The art of tuning a nitro engine for performance is one of those things you can either do, or you can’t. Those who can’t can learn as it’s actually not such a mysterious gift given to the chosen few, it is a lot to do with feel and knowing the symptoms. There’re many tuning guides on the web but with so many new kits being sold at Christmas, once you engine it broken in we thought you’d appreciate this user friendly version which you can keep close to hand, the MaxBashing magazine is after all designed to fit in your toolbox!

Model cars that have nitromethane based fuels can be a daunting and frustrating proposition when it comes to setting the carburettor on the engine. This is partly due to some engines being temperamental, but new RC owners are often put off by lack of assistance. Everyone wants to get the best out of an engine and with correct tuning, that’s what you’ll get. There is a certain ‘sweet spot’ that you’re looking for, a certain pitch in the sound from the exhaust when it’s reached and then you get the ‘feel’ of tuning, it’s almost like having a sixth sense.
There is no universal setting that works for all; every engine will be different with factors such as the outside temperature and what brand and percentage of fuel used will play a big part in each engine’s exclusive requirement. This guide will work just as well on a rotary or a slide type carburettor.

The Carburettor

Carburettor settings are decided by adjusting one or two screws (or needles), these being called the High Speed Needle (HSN) and Low Speed Needle (LSN). These settings are quoted by the number of full turns out each needle is from a closed position. To discover your current setting, simply turn the screw in a clockwise direction gently until it stops and count how many turns it took, then return it to its original place. Never over tighten the screw as this could cause lasting damage to the rubber O-ring inside. There should also be an idle screw, which will raise or lower the idle speed of the engine; it plays little part in the actual tuning.

The Procedure

The correct procedure to setting the carburettor is to start with the High Speed Needle; this will be the larger screw usually pointing upwards and located at the point where the fuel enters the barrel of the carb. The engine must be allowed to reach its normal operating temperature before tuning begins so start it up and leave it to idle or gently drive it about till it is ready, a couple of minutes should be fine.

The idea is to get just the right amount of fuel flowing through so that when the engine reaches its full rev potential, it isn’t bogging from being too rich (too much fuel) or starving from being too lean (not enough fuel). The easiest way to achieve this is to have the truck wheels raised off the ground so there is no load placed on the engine and it can rev freely, this can be made easier if there is someone to help you. With the engine running and your tuning screwdriver in one hand, give it a full blooded rev and hold it, open the HSN up (anti-clockwise) until it bogs and stutters, then turn back (clockwise) until it reaches a full and clean high pitched rev yet still giving out a good puff of smoke from the exhaust, then release. You don’t want to be over-revving the engine, so only keep it fully revving for approx five seconds at a time and always make sure there is smoke coming out of the exhaust, as this is the engine burning the oil content of the fuel which keeps it lubed and cool. Still off the ground, rev it again to be sure.
Once the engine is ‘clearing out’ at high revs you can move on to the LSN.  By clearing out I mean there’s a good plume of smoke coming from the pipe for the entire acceleration range and expels any unused fuel, when it reaches its maximum RPM, it has cleared out.

The LSN adjustment is located at the end of the carburettor on the part that opens and closes (where your throttle linkage attaches). It is a highly sensitive needle to adjust so requires a bit of respect when doing it. This needle controls the amount of fuel used at idle and take off and will improve the initial punch and acceleration. Have the truck back on the road and leave to idle, then hit it with a full throttle rev, if it stalls with a puff of smoke it is set too rich and will need to be leaned in (clockwise), but a little at a time with this needle as it is always better to be a little rich than a little lean. By a little at a time I mean turning the screw no more than five to 10 minutes from a clock perspective . On the other hand, if the engine cuts with no smoke it wants richening up. The ideal result is to have the truck sat at idle for approximately ten seconds, then when the throttle is applied it will launch into its acceleration like a rocket carrying on into the high revs and a full speed run.

To check the LSN setting is good, pinch the fuel line in the region of roughly an inch away from where it meets the carburettor, your engine should idle smoothly then cut out after three or four seconds. If the idle raises it may still be a tad rich, if it cuts out before three seconds it may be slightly lean.

Conclusion

You should now be able to competently tune a nitro engine to a good level of performance. If you do still have any queries or problems, be sure to check out the online help available on the forum at www.maxbashing.com

Key Points

It’s better to be too rich than too lean
Turn needles five to 10 minutes from a clock perspective at a time
Know your stock needle positions
Turn the needles anti-clockwise to richen
Turn the needles clockwise to lean in
Always adjust the HSN before touching the LSN
Engines normally run best between 220 and 250 degrees Fahrenheit
There should a constant plume of smoke from the pipe
A clean air filter is essential to the performance of the engine.

Important

- Not all carburettors have both a HSN and a LSN, if yours only has one it will be the HSN.
- Some carburettors are also fitted with a 'third needle', which isn't a needle at all; it is an adjuster that allows for the position of the jet assembly to be altered in relation to the flow of air through the carburettor. It is highly recommended to leave this adjustment alone.
- If you have a temperature gauge… use it. Most engines will be happy at a running temperature of between 220 to 250 degrees Fahrenheit maximum. Running an engine hotter than this for any length of time will dramatically shorten the engine’s lifespan.  Having said this, some engines like the Traxxas 2.5 do run slightly hotter and can operate between 250 and 270F.

- If you don’t have a temp gun you can use the spit test.  Put simply; if a bit of your finest flem sizzles off immediately as it touches the top of the cooling head, it’s too hot so richen it up quick smart. As a rough guide, it should take 5 -7 seconds to go.

- The terms rich and lean are simplified by explaining that it is the mixture of fuel and air to the carburettor, too rich means there is too much fuel compared to the air and too lean is obviously too much air compared to fuel.
- There should always be a trail of smoke coming from the exhaust when the engine is in use, although it is sometimes hard to see that at idle speed or max RPM.

- Always ensure you have a clean yet slightly oiled air filter, this will ensure no grit or small particles of dirt can enter the carburettor.