Tuning the Traxxas TRX 3.3 Engine
Dec 24th 2009carltonGeeky & Hobbies
First off, let me say you should follow the instructions both written and on the DVD that accompanied your model. The directions given by Traxxas will both help further your understanding of this machine and avoid the worst abuses you can otherwise unknowingly impart on your engine. What I write is no substitute for their information and should never be done until AFTER your completed break-in following the Traxxas break-in procedure. I also recognize there is a certain amount of gap between that information and a working understanding of how to tune your engine in the real world.
I was forced to learn a bit more about tuning because of the cold weather this winter and I wanted to take a chance on passing along what I have learned. First, a short lecture about what it takes to tune any engine. You’ll read about fuel mixtures and how to set those, idle speeds and settings, and other various things when it comes to tuning an engine. The fact is no one aspect exists in a vacuum but they all interact and affect each other. Don’t expect to get the best performance from an engine JUST by gapping the spark plugs or just adjusting timing or just changing fuel mixtures. You have to level set everything in rounds, check and readjust until you get everything set up right. Most mechanics will get everything in the ballpark and let it ride. In modern passenger vehicles, the computer is typically relied upon more to fine tune in real time, resulting in huge leaps of power and efficiency previously unobtainable. In the TRX 3.3, you have no computers, so you have to tune it old school.
There are five items you must be concerned with. These are (1) general running condition, (2) Factory settings, (3) high speed needle, (4) low speed needle and (5) idle air gap.
General Running Condition
This references the condition of all the parts in and around the engine. The gas tank needs to be free of cracks or leaks. Check the O-ring around the lid and adjust it’s sealing properties using the alan head screw. Replace your glow plug without hesitation. Check all your fuel and pressure lines for splits or cracks. A roll-over accident can cut these lines pretty easily especially if running on asphalt or concrete. Check your exhaust system and the rubber connector between your pipe and your header. Make sure all these connections are sealed up tight and are cinched with zip ties. Clean your air filter and filter assembly. Re-oil the filter and get it back on the engine. Check the engine mounts for loose or missing screws. I’ll say here that the magnets in your EZ Start motor will attract loose engine mount screws. This is one demonstration of the fact that your success or failure will depend in part on how well you clean and inspect your model before and after running it. Check wire connections such as the block ground connection and the glow plug connector. Check mesh with the spur gear and make sure your transmission is properly seated and all of it’s screws are tight and present. Correct any issues you find in this area. I use a strip of notebook paper folded to double thickness as a way to set my spur gear mesh. If it’s tight with the notebook paper it will mesh perfectly when that paper is removed. A thicker paper doesn’t have to be folded in half. I wouldn’t use construction paper though. Basically what you want is a completely non-binding mesh such that when the teeth come into contact with each other their entire faces touch all the way across, transferring the most power possible from the output shaft of the engine to the input shaft of the transmission. Make sure the motor is squared perfectly and that the teeth mesh all the way across and not just on one side. Also check your throttle and brake linkages. Follow the directions from Traxxas to a “T” and you will not have issues. A fouled up linkage adjustment will cause your engine to act in unpredictable ways so do not ignore this part. Your internal engine components play a part as well, obviously, but I am assuming you have good compression and aren’t needing a rebuild. Once you are comfortable with the general condition of your rig, you can move on to the other steps. I highly recommend some sort of temperature monitor, either a infrared gun or the traxxas on-board temp gauge. The on-board monitor records the max, min and current temp so you can be sure you didn’t overheat during a run and just miss the peak temp by getting to your gun and checking. These engines cool very quickly from their peak temp. Bear in mind that your real goal is to make sure you aren’t overheating. There is no set temperature you should run at, but in general you don’t want to see the max temp rise above 270. I’ve seen mine hit over 300-311 when I had an issue… but when things are right, I see it around 230-240. It doesn’t start running right until it’s over about 150-160 though. The temp gauge helps ensure you don’t fry your engine and get to start over with a new one. Something you may want to check periodically is your clutch bell bearings and clutch shoes/springs. These things do wear out eventually and need to be replaced. I have heard of people replacing these things every 2 gallons of fuel run through their engine. I have replaced my bearings once and my engine has 1 gallon on it, almost. I replaced mine with sealed bearings, so they should last longer this time around. Put your engine in a large zip lock back before you take off the E-clip holding on the bell housing. That way you won’t lose it when it shoots off. If you are careful it won’t shoot off anyway, but better safe than sorry. Hey, be sure to trim all zip ties prior to running your engine. You don’t want these getting in the way of moving parts or getting snagged by a wheel or something in your operating environment.
Factory Settings
This is an optional step, but if you have any doubt about the settings of your engine being someplace close or not, it’s a great idea to reset to the default factory settings and move on from there. These are detailed in a couple different places in your included documentation. Basically, move the carb slide to it’s resting place and measure the air gap between the slide and your air horn. That gap should be 0.4-1 mm. Next, check your low speed needle. The raised parts of the screw head should be even or flush with the part of the carb slide immediately surrounding the screw. Next, GENTLY turn your high speed needle clockwise until it seats to fully closed. Do not tighten it here, just get it to touch. Next, back it out counter-clockwise four (4) full turns. If it helps you count turns you could put a dot of white fingernail polish on one side of the screw head and use it as a point of reference. These settings bring your engine back to very close to where it was coming from the factory. Check your Traxxas information and look for the chart detailing how adjustments will have to change based on elevation and ambient temperature. In cold air, you’ll want to run more rich. In warmer air you’ll tune leaner. In higher elevations, you’ll run leaner than in lower elevations. It’s all about balancing how much fuel to shoot into the carb with how much air is getting sucked in. Once you are all set, fire your engine up and get things warmed up. Make a few speedy passes to bring your engine temperature up to a good level. You can’t tune an engine that is not running or that is too cold. This is a fact: running your engine too lean will cause premature engine failure due to a lack of lubrication. This is also true: running your engine too rich might affect performance but will not damage the engine. People who contradict those two statements go against what the people at Traxxas have said. Once your factory reset is done, make sure your clean air filter is strapped on with a zip tie.
High Speed Needle
Your high speed needle controls how much fuel is mixed with the incoming air when the throttle is pulled past a certain point. I had to richen (counterclockwise/open) my HSN another full turn from the factory setting (4 turns) to get close to my ballpark in these cold temperatures outside. Remember, too rich is ok, too lean is too bad. Dad always said an engine will run without gas, but it won’t run without oil. It’s obvious you need gas, but it’s no good unless there is lubrication. Anyway, basically, you’ll be tuning the HSN based on how your engine performs when you accelerate. So accelerate and observe what happens. Does your engine bog down? Open the HSN 1/4 turn. Does your engine scream like a scalded dog? Close it 1/16 turn. Recheck. If you lean the needle (close it) to a point and do not notice an improvement in performance, back the needle back out 1/8-1/4 turn and retest. This is where that needle needs to be.
Low Speed Needle
Your low speed needle controls how much fuel is fed to your engine when the throttle is mostly closed or in the process of opening. This needle is generally tuned by doing a “pinch test”. What you’ll do is with the engine warm and at idle, pinch the fuel line close to the carb and observe what happens when you cut off the fuel flow. If the engine dies immediately without changing speed, open the LSN up 1/8 turn and recheck. If the engine runs more than 3 seconds, then speeds up and dies, close the needle 1/16 turn. You want to aim for 2-3 seconds, then a speed up and die situation here. In the cold air in Northwest GA, I had to open my LSN quite a bit to get a good pinch test. Don’t be scared if you are not sitting right there at the factory settings, but go by what your engine is doing. Here’s something I did learn, you can run a really rich HSN but if your LSN is too lean, your engine will overheat. Do the pinch test and make sure of where you are. When you get your LSN set right, your engine will take off really quick without sputtering or hesitation. Keep in mind that a cold engine will not run right anyway, so make sure you warm it up before you test. After each needle adjustment, make a couple of passes to clear the engine of excess fuel, then assess the condition of your latest adjustment.
Idle Speed
After your LSN and HSN are dialed in, you can back your idle speed down to the lowest reliable speed possible. This will keep your transmission from slapping the clutches and jerking your model while you are idling.
Now, go back through the HSN and LSN and recheck idle. The transition between LSN and HSN should be smooth as silk. In other words, you should be able to operate the throttle at any position and have a smooth response from the engine. It should punch hard and idle smooth and have a good attitude about running at lower speeds as well as higher speeds. I have seen where if I had to make significant adjustments to the HSN or LSN that it affects this transition. At the end, your engine should be extremely easy to start, quick to warm up and should never overheat. Your problems should be in learning how to properly control all that power you coaxed out of your engine.
I will reiterate once again that you should first reference the information from Traxxas. A lot of what I have said is straight from their documentation but I may have something wrong or off. When in doubt, reread their documentation, rewatch their DVD and follow their advice. After all, they designed it, so they should know best.
