View Full Version : Nitro Engine Tech & Tuning

10th November 2009, 08:08 AM
Want to discuss nitro engines, or need help tuning, this is the place for you to ask.

10th November 2009, 08:59 AM
Like all combustion engines, we need a means of delivering a fuel mixture to the combustion chamber, and a nitro R/C engine is no different. A new age nitro engine, usually utilizes a 2 or 3 needle carb, constructed in either aluminium or a form of composite.

Some of these carbs can be quite tricky to tune, and most new comers to the hobby, struggle to get past this, and it is what usually kills any love they had for the hobby, after they lunch an engine, and are up $3-400 for a new mill.

In this article, I will outline the basics to tuning your R/C Nitro engine, whether it be a .12 or .21.

The Barrel

The slide or barrel regulates the amount of air to enter the engine controlled by the throttle servo. It simply blocks airflow to the engine proportional to how far it is open or closed.

Idle Speed Needle

The idle speed needle, is a small blunt need located in the side of the carby, which comes in contact with the slide or barrel, Adjusting this needle will determine how far open the slide or barrel is, usually 1-2mm.


The HSN or high speed needle, controls fuel delivery at any throttle %. The HSN is located opposite the throttle slide or barrel, where the fuel enters the carby, via the nipple. Tuning the HSN is usually the easy bit.

The Low Speed Needle

The LSN or Low Speed Needle, determines how much fuel enters the engine at idle. In the center of the slide or barrel is where this needle lives. While looking down the throat of the carb, you will see a small needle poking out the end of the slide, opposite this, is the fuel inlet. While at throttle this needle nestles itself into the fuel inlet, causing a restriction, and allowing us to tune fuel delivery at idle.

A common issue with a lot of R/Cers, when tuning the idle speed, and LSN, is they set the idle speed to high, and the LSN too rich, this will cause all sorts of problems. The biggest is disguising a lean HSN, due to the extremely rich low end mixture, residual fuel is burnt at wide open throttle, but when the residual fuel is burnt off, we are left with a lean HSN, which usually will result in a failure sooner or later. Spluttering or bogging are both symtoms of an incorrectly set LSN.

The Mid Range Needle

The mid range needle regulates fuel to engine after idle and before full fuel position. This spray bar is were ALL the fuel enters the airflow stream regulated by the high speed, mid range and idle/minimum adjustments.
Normally somewhere between 1/2 and 3/4 throttle open position the needle is completely out of the spray bar, This is what is called full fuel position or FFP, at this point 100% of the mixture is controlled by the high speed needle.
On many carbs the low speed and midrange are not independently adjustable so the mid rage is a factor of the needle taper and is engineered by the factory. On some SLIDE carbs there is both independent spray bar and mid range needle adjustments.

Setting the Carb

As I have said, there is no one way to set the carb, every engine is different, and they are very susceptable to changes in climate, and altitude. Anyway, first off, we need to set the idle speed.

Setting the idle speed can be tricky as I have mentioned, care needs to be taken. So your about to start the engine for the first time (we'll get into break/run in later) we need to set the carb to get the engine to run and idle, that is all we need at this point. So with your air filter off, have a look down your carb, and you will see a closed slide or barrel, we need to adjust it so it is open approx. 1-2mm, we'll go with 2, as we want a rather fast idle speed. Now at the end of the slide, is our LSN, for now we are going to set this to flush. So we try to start it, it does, and it stalls.

It has stalled, 1. Because it too rich, or 2. It was too lean. Now you need to ask yourself what happened prior to it stalling, one of 2 things happened. The engine idled up and died, a result of a lean mixture, turn the needle out a 1/4 turn, and restart, it should now be rich, if not repeat. Or the engine idled, fast and then slowed and died, due to a rich mixture. We need to make 1/12 or hourly turns in, and restart, Once we have a good low end mixture, we need to lower the idle speed, back down to the 1mm mark. By doing this it should give us a good idle speed mixture.

Now, we have the engine idling nicely, and we can putt our car/truck/buggy around, we need to set the high end mixture. Again this needs to be set flush, and we want to give it a squirt, and see what it does in the top end. We are looking for a trail of blue smoke, and temps of around 110 degrees. So you have a lot of smoke, and not much action, your high end mixture is too rich, again at hourly turns, turn your HSN in, after each turn, make another pass/run, and adjust, until you have a thin trail of blue smoke, and temps of 110-115 (some engines, and fuel runs hot, again another thing to be aware of) If you have no smoke, and its hot, its too lean, and turn it out a 1/4 turn, until you have a rich mixture, never tune from the lean side, always tune rich to lean.

So we just have the Mid range needle, alot of people suggest you don't touch this, but I love it, basically this is the transition time between idle, and going fast. It is quite easy to use, but will destroy your tune, if you don't understand it. Most are set flush from factory, usually best to leave them there any way. So if you remove your MRN you will notice it has 4 holes in the side of it (like most jets) now we need to line these up with the fuel inlet, so basically we can only make 1/4 turns in either direction. I do think the need to line them is a bit odd, and have questioned it, but no one has ever given me a straight answer, you see where the jet accepts the fuel, there is a cavity, and all holes receive the same pressure, but this is what manufacturers suggest, so I have always lined them up, its good pratice to line up and mark 1/4 turns on the side of the carb. Anyway once all that is sorted, you can use this to tune just how had you can get t to punch out of the corners, but as I said best to usually leave it alone, till you are good at tuning.

Hopefully , that will some of you out with tuning, and understanding what does what on your carb. There is no set tune for any particular engine, after all they are just engines, and none are ever the same.

11th November 2009, 03:14 PM
So it's time to break your new engine in. First things first, this can be an easy and stress free process, or an absolute prick of a thing to do, which can result in ruining your new engine.

There is ALOT of debate regarding the right way to break in an engine, many people have their own way of doing it, including myself. But in this article, we are going to look at the easiest method, The Heat Cycle Method.

During the heat cycle method, we will b heating our engine to operating temperature, and then cooling it, which we will then repeat, then move on.

First off, before we even think of starting your new engine (I know you want to, but don't) we need to do a couple of things.

I find it good practice to open an engine up and inspect it before starting, if this is your first engine I wouldn't bother, you may do something wrong and ruin it. By doing this we can insure nothing has been left behind during the building process.

We also need to heat the engine, using a blow drier, or heat gun, to approx. 90*c. This allows the engine to have a bit of clearance, which in turn puts less stress on the rod, crank pin, and bearings. Also we need to flush the engine with fuel (a dribble in the carb, and down the glowplug hole), to ensure everything is lubricated, some people suggest using after run oil (more on after run oil later) to initially lubricate the engine, I have found that after run oil carbonizes when burnt, and may damage the glow plug, I have also seen bearings damaged as a result of this, so be warned.

Next we need to look to the carb, as stated in the carby article, we want all the mixture needles at flush. Also we need to set your Idle screw so that our carb is open to about 2mm, this will allow a higher idle speed, to compensate for our new engine being to tight.

We now need to prime the engine, so with your engine at 90*c block your exhaust, and give the engine a few pulls, do this till all the air bubbles are gone from the fuel lines. If during this the engine is to tight to turn over, remove the glow plug, and give it a couple pulls, it may be flooded, so watch your eyes.

Now with your engine at 90*c, tank primed, glow ignitor on, and the car on a box or similar with the wheels off the ground it's now time to start your new engine. We need to pay special attention to what happens when we start the engine, chances are it won't run long, and what it did before it stalled, is what we need to know to tune it accordingly. One of 3 things will happen, it'll splutter till it stalls (too rich), RPM will flare and it will stall (too lean), or it will idle (Good enough for the first tank)

So going back to tuning the carby, if it is too rich, we want to make hourly turns in of the low end needle. If it is too lean we want to make a 1/4 turn out of the low end needle.

Now with the engine running, we want to let it idle high, with the occasional blip on the throttle. Once the first tank is almost burnt through, we want to block the carb and shut it down. Now its time to let it cool, I wouldn't let it get past 40*c, then heat it up with the gun, to 90* and go through the same process. We will burn a total of 3-4 tanks of fuel.

The next step, we want to run the car around, I choose to do a figure 8 pattern. You will most likely find when the car is on the ground, and you give it some throttle it will stall, again we need to pay attention to what is happening, if it splutters and bogs it is to rich, and we want to turn in the HSN an hour at a time. If it sounds crisp, and stalls, its probably to lean, so turn the needle out a 1/4 turn. While putting around do not go over half throttle, for the next 2 tanks, remember after each tank let the car cool to 40*c, and then heat with the blow drier or heat gun to 90*c.

Once we have cleared those two tanks, we will want to move upto 3/4, never hold it at one rpm for too long.

Then we want to do the same, but with short burst of full throttle, no more than 2-3sec.

Now we need to lower our idle speed, so screw your idle screw out, so the carb is open about 1mm, we don't want the clutch engaged, so set it just prior to where the car wants to move, this needs to be set with the car on the ground. Next we need to perform a pinch test, this is where, just prior to the fuel nipple on the carb, we want to pinch the fuel line and run the engine dry, the time it takes from pinching the line to it stalling, should be around 3-4secs, any longer, and your LSN is too rich, any less and its too lean.

Also pay attention to the car while it is idling, if the idle slows a short while after being at idle, your LSN is to rich, this puts undue stress on the engine, and is good to avoid it.

Overall, we want to run through 1.5 ltrs of fuel during the initial break in period.

With the last .5l you have in your fuel bottle, we want to give the engine a tune. Basically we want it to punch out of the corners, and carry good speed. We always need to see oil smoke, just a thin trail behind the car, and we do not want temps over 115*c.

And we are ready to race.

15th November 2009, 01:07 PM
After run procedures, and regular maintenance, of your nitro engine are extremely important, and have a dramatic on the life your engine, and long term performance.

The nitro-methanol we run our fuel on is extremely hygroscopic, and basically inhales moisture from the atmosphere, leaving unburnt fuel in your engine will accelerate corrosion, and in turn wear. Like all things in the R/C world there is a lot of debate surrounding this subject, but we will just go with the commonly known way to do things.

After running our nitro engines for the day, we need to do few things, first off we need to perform a pinch test, this will run our engines dry of fuel, it is also good for checking our idle mixtures are still correct, if not this may be symptom of something like an air leak. Then it is a good idea to put a few drops of after run oil down the carb, and the combustion chamber, then give the engine, a few pull, to get it around the engine. After run oil provides the engine with an oil film, to help prevent corrosion. I always suggest flushing after run oil, out before running the engine, just add a few drops of fuel, and give the engine a crank.

Regular maintenance of your nitro engine is something, every hobbyist should learn, for a little time you can save yourself a lot of grief, and a shitload of money. I pull all my engines down for inspection, after every 2 litres. But I am a little obsessive. We should be checking bearings, crank, and crank pin wear, piston and liner wear, wrist pin wear, and conrod bushings, and straightness during this time.

Before we disassemble our engine, we need to check compression, we simply turn the engine by the flywheel, it should be fairly hard to do, secondly remove the plug, and check the pinch, this is how tight the piston is in the sleeve, it should feel tight at TDC, not so its hard to move, but you should be able to feel some resistance at TDC. If you do not have pinch, you can have the sleeve repinched, this is risky, and isn't a long term solution.

While the engine is at top dead centre, we want to check the play in the bottom crank bush, this is also an indicator of a stretched rod, both of which will require you to replace the rod, with cheaper engines this is a common issue. So with the engine at TDC, rotate the flywheel back and forth, you will feel a bit of play, no im its a small amount, say 1mm or less, it's not bad, though if you have more, there is a chance that the rod will be stretched, or the bushings and/or crank pin may be worn.

So with the backing plate and head removed, we can now see inside the engine, first off just have a squiz, see if there is any debris inside of the engine. Next take a small cable tie, and slide through your exhaust port, then turn the flywheel and as the piston catches on the cable tie this will lift the sleeve out of the block, it will lift it about 4 or 5mm, simply slide it up with your fingers.

Now we need to remove the piston and rod, we need the engine at TDC, then take a pair or long nose pliers, with some fuel line iver the ends, so you do not mark or damage the rod, now just pull it out off the crank pin gently. We can now remove the crank, take a small rubber hammer and give shaft a bit of a love tap, and push the crank out of the crank case.

If you have any issues removing anything, just stick the engine in the oven at 150*c, remeber to wear gloves when handling it hot.

So with the crank out of the engine, feel the motion of the bearings, they should feel smooth, if they feel as though they are notchy or gritty, they will most likely need to be replaced. There are special tools for doing this, bearing pullers. I suggest you use them, as it will ensure no damage to the bearings during removal, and fitting. Also if you notice it looks as though the conrod, has made contact, on the bottom of the crankcase, your front bearing will need to be replaced

We now need to visually inspect all of our components. Start of with the crank, first we want to look at corrosion, this can be rectified by polishing the crank, or performing something similar. Next look at the shaft itself, we should not have any scratches or grooves, these are symptoms of a lack of lubrication, if you see this, most of your engine will be ruined. Then inspect the crank pin, again we should not see any lines or grooves, also kep an eye out for uneven wear.

Next take the piston and rod combo, we want to check the play in the little end bearing, so grab your rod and try and move it up and down, if there is any play that you can feel replace the rod, as this bush is worn. Next we need to remove the wist/gudgeon pin, again check for wear marks, anything noticible, we should replace the part.

Inspect the skirt and edges of your pistons, for uneven wear lines and gouges. This is a direct relation, to how you have been running your engine, and the lubrication, you will see more wear if you have been running it hot, or have been using inferior fuels.

The piston liner should also be checked the same way we checked the piston, again the same rules apply.

So we have inspected our engine, we have replaced anything worn, and it is now time to reseal the engine, remember during assembly, everything should be lubed, with either 2 stroke oil, run in oil, or assembly lube, I use assembly lube, the way I figure it, is, if it is good enough for a 500hp engine its good for my nitro. Anyway back to assembly, with all of the internal components back where they belong, we need to look at sealing the engine. I recommend using a high temp fuel safe silastic, like the 3 bond stuff, or similar, this will insure we do not have any airleaks, which will in turn lean the engine out.

15th November 2009, 06:28 PM
i need an new engine for my nitro car. if i buy it wanna run it in for me? for a little $? you seem pretty good at it

15th November 2009, 07:28 PM
yeah send me a pm

16th November 2009, 03:38 PM
Thanks dude, not finished yet. I understand why people go to electric, but wheres the excitement the noise, the smell, and hey 40,000+ rpm, is fucking cool, the really high end modders are pulling 55,000rpm out of a 21, pretty impressive.

16th November 2009, 04:58 PM

16th November 2009, 05:05 PM
nah centrifugal clutch, to a centre diff, then split to front and rear. drive ration is usually around 11:1. I have got to 47,000rpm, but should be able to get near on 50k with my new crank mod.

I use one of these as a dyno


I had a real dyno on order, but shit came up so i had to cancel it.

17th November 2009, 08:50 AM
done, as I have said earlier, if anyone has questions just ask, don't worry about looking stupid, 90% of people dont know either.

25th December 2009, 01:45 PM
hi gunner,

i put a victory rc pipe on my baja 5t and dont want to run it cause im worried its too lean. i cant get it to the shop till tuesday to get it sorted. so i want to know is there anything i can do now to make it safe to play with until tuesday?

25th December 2009, 11:08 PM
I don't have much experience with the petrol stuff, only really nitro. But unless you are familiar with the carb I wouldn't suggest mucking around with it too much.

There is a heap of info regarding 1/5 scale out there, try some of the forums in the links section, like ausrc.com. There is a few dedicated baja forums too.

26th December 2009, 12:02 PM
cheers for that gunner

27th December 2009, 11:17 AM
No worries mate, sorry I couldn't be more help.