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Nitro Tuning Tips and Tuning Flow Chart

Discussion in 'Nitro RC Talk' started by Rolex, Oct 27, 2004.

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  1. Rolex

    Rolex I've fallen but I can't remember Staff Member Veteran

    Hopefully, the following posts, and others that will be added, will help you to understand your nitro engine and the proper procedures for break in and accurate tuning. There are several things to look for and a few tips that will help you isolate your problem. If you need any information that's not listed here, post your question in the Nitro RC Talk forum.

    Quick Links to Topics in this Tread:

     
    • Informative Informative x 1
  2. Rolex

    Rolex I've fallen but I can't remember Staff Member Veteran

    RC Nitro Tuning Flowchart

    [​IMG]

    View or print out this FULL SIZED flowchart for future reference. Click for printable PDF file


    What you need to know about carbs:

    [​IMG]

    Idle speed adjustment:

    This is an HPI Savage 4.6 engine, but that is the common location for the idle speed adjustment screw.

    [​IMG]


    Here are some symptoms of an engine that is running too lean:

    1. Dies or flames out at full throttle.
    2. The glow plug wire or element turns white.
    3. A drop of water or spit on the engine heat immediately sizzles. Too hot.
    4. Dies while just idling.
    5. Dies while accelerating from idle. Note this can also be caused by a rich engine.


    Here are some symptoms of an engine that is running too rich:


    1. A lot of blue smoke from the exhaust pipe.
    2. A lot of unburned fuel exiting the exhaust pipe.
    3. Idles properly, but bogs down an dies when the throttle is full applied.
    4. Never reaches full top speed.
    5. Engine temperature is too cold (e.g. below 200 F degrees)


    Setting your carb:

    These directions are ONLY for an engine that has been fully broken in:

    Start with the needles at factory recommended settings and make sure all your batteries are fully charged; The starter battery, if you use one, the glow ignitor and the on-board receiver battery. Make absolutely sure you have a good glow plug with a bright glow, or install a new one.
    Be very precise about closing both the LSN (Low Speed Needle) and the HSN (High Speed Needle) and carefully counting the turns to factory recommended openings.
    Turn the Transmitter on, then the Receiver. (Tx and Rx)
    Turn the throttle trim knob on the Tx all the way down and hold the trigger to full brake when adjusting the idle screw. That way, there's no way the carb can close completely and stall the car when you hit the brakes.
    Remove the air filter and look into the throat of the carburetor. You should have about a 1 to 2mm opening that looks like this:

    [​IMG]

    Start the engine. If it won't start without a little throttle trim applied, that's okay for now, since it's probably a little on the rich side. Let it idle, giving it little blips on the trigger to clear out the oil till it warms up. When it's close to running temp, check the takeoff response by first using normal throttle, stopping it, and using quicker throttle as the engine starts to clear out. If you get a sluggish start, you'll need to start leaning the LSN by no more than 1/16 turn at a time, and test it several times before you lean it any more.
    It should start to take off with a more instant response as you get the LSN to the proper setting. You will most likely have to compensate for the leaner LSN setting by lowering your idle adjustment.
    When your idle is good, and you're getting good response off the line, your LSN is set. Just make sure you monitor your temps closely, since a too lean LSN will will raise your temp to dangerous levels.
    Start running the car at higher speeds, and adjust you HSN for wide open throttle. (WOT) You will need to make slight adjustments to the HSN regularly, for changes in the ambient temperature and humidity.



    What you need to know about glow plugs:


    Glow plugs are very important to the performance of your engine. You can also read your glow plugs to determine if you are running too rich or too lean.

    A brand new glow plug has a wire or filament that is shiny. If after running the wire is still shiny and the bottom of the plug is wet, then you are running too rich. You are probably getting only 85% of the maximum engine power.

    If the wire turns starts to turn gray, and the bottom of the filament is slightly wet, then you are almost near the peak engine power.

    Once you have a gray wire, and the bottom of the plug is dry, you are at 100% maximum power.

    Once the glow plug wire starts to distort, you have exceeded the maximum power and are running too lean. Adjust to a slightly rich setting.

    Hot plugs are used for hot summer days, while cold plugs are designed for cold & winter days.


    How Does a Glow Plug Work?
    [​IMG] Engines need three things to work: fuel, oxygen, and an ignition point. The glow plug provides that ignition by heating the element (which is the small coil of wire inside the plug). This is usually done with a 1.5v battery contained in a glow ignitor. Some of the more recent vehicles have on-board batteries that ignite the glow plugs as part of an electric starting system. Either way, once the element is heated and the engine is started, no more power is needed to keep the element hot and provide constant ignition to the engine.
    How the element stays hot after the battery is removed all hinges on the fuel we use in glow engines and on the material the element is made of. The fuel contains methanol, which is a type of alcohol. The element is made of several different metals, which when alloyed together, make it strong enough to handle the heat and vibration. But one of the metals, platinum, is special. When the platinum in the element comes in contact with the methanol in the fuel, there is a catalytic reaction between the two. This heats the platinum while causing the methanol to ignite. This is the foundation upon which the entire nitro hobby is built.
    What Determines the Ignition Point if the Element is Always Hot?
    The catalytic reaction depends on two things to work: heat and pressure. The hotter the element is, the easier it will ignite. Similarly, the higher the pressure inside the combustion chamber, the easier things will ignite.
    Glow Plug Temperature - Hot, Medium, or Cold?
    Glow plug temperature is controlled by using different heat range plugs. There are many different heat ranges, but most fall in one of three categories: hot, medium, or cold. If you're not sure which kind to use, consult with the engine manufacturer to determine what they recommend for their engine. Using a hotter than normal glow plug will advance the ignition point, and using a colder than normal plug will retard the ignition point.
    Combustion Chamber Pressure
    There isn't much you can do to alter the pressure within the combustion chamber, as this is usually set by the manufacturer. You can add head shims to increase or decrease the size of the chamber, but this is something only experienced nitro users should attempt, as you can easily brick your engine if you make a mistake.
    Glow Plugs and Nitro Fuel - What You Need to Know to Pick the Right Plug

    There are very few glow plugs that are considered universal plugs (OS' #8 plug is an example). For the most part, the type of fuel you are running will impact the kind of glow plug you need to use. Another component in the nitro fuel is nitro methane.
    The Nitro Percentage of your fuel determines the ignition point as well; in other words, the more nitro you run, the more you advance the ignition point. Ideally, the ignition point will be when the engine is at top dead center (TDC). This will force the piston down and back up again for another stroke. But when you run higher nitro content and don't switch to a colder plug, you will advance the ignition point and result in less-than-optimum performance, since the piston is still on its compression stroke (the upward stroke) when the air/fuel mixture is ignited.
    In general,
    The higher the the nitro percentage, the colder the plug should be. Conversely, the lower the nitro percentage, the hotter the plug should be.





    [​IMG]
    Glow plug and compression washer
     
  3. Rolex

    Rolex I've fallen but I can't remember Staff Member Veteran

    Giving the engine a good once-over


    • Screws - Make sure that all the screws are tight and in place. Loose or missing screws are a very common cause of engine problems yet are often overlooked.
    • Flywheel - Spin the flywheel to see whether it’s firmly attached to the crankshaft. A loose flywheel, though difficult to detect, can make an engine nearly impossible to start.
    • Idle-speed screws and mixture needles - I know this sounds crazy, but be sure to check whether your idle-speed screw and mixture needles on the carburetor are still there. Typically, the mixture screws loosen over time, and in extreme cases, they can even vibrate completely out of the engine.


    Engine wear

    More than half of the engines I’ve been asked to help start are so badly worn that I’m amazed they started in their owners’ previous 100 attempts. Engine wear makes starting increasingly difficult, and eventually, the engine won’t start at all. The critical components here are the piston and sleeve; more specifically, the fit between the two. An excessively worn piston and sleeve won’t create enough pressure in the crankcase to force fuel through the transfer ports and into the cylinder. After combustion, much of the cylinder pressure above the piston bleeds past the piston, too. More simply, the engine won’t run well, if at all, if the piston and sleeve are excessively worn. Check for wear by rotating the flywheel counterclockwise. With the glow plug in place, if you can easily rotate the flywheel with one finger, it’s time to start shopping replacement parts or a new engine.


    Defective glow igniter or plug

    Above: The gauge on top of this glow igniter tells you whether the glow plug is defective or whether the glow igniter’s battery is dead.
    You should check both the glow igniter and glow plug for possible problems. Here’s how:


    • Glow igniter - Before you start, make sure that the glow igniter is fully charged and working properly. Partially install a spare glow plug to check the glow igniter’s condition and the state of its charge. If you fully install the glow plug in the igniter, you might find it difficult to remove, especially when it starts to get very hot. Simply hold the glow plug by its housing (keep your fingers away from the element), and lightly press it into the igniter’s contacts. The plug should glow bright white/orange. Also check whether the end of the glow igniter is clean and allows a good contact between it and the glow plug.
    • Glow plug - If the glow igniter checks out, move on to the glow plug you’ve installed in the engine. Clean out the area around the glow plug (very important) and remove the plug from the engine. Insert the plug into the glow igniter (which you already know works properly), and make sure that the plug’s coil lights up properly. If it doesn’t, you need a new plug. If the glow plug does light but is a deeper orange color than it should be, it might be fouled or have some other flaw that will prevent it from working properly. A tip-off is that the engine dies as soon as you remove the glow igniter from the glow plug; don’t confuse this with problems caused by having an excessively rich fuel mixture.

    Some glow igniters are designed to make it easier to diagnose a plug problem. They have a built-in gauge that can help you with the two most common plug problems: the glow igniter is not charged, or the glow plug has failed completely. If you know you have a full charge on the glow igniter and the gauge is still in the red, you know the plug is the problem.

    If you use an on-board electric starter, you might have problems heating the glow plug completely. When the engine is new or flooded, the starter motor works harder than normal to get the engine cranking. This heavy power drain results in a measurable drop in the power remaining to ignite the glow plug. This power drain reduces the glow plug’s effectiveness and makes it harder to start the engine. Use a separate glow igniter as an independent power source to help get the engine started.


    Stale fuel

    Take care of your fuel; don’t let it sit around too long. Condensation and evaporation are leading causes of problems with old fuel. If you leave the cap off the fuel bottle for too long, moisture will literally be sucked in, and the methanol will quickly evaporate. Engines don’t run too well on a water/nitro blend! If you aren’t sure how long your fuel has been sitting around, it has probably been too long. Buy new fuel


    Fuel-system problems

    The fuel system is important when it comes to getting your engine fired quickly. A typical fuel system includes: a tank, a fuel line that connects the tank to the carburetor, a pressure line that feeds exhaust pressure into the tank for consistent fuel delivery and, possibly, a fuel filter.

    • Fuel tank. The fuel tank should be clean and free of any obvious defects. The most common fuel tank problems are a faulty seal between the filler cap and the tank, a worn O-ring seal on the primer pump (if it’s so equipped), plugged or restricted fuel flow through the pick-up, or a crack in the tank. Any of these problems can mean an air or fuel leak that will compromise the engine’s ability to start and run properly.

    The key is that the fuel system not only be free of fuel leaks but also be airtight. Air leaks in the fuel system cause uneven pressure in the fuel tank and excessive air bubbles in the fuel lines. Both of these can make starting and tuning the engine more difficult. It isn’t unusual to have a few air bubbles in the fuel line, but you might have problems if you have excessive, persistent bubbles.


    • Fuel lines and pressure lines - These are often overlooked during troubleshooting because damage isn’t always obvious. The most common problems are small cuts in the lines (sharp or abrasive edges or rotating drive components are often the culprits here). Though hardly visible, these cuts in the lines can adversely affect engine starting and tuning, just as a faulty fuel tank can.
    • Fuel filter - Finally, do you have a fuel filter? A filter can protect your engine by preventing debris from getting inside it from the fuel tank. But if it’s poorly maintained or badly designed, a fuel filter can cause as many problems as it solves. Debris can gradually clog a fuel filter to a point that the fuel won’t flow sufficiently even to start the engine. A clean but poorly designed filter can also restrict fuel flow and cause many of the same problems as a dirty, well-designed one. Check your fuel system for sufficient flow with the filter installed to ensure that it’s not the cause of your starting problems.

    To test the entire fuel system, remove the fuel line from the carburetor and insert it into your fuel bottle. Next, remove the pressure line from the muffler or tuned pipe and gently blow into it. Fuel (or air, if the tank is empty) should flow freely from the fuel line to the fuel bottle. If it doesn’t, follow a process of elimination:


    • Carburetor. OK; the fuel flows freely through the fuel system, but your engine still won’t start. Look at the carburetor, and eliminate the obvious problems: a loose carburetor clamp holding the carb to the engine, screws missing, etc. If everything checks out, move on to the next few steps.
    • Idle speed. This is the most common cause of engine problems. You might use the radio trim feature to set the engine’s idle speed, and then you can’t figure out why the engine dies when you apply the brakes. Adjust the idle-speed screw to prevent the carburetor from closing more than 1mm under any circumstances.
    • Check whether your engine has enough compression by turning the flywheel with one finger. There should be some resistance when the piston reaches top dead center.
    • Mixture-needle settings. These are the second most frequent cause of engine headaches. If the mixture needles are too far out of adjustment, the engine will either flood quickly or will never get enough fuel to start in the first place. Either way, you need to adjust the mixture needles correctly for the engine to start. Engines vary, but generally, you follow the same steps: first, close the needles by gently turning them clockwise until you feel a slight amount of resistance; this is the closed position. Then turn the needles counterclockwise. Open the main needle 2 1/2 to 3 turns and the low-speed mixture about 1 to 1 1/2 turns. Once the engine starts, you can fine-tune the mixture settings for performance.

    Starter Problems

    You need to use either a starter box or a hand-held starter to get your engine started properly. RC engines have been known to start and run in the wrong direction. If your starter runs in the wrong direction, reverse the positive and negative leads that connect its motor to the battery, or if you use a hand-held starter, rotate the starter 180 degrees.

    Don’t think that you’re immune from starting your engine backwards if you have a pull-starter, either. You may have installed the one-way bearing in the pull-starter or electric starter backwards during assembly or maintenance, and that will make the engine crank backwards! Before you eliminate this as a possibility, give the pull-start a quick pull to see whether it is cranking the engine in the right direction.

    These tips describe some of the barriers to starting your engine. This might seem to be a long checklist, but when your engine is giving you trouble, these checks actually take very little time to do. The problem areas will become even easier to recognize as you gain experience.
     
  4. Rolex

    Rolex I've fallen but I can't remember Staff Member Veteran

    Heat cycling method of breaking in a new engine

    The following information is from a world class racer:

    If you are still idling at least a tank through during break-in, you're using the old-school accepted method (still works well for some!) but it's not the method that the top engine guys recommend anymore.

    From the very 1st time you start your engine, plop the car on the ground & begin running it in a parking lot in 2-3 minute intervals, tuned only *slightly rich* getting the temps up in the 200F range on a normal day. Every 2-3 minutes, shut the engine down & let it cool completely with the piston at BDC, and then fire it back up; continue this cycle until you've run 15 min or so, and then bump up to 3-4 minute intervals. Vary the RPM and don't be afraid to get the temps in the 200's. What you want is heat cycling of the components without the incredible stress that comes with breaking an engine in when it's overly rich & cold. After cycling the engine in this manner for about 20-25 total minutes, it'll be ready for the track and race tuning. I realize this method goes against the old-school "idle on the box" routine, but you'll be amazed once you've completed this break-in routine, your engine will still have amazing pinch w/out sticking at the top AND your compression will last far longer than it will with the "old school" method.

    You say you run the engine at "factory settings" for the first FIVE tanks? That alone causes lots of stress, as the factory settings are very rich on every engine I've ever owned or tuned. The piston & sleeve haven't expanded to operating temps, and every time the engine turns over, the piston slams into the pinch zone at TDC. The not-so-surprising result can be a cracked con-rod at the crank pin--that's where the majority of the stresses occur as the engine turns over. I've only heard of about 6-8 OS engines breaking con-rods, and they're ALWAYS during the first gallon...and almost every time it's because the guys have performed the break-in using the "old school" method. Doesn't seem like a mystery as to why it's happening. Drawing out the break-in routine really stresses the engine & actually wears away compression along the way. This method I've outlined will feel weird at every step, but after you try it once, you'll notice a big difference in your engine's performance & lifespan.


    BDC means Bottom Dead Center. This is when you roll the crank until the piston is at the very bottom of the crank case. Before you install your new engine look at the piston through the exhaust and when the piston is at the bottom put a mark on the flywheel. This will make it easy to set the piston to BDC.

    [​IMG]


    [​IMG]
     
  5. olds97_lss

    olds97_lss RCNT VIP Supporter

    Sealing a Nitro Engine for air leaks.

    I borrowed this from savage central. It is a well done tutorial of sealing up an engine with RTV. I'll go through the sealing process and make one of my own to replace this:

    -------

    Today I will take you step-by-step through the process of sealing your engine. Whether your engine is brand new, or you have been running it for a year or more, if you have never sealed it, now is the time. By sealing your engine from the beginning, you are eliminating most of the tuning problems that plague nitro engines. If you are having trouble keeping your engine tuned properly, chances are you have an air leak somewhere on your engine. By following this easy to follow guide, you will be well on your way to having a trouble free engine. The engine I am sealing up today is a new STS .28. Now your engine may be slightly different, but the process is essentially the same for all nitro engines.

    First off you will need a clean work area. I like to use this Hobbico cutting mat. Give the area a good wipe down to eliminate the chances of foreign debris entering your engine. Here are some things you will need: A tuning screwdriver, a couple of good hex wrenches, blue loctite, silicone sealer (I like Permatex Utra Copper), your engine, piece of scrap thick paper (I use package tops) and a couple of things that didn't make it in the pic is some Team Associated's Green Slime grease, and some good nitro cleaner.
    [​IMG]

    Another good thing to have with you as you dissassemble your engine is the manual that came with your engine to refer to so there are no suprises along the way.
    [​IMG]

    Now go ahead and take off the pullstart, backplate and head.
    [​IMG]

    I like to use the lower part of a fuel container to put metal parts into for cleaning. You want to clean the areas you will be sealing so that the silicone can get a good bond to the metal surface.
    [​IMG]

    I also like to go ahead and pull the sleeve out for a close inspection. Now be careful doing this because you don't want to damage this important part of your engine. Check it carefully for any burrs, or loose shavings. Do the same for the rest of the engine. look inside the crankcase for any burrs and/ or shavings and remove carefully. My other STS .28 engine had a piece of metal shaving in the crankcase.
    [​IMG]

    [​IMG]

    Now take a clean cloth with some nitro cleaner on it and carefully clean the back of the crankcase (where the backplate bolts on). This will ensure a perfect seal for the backplate.
    [​IMG]

    Most engines have an o-ring on the backplate. There is no need to mess with this- let it do it's job. We are just going to add some extra insurance by sealing the backplate. Squeeze a small amount of silicone sealer onto the thick paper. Dab a litle on your finger and put a very thin coat on the mounting surface of the crankcase.
    [​IMG]

    [​IMG]

    Now place the backplate back on the engine being careful to align the crank pin on the crankshaft. It is important in this step to use loctite on the screws that attach the backplate. I like to use blue loctite, but HPI reccommends red. Red loctite is HIGH STRENGTH and makes it very difficult to remove if used on small screws like the ones used on our small nitro engines. Also, loctite the screws that hold your pullstart on as well. remember you don't need to crank down the pullstart bolts, they just need to be snug.
    [​IMG]

    Here the backplate is loctited into place. Rememer to use the X- pattern when tightening the screws.
    [​IMG]

    Next we will seal up the carburettor and pinch bolt. Start by applying a thin layer of silicone around the inside edges where the pinch bolt goes. Do this on both sides of the engine.
    [​IMG]

    Next add a small amount of silicone to the upper neck of the carb. This engine has an o-ring down in the block to seal the carb opening. We are not going to mess with that- let it do it's job. We are just going to add more insurance by sealing the upper neck of the carb. Slide each half of the pinch bolt together from both sides and get the bolt started.
    [​IMG]

    Now at this point, you want to go ahead install the engine mount and set the engine on the truck so you can set the angle of the carb and make sure there is no interference with the fuel tank when the carb is fully open.
    [​IMG]

    Once you are happy the angle of the carb, tighten the pinch bolt and seal up both sides of the pinch bolt by completely covering them. Do this on both sides.
    [​IMG]

    Now we are going to seal the carb needles. Before you begin, hold the carb closed with one hand and tighten the high speed needle until it stops carefully counting how many turns until it is fully closed. Write the number down and do the same for the low speed needle. These are your base settings from the factory. This is a good thing to do any time you get a new engine because alot of engine manuals don't mention the factory carb settings.settings.
    [​IMG]

    Ok, go ahead and un-screw both needles and the idle speed screw. Look into the needle housing for anything unusual like o-ring shavings or any foreign debris. The Wasp .26 engine was notorious for having o-ring shavings or "worms" as they were called. They will give you headaches when you are trying to tune your engine.
    [​IMG]

    Now grab your tube of Associated's Green Slime and put a small bead on the o-rings of the needles and re-install them by screwing them all the way in and backing them out to the settings they were at before you touched them. Be careful to to tighten them too much during this process, you can ruin them by cranking down to hard.
    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]


    Now we are going to re-seal the fuel inlet fitting. Most carbs have a gasket on both sides of the fitting. If it does, I like to take the gaskets off and apply a thin layer of silicone on both sides of them and re-install. Now this particular engine does not utilize gaskets, it just uses a tapered seat, so I will apply a small amount of sealer to the mating edges of the fitting and housing being careful not to use too much because this fitting can be easily clogged by too much silicone and cause you all kinds of trouble. Be very careful with this brass fitting when tightening as it is very soft and can be easily broken. You don't have to wrench down hard, just snug. Be sure to point the fitting rearward for the Savage.
    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]


    Well that's pretty much it. Just let the silicone cure for 24 hours before firing up the engine. By doing this you will save yourself from alot of headaches concering your engine.

    Source: sfgascott from SavageCentral
     
  6. Rolex

    Rolex I've fallen but I can't remember Staff Member Veteran

    The only info I would change in the above post is how to seal the back plate. I recommend NOT tightening down the screws for 24 hours.
    Place the back plate against the siliconed block and start the screws to keep it aligned.
    24 hours later you tighten them down with locktite. That way you don't 'squish' out the fresh silicone, but you will have actually created a thin gasket that will compress and give you an air tight seal.
     
    Last edited: Jun 14, 2009
  7. Rolex

    Rolex I've fallen but I can't remember Staff Member Veteran

    CARBURETOR 101

    [​IMG]




    [​IMG]
    There are only three moving parts in a 2-stroke engine. Total parts count is under a dozen!

    How does the carburetor work and how do you adjust it?


    [​IMG]
    Carburetor Theory
    The carburetor has one main function, to regulate engine speed. It accomplishes this by metering the amount of air and fuel as required, to sustain combustion per the input of the throttle servo. Thus for a low-speed idle you would have a small amount of air and fuel entering the engine. This would in effect lower the chemical energy entering the combustion chamber and thus lessen engine power and subsequently lower the RPM. As we open the throttle the carb will allow more air and fuel into the combustion chamber, thus increasing engine power and RPM's (revolutions per minute). Now that we know what the carb. has to do lets explore the underlining fluid mechanic properties that allow the carb to function effectively at different throttle settings.

    The Venturi-Effect
    What allows the carb to pull fuel from the fuel tank is the venturi-effect. This states that in a converging funnel the entering fluid velocity increases as it passes through a reduction in the funnels throat diameter. This increase in fluid velocity decreases the localized pressure at the venturi throat to below atmospheric pressure. This low pressure region is precisely where fuel enters the carburetor throat. This is what allows the engine to "suck" fuel from the gas tank. The truth is that the venturi-effect is all that is needed for the engine to get fuel. Pressurizing the fuel tank is really only done to decrease the effects of fuel level on the mixture setting of the carburetor.

    Fuel Metering Devices
    The venturi-effect draws fuel from the tank but does little to regulate it's flow. It's true that as the engine accelerates the amount of air that moves through the engine increases. The increase in air velocity also increases fuel flow into the induction port, this helps the engine self regulate the fuel up to a certain point.

    This is not the only means for the carburetor to meter air and fuel. Engines need a metering device to help regulate the amount of fuel that enters the carburetor. This is accomplished with an adjustable orifice, typically we call them needles or jets. Most engines have a second adjustable needle that helps regulate fuel at low throttle settings. By adjusting these two needles we can control the transition from low to high speed operation of the engine.

    [​IMG]
    How do we adjust a carburetor?
    The carburetor is typically adjusted with a long flat-head screw-driver. Carb adjustments are then done by rotating the needled in, our out of the needle seat. The idle speed is adjusted by a screw at the base of the carburetor. This allows the throttle barrel to only close to a preset position.

    [​IMG]

    The carb has three main adjustments that allow you to set the following:
    1. Set the idle speed.
    2. Set the mixture at idle (Adjustable on 2-needle carbs only).
    3. Set the high speed needle mixture and control engine temp
    How to make carburetion adjustments:

    Idle Speed:
    The throttle stop screw or idle-speed screw (same thing) determines how far the carb barrel will be able to close when the servo is in the neutral position. Typically you set the servo/throttle linkage so that the carb will go from fully open when the trigger is fully pressed to fully closed when the trigger is in neutral. Then you would adjust the idle-stop/speed screw so that there is a 1-2 mm gap when the servo is in the neutral position. You might need to readjust the spring collars on the throttle linkage to force the throttle arm against the idle speed screw.

    Tip #1: If you completely mess up the carb setting and you want to go back to the factory recommended needle setting then you must have the carb fully (Yes I mean fully closed) before you can set the low-speed needle to whatever turns the engine manufacturer suggests. Before you close the carb fully back the low-speed needle a bit to make sure you wont put un-needed stress on the needle seat.

    Tip #2: There should be no speed change whatsoever when the car is in idle and when you hit the brakes. If the engine's RPM drop either your linkage isn't set right or the idle-speed screw is set too loose. Tighten clockwise until the carb barrel doesn't move when you go from neutral to full brakes.
    Tip#3: Some RTR kits have servo horns that are too small. There is not enough servo throw to open the carb barrel, if you use servo trim to be able to open the carb fully, then when you go to neutral the carb doesn't close enough. To compensate for this the novice engine tuner opens up the low speed needle to drop the engine RPM so the car will stay still when at idle... The drawbacks of correcting the linkage problem with the mixture control is that now the low-speed is too rich and the car won't idle for more than a couple of seconds before the engine sputters and dies.

    To fix this problem you need to get an after market servo horn that is larger yet still fits your particular servo brand. Now you can go from fully open to fully closed, without using trim. Now you wont have to compromise the carb settings because of lack of servo throw.

    Low-Speed Needle:
    At this point you would start the engine warm it up and commence tuning. Adjust the low-speed needle clock-wise until the engine doesn't sputter when at idle. You want a fast idle, if the car wants to move forward a lot, then turn the idle-speed screw counter clock wise to lower RPM until the engine just barely want to engage the clutch. It may take a little time to get the settings right.
    Remember you want the fastest idle you can get away with. It will make the engine more stall proof. Some engine will overheat if the idle isn't rich enough, you need to experiment to determine what's the right setting for your particular engine. When every thing is set right the engine will be able to idle through an entire tank without missing a beat.

    High-Speed Needle:
    The high speed needle will control fuel flow into the carb from 1/2 to full throttle. Typically the high speed needle is set to allow the engine to reach it's peak power point, then you open the needle slightly and go racing. On very hot and humid days you will probably have to make a compromise in the tuning department. For most this will mean you will richen up the high-speed needle to lower engine temperatures to acceptable levels. Everyone has their own interpretation of what an acceptable engine temperature is, for me anything under 260 is acceptable. Going higher will typically mean shorter engine life-span and less reliability.
     
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