back a little bit. Keep trying and testing the brakes until you get the balance right.

That is, of course, the final tuning issue – the kit not properly stopping when you brake. If the wheels keep rolling then it means your idle screw is too far in, and you gotta pull it back out a bit! This will lessen the amount of fuel getting through, and hopefully hit that balance where your engine doesn’t struggle or die, but sits at a nice happy idle instead.

So that’s all of your major problems fixed, hopefully! Obviously there are a few more things you can do and a few more tests you can run if you want a really fine tune, and checking out a few of these can help stop the major tuning problems from returning. Which can be no bad thing, right? If you’re interested, check out our other tuning article for the perfect fine tuning routine – and never let it get this bad again!

Comments are closed.