Engine Tuning and Troubleshooting

This article describes common troubleshooting and tuning of glow engines (commonly referred to as Nitro Engines due to the common addition of nitro methane in glow fuel). The purpose of this article is not to discount the Owner’s Manual instructions for any engine, but to add further information that may be beneficial, particularly aimed at the newcomers to the hobby. Special thanks to Savage-Central member, PureNitro, for his contribution on performance tuning. The information has been obtained from contributions by many Savage-Central Forum members as well as other sources on the internet. This article makes use of several acronyms and abbreviations which can be found at the abbreviations article.

Contents 1 Carburetor Adjustment Basics 2 Basic Problems 2.1 Glow-Plug and Glow-Plug Drivers 2.2 Flooding 2.3 Bad Fuel 2.4 Stalling at Idle 2.5 Unexpected High Idle 2.6 Flame-outs 2.7 Temperature Guns 2.8 Compression and Pinch 2.9 Air-Leaks 2.10 Overheating 2.11 Break-In 3 How to Start a Temperamental Engine 3.1 One Way Bearings 4 Rough Tuning 5 Tuning for Performance 5.1 High Speed Needle 5.2 Low Speed Needle 6 Helpful Tuning Videos

Carburetor Adjustment Basics

The High Speed Needle (HSN) controls the amount of fuel to be mixed with air in the carburetor and depending on the engine, usually affects the air/fuel mixture from 1/3 to full throttle.

The Low Speed Needle (LSN) controls the amount of fuel to be mixed with air in the carburetor and depending on the engine, usually affects the fuel/air mixture from idle to approximately 1/3 throttle.

The Idle Stop Adjustment Screw controls the idle speed of the engine, but has no affect on the air/fuel mixture.

It should be noted that the HSN is in line with all fuel entering the engine, and therefore has some effect on the LSN adjustment. As the HSN flows much more fuel than the LSN, it is not clear how much of a restriction the HSN adds to the LSN needle restriction. Research on tuning generally recommends tuning the HSN first, although some reputable sources suggest tuning the LSN first, as establishing a stable idle makes further tuning easier. However, it is generally agreed that setting the LSN to the Factory recommendations and then tuning the HSN first is the best approach. The LSN can be tuned after that. In any case, it is usually necessary to go back and forth between the two and make minor tweaks to gain full engine potential. Once properly tuned, re-tuning the LSN is not that common except for those involved in racing.

Basic Problems

These brief descriptions of typical engine problems should be read and understood in order to achieve the basics of engine troubleshooting abilities.

Glow-Plug and Glow-Plug Drivers

The proper glow-plug (size, style and heat range) should be used, however merely using a different heat range plug should not prevent the engine from starting. The two most significant factors that prevent engines from starting is a bad glow-plug or a discharged battery in the glow-plug driver. Removing the glow-pug and inserting it in the glow-plug driver will result in a the glow-plug coil glowing bright orange. If it does not, then try another glow-plug, preferably a new one, or one otherwise known to be good. If there is still no orange glow, charge or replace the glow-plug driver battery.

Flooding

Flooding is a term indicating too much fuel is in the engine. Minor flooding results in an air/fuel mixture that is too rich to ignite and can also absorb heat from the glow-plug element so that it won’t glow. Major flooding does the same thing, but can be further identified as the engine will become difficult, if not impossible to turn over. The solution is to remove the glow-plug, turn the vehicle upside down and crank the engine using the Pull-Start or Roto-Start to force the excess fuel out the glow-plug hole.

Bad Fuel

Difficulty in starting an engine is often blamed on bad fuel, however this is usually not the reason for difficult starting. Fuel can go bad if stored improperly with the usual problem being the absorption of water from the humidity in the air. Keep the fuel jug tightly capped when not decanting fuel to your fuel bottle. Do not leave fuel in your fuel tank after the end of a days session. Discarding any fuel left over in your fuel bottle at the end of a session is recommended, but if it contains a lot of fuel (not much air) and is tightly capped, it will usually be fine for a while.

Stalling at Idle

Stalling at an idle can be caused by either improper tuning of the carburetor, an improperly adjusted Idle Stop Screw, or an air leak. If the engine stalls when applying the brakes, it means the throttle servo has control of the idle speed instead of the Idle Stop Screw. When the brakes are applied, the throttle continues to close the carburetor and the engine stalls. In this case the Idle Stop Screw needs to be turned in (CW) to prevent the carburetor from closing further when the brakes are applied. A general rough setting is for the Idle Stop Screw to maintain a 1mm opening of the throttle as viewed down the carburetor throat with the air-filter removed. To make a rough adjustment of the LSN, listen to what the engine does before it stalls. If the idle speed is rising, the LSN is set too lean and the engine basically runs out of fuel. If the engine starts to slow down or stops with little warning, the LSN is set too rich. Air leaks will be covered below.

Unexpected High Idle

If your engine idles high after throttling down from a high speed run, the HSN may be set to lean. This can be confirmed if the idle eventually settles down on its own after a few moments. An idle that begins to increase on its own indicates a lean LSN setting. See the section on Performance Tuning below for more information.

Flame-outs

A flame-out is a term used to describe an engine ceasing to run while at an rpm higher than the normal idle. This usually is because the HSN is set too lean. This will be evidenced by holding the throttle wide open and observing the vehicle. It will start to accelerate very nicely and then suddenly quit. When this happens, a first move would be to richen the HSN approximately ¼ of a turn (CCW), re-start the engine, and try again. If the vehicle takes off sluggishly, or wants to stall as soon as you operate the throttle, the HSN is probably set to rich and you need to lean it. Try 1/8 turn CW, and repeat a few times if necessary.

Temperature Guns

There are pro’s and con’s of using temperature guns. Trying to tune an engine to run at exactly the same temperature every time is a mistake. Driving style, terrain, and weather are just some of the factors that will change the operating temperature of the engine, even though a good tune is performed in each case. Tuning by ear and sight is the best approach. As long as the engine isn’t screaming like it wants to explode and a visible smoke trail is evident, the engine isn’t running too lean and shouldn’t be damaged. If the engine runs too cold, it will be very noticeable due to the sluggish performance of the engine as the carburetor is set too rich. All this aside, the Temperature Gun can be a valuable tuning aid, especially for inexperienced engine tuners. HPI states that reasonable temperatures for an operating engine, range from about 205F to 280F although there are exceptions, especially at the high end. If your engine is running near the low end of this range, it is likely, but not certain, that your engine is running to rich. If your engine is running near the high end of this scale, it is likely, but not certain, you are running too lean. Once you get an idea of where your engine normally operates, a quick temperature check can tell you if it’s running too rich or too lean. This is only a guideline for those that have their engine running, require tuning, but aren’t certain where to start. It is fairly well agreed that an engine running over 300F may lead to engine damage, so newcomers to the world of engine tuning may appreciate that check.

Compression and Pinch

Pinch is a term to indicate that the piston will start to bind up in the cylinder bore as it approaches the top of its stroke, i.e. at Top Dead Center (TDC). This is due to the cylinder being slightly tapered so that a tight seal occurs as the piston approaches TDC and combustion is about to take place. To test the pinch, remove the glow-plug and rotate the flywheel with your fingers. Resistance should be felt as the piston approaches TDC. A worn out engine will not have any Pinch left, however a good engine may also appear to have no pinch, so checking the pinch may not tell you a lot. To check compression, re-install the glow-plug and rotate the flywheel with your fingers. Again resistance should be felt as the piston approaches TDC. An engine that does not have any compression or pinch is either worn out or has a massive air leak around the cylinder head or glow-plug. A worn out engine will often stall once it warms up. As the cylinder sleeve expands, compression is lost and the engine ceases to run, although it may re-start once it cools down.

Air-Leaks

Air-leaks can cause difficult starting, inconsistent idle, and over-heating of the engine. The most common sources for air-leaks are at the carburetor base, around the carburetor pinch bolt, and sometimes the engine back-plate. Leaking front bearings and starter drive shafts and bushings can also develop air leaks. The fuel lines, pressure lines, fuel tank, fuel tank lid O-ring, and exhaust couplers and gaskets should also be checked. Air leaks usually are small and do not affect the higher rpm range of the engine. However, at lower speeds, the leaking air is more significant to the air/fuel mixture ratio. Often, the air leak is not discovered and the LSN is adjusted rich to compensate for a poor idle. In this case, the engine starts to run rich as the rpms increase, so the HSN is leaned out. The end result is an engine that overheats.

Overheating

Over-heating can be caused by too lean a mixture (improper tuning which can be compounded by air leaks), insufficient cooling air to the cylinder head (cut out the body more), driving under high load (long wide-open-throttle (WOT) runs), running through thick grass, sticking brakes, or binding of other driveline components. Improper glow plug selection, too low a percentage of nitro in fuel, and incorrect cylinder head shimming, are other contributing factors. Too high an oil content in the fuel can contribute as well, as the engine can become touchy to tune and easily leaned too much in the quest for peak power.

Break-In

Following the manufacturer’s recommended break-in procedure is always wise, although many hobbiests do not. The manufacturer’s recommendations will work. Following them may also help if warranty recourse becomes an issue. Essentially a break-in allows parts to work together as a whole. This includes the fit of the piston and cylinder sleeve as well as the seating of all bearings and bushings. Additionally, the heating and cooling of the engine helps relieve internal stresses in the metal parts making them more resistant to failure. All engines will break in without a procedure provided they have enough run time and are not allowed to rev high or work hard. A proper procedure speeds things up so that full enjoyment and longevity of the engine is assured. There are far too many break-in procedures and even more variants than can be discussed here, however the reader is encouraged to research the Heat Cycle Method using an internet search engine..

How to Start a Temperamental Engine

1. If your mixture adjustment needles, are in question, put them to factory suggested settings. This is often a bit too rich with respect to the HSN, so you may have to lean the HSN 1/4 to 1/2 a turn in (CW) from factory setting to get it to keep running.
2. Remove the glow-plug and check the glow-driver by putting the glow-plug in it and make sure the coil element glows bright orange. Re-install the glow-plug.
3. With Glow-Driver not connected, prime engine by plugging exhaust and cranking engine until fuel travels up the fuel line to the carburetor. Continue to crank for 1 more second, or two more pull-start pulls to fill carburetor and get some fuel into the engine.
4. Attach the glow-driver, and turn up your throttle trim on your transmitter until you can see the carburetor open just a tiny bit. Stop just as the bellows start to move. Advancing the throttle in this way, is perhaps the most beneficial thing you can do.
5. If it’s very cold outside, the engine should be pre-heated with a hair-drier or similar hot-air device. Never use an open flame around the vehicle due to the danger of fuel catching fire. Starting the engine in a warm place, or starting it immediately after moving it from a warm place to the outdoors is another way to deal with cold weather.
6. Attempt to start engine. If you are using a Roto-Start, don't crank and crank and crank....crank for only 3 to 5 seconds at a time. Pause for about 10 seconds between attempts. This allows the Roto-Start to cool down, but more importantly, it allows the glow-plug to burn off excess fuel if the engine is slightly flooded. If you are using a pull-start, give it a rest somewhere between 5 to 10 pulls.
7. As soon as it starts, turn down the throttle trim a bit if its revving too high. As the engine warms up, you can continue to turn it down until you get to a normal idle (as set by the Idle Adjustment Stop Screw).

• If the engine is flooded, indicated by difficulty in cranking the engine over, refer to the previous section on flooding.

• A Note on Roto-Starts

Many of the mechanical as well as the electrical components of the Roto-Start internal electric motor are held together with solder. Allowing the Roto-Start to overheat can severely damage it. HPI recommends the Roto-Start not be operated for more than 5 seconds at a time.

One Way Bearings

The one-way-bearing used in engine back-plate housings that use pull-starters or Roto-Starters, can slip and not allow the engine to crank over. To cure this, remove the back-plate housing and then the one-way-bearing. Clean it thoroughly with alcohol. Remove built up varnish from the starting shaft that holds the one way bearing, by polishing it with fine emery paper, 800 grit or higher. Clean the shaft with alcohol. Re-install the one-way-bearing and starter housing.

Rough Tuning

Rough tuning is required in order to achieve reasonable response from the engine, and is used once the engine is near the end of its break-in period, but before you proceed to performance tuning.

1. Keep the LSN at the factory recommended setting, which is usually flush with its housing on the carburetor.
2. Warm up the engine and then do some high speed WOT runs. Lean the HSN (CW) a small amount at a time (1/8 of a turn and then down to 1/12 of a turn as the engine performance increases) until decent performance is observed. The engine shouldn’t be screaming and a good smoke trail should be visible.
3. Next do the Pinch Test to establish a rough idle mixture setting. Pinch the fuel line as close to the carburetor as you can and listen to the engine. It should start to increase in rpms after few seconds. You can let go of the fuel line when this happens, or the engine will stall. If the engine dies immediately, the LSN is set too lean. Richen the LSN about ¼ of a turn and repeat the test. If the engine takes a long time to stall (more than 10 seconds, the LSN may be too rich. Lean the LSN by turning it CW ¼ of a turn. Repeat the test. Typically, the LSN should finalize between the factory setting of “flush” and no more than 1 turn in (CW).
4. Next, give the engine a good WOT run and bring it in and let it idle for 10 seconds. Now punch the throttle. The vehicle should accelerate quickly. If it doesn’t, richen or lean the LSN in small increments looking for improvement. Obtaining decent acceleration should not require the LSN to vary from the pinch test setting by more than ¼ of a turn.
5. Adjust the Idle Adjustment Stop Screw for an idle that is high enough to prevent stalling, and low enough it doesn’t start to engage the clutch and encourage the vehicle to move forward on its own. Typically this calls for a 1mm opening of the throttle as viewed down the carburetor throat with the air-filter removed.

Tuning for Performance

This technique is for a fully broken in engine that is ready for a full performance tune. This tuning procedure will help you set your carburetor mixture needles for optimum performance. This is adapted from an original post by Savage-Central member Purenitro, special thanks to him for the guide.

Start the engine and ensure it is fully warmed up. The engine should be roughly tuned as described in the section above.

High Speed Needle

1. Perform a few WOT runs and listen to what the engine does when you let off the throttle. As long as the rpm drops right to idle, the HSN is either good or on the rich side. At this point you can lean the HSN in 1/12 increments (CW) until the idle seems to stay high for a short period after the WOT pass, when it starts to idle high after a pass it is just on the lean side of a perfect tune. Richen the HSN (CCW) 1/12 turn at a time after a WOT pass until the engine drops right to idle after a pass. Adjustment of the HSN is now complete.
2. If your HSN is too lean after a WOT run, when you let off the throttle the engine is still lean causing the idle to hang and idle high until the LSN has a chance to take over and meter the right amount of fuel to bring the idle down to normal running speed (given the LSN is set correctly).

Low Speed Needle

1. If after a WOT the opposite happens when you let off the throttle and it drops to a good idle right away and then starts to idle back up too high, this is a sign that the LSN is too lean. If it comes off WOT with a good tune and will drop rpm nicely then the HSN is metering the fuel properly but once it hits idle the LSN being too lean will quickly take over causing the rpm to go back up.
2. It has been stated not to tune for specific temperatures, however a temperature gun is very handy for this part. After some WOT runs with the engine fully warmed up, bring it in and let it sit for about 10 seconds. At this point take your temp gauge and hold it as steady as possible over the cylinder head, pointed at the glow-plug. The temperature should drop a degree every 4-5 seconds. It is desirable for the temperature to drop very slightly at idle because when you’re off throttle the engine should be cooling. If not, the temperature will keep increasing with on-off throttle running and overheating may occur.

Helpful Tuning Videos

Nitro RC Engine Tuning 1 (using a Savage 25)

Nitro RC Engine Tuning 2 (using a Savage 25)

K5.9 Engine Break-in and tuning