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Old 03-17-2010
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Carburator Tuning By Ron Paris!

Carburetor Needle balance:

It has come to our attention some racers are making the mistake of setting the idle speed opening to wide [high idle] and setting the bottom end too rich!

This will give a false normal idle speed even though the speed is set to high because it “loads up” the engine with excess fuel causing the idle to be lower than set! The end result is a very unstable idling engine that surges and may cut out as full throttle is applied because the over rich bottom end can disguise a too lean TOP end setting!!!

Let’s address this a little more in depth! Idle speed opening set too wide:

It is possible too set the idle screw adjustment in to far but yet the idle speed is not high! Even though the air regulation [carb barrel or slide] may be set to a position that would normally equal a vary fast idle, the idle is low because the low speed mixture adjustment is set so rich that the engine loads up with excess fuel and the engine goes into what’s commonly called a four cycle idle. One tell tail sign of this is if after revving up the completely warmed up engine it tends to idle fast for a few seconds then drops to lower idle speed. [Here we go again with the written sound effects :-)]

Something like da..da..da..da..da..da..da..da then it drops to da….da….da….da [if it were sheet music it would be like dropping from 8 beats per measure to 4 beats per measure] If you start leaning the bottom end a little at a time [then repeat the reving up and idle -*test*-('")] and it takes longer before the idle drops your going in the right direction! Eventually as you keep leaning the bottom the idle will stay to high, now it is time to lower the idle to were it belongs by re adjusting the idle screw! CAUTION!!!! Now that you have the idle set correctly the top end may be too lean!!! Keep in mind the fuel does not directly enter the cylinder area like a 4 stroke engine, it enters the crankcase area first then is transferred or pumped up to the cylinder area by the piston movement. Simply put, the crank case volume can hold much more capacity than the cylinder so it takes some time to burn off the residual fuel.

In other words if the bottom end is to rich the engine will be supplied by this residual fuel briefly and depending on the demand you may be actually be experiencing a lean condition on the top end that can range from:

1. Seems to run well but engine life is short

2. Seems to run well but car continues to get hotter the longer you run to the point of overheat!

3. Seems to run ok on the bottom but sputters starves or strains to gain rpm

4. Seems to run ok on the bottom but when I give full throttle it cuts out or stalls [An overly rich top end can act the same as 3 & 4 but excessive smoke and oil are usually present with a distinct blubbering sound] There is no reason for an engine to continually get hotter unless the tune is wrong [classic #2. symptom] or there is a mechanical problem causing more load or drag on the engine as the run continues.

Exception Note: if the weather or track conditions change DRAMATICALLY.

{Examples** A light drizzle starts and the off road track goes from a very dry loose to high traction condition, or during a race a rapid weather front like a ten degree change! It is very important to fully warm up your engine, clutch and chassis before making finale adjustments.

The chassis in most applications also works like a heat sink to the engine so it is important to fully saturate the chassis!!! I like to start the engine at least 3 or 4 minutes before our qualifier to get some heat in the engine. [Operate the throttle by hand until you have radio frequency clearance] It still takes at least 2 to 3 minutes of hard running on the track to fully saturate the chassis! There is a series of restrictions to control fuel flow at different throttle/air flow positions called needles These are the five basic parts of the carb to concern yourself with:

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

2. The idle/air speed screw [sets the absolute minimum air the barrel/slide can control to maintain idle speed] It simply is an adjustment screw that comes in contact with the side at the nearly closed/idle position.

3. The high speed needle [regulates maximum fuel flow allowed to enter engine at any throttle position] It simply is a tapered needle that screws into the fuel flow orifice [an adjustable restriction] This maximum fuel flow ideally is adjusted to the correct mixture ratio for the surrounding conditions at WIDE OPEN THROTTLE or WOT There are two more devices’ the low speed or minimum spray bar and the mid range needle that restrict or control the fuel further at less than full throttle.

4. The low speed needle [regulates fuel to engine at idle] The low speed adjustment simply restricts the flow at idle speed. If you look down the bore of the carb you will see a long tapered needle [except for Picco torque carbs] When the carb barrel/slide is closed the larger part or diameter portion of the long tapered needle is inserted into the spray bar, this is what’s adjusted when you turn the low speed/minimum adjustment. It literally moves either the tapered needle OR spray bar farther in or out changing the restriction independent of the barrel/side position. This leans [more restriction] or richens [less restriction] the flow from the spray bar at idle. NOTE: some carbs the spray bar is moved and others the needle assembly is moved, both have the same effect.

5. The mid range needle [regulates fuel to engine after idle and before full fuel position. Notice as you open and close the carb the tapered needle [mentioned above in item 4.] enters into a small tube this is called the spray bar/jet. 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 ½ and ¾ 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. CAUTION: be very careful with these type carbs!!! You can identify them easily because there are four adjusting screws! {Continued on page 2** Page 1 One each for the: High speed/top end [normally sticks up some what vertical] Idle/air speed [normally a much smaller screw entering the carb at an angle] Low speed/minimum [located in the end of the slide OR on the opposite end of the carb body] Midrange [also located in the end of the slide OR on the opposite end of the carb body] Note: if there are adjustments on both the slide and the carb body one is an adjustable mid range and the other the low speed!!! Check with the engine manufacture before attempting to adjust these types of carbs!

It is very important not to use the mid screw to adjust the low end by mistake; it is very easy to get the carb way out of sync. Normally I recommend not attempting to adjust the mid range even if your carb is so equipped, the gains are VERY small and mostly limited to minute midrange drivability/economy changes that only the most sophisticated driver will recognize!

The negative is a carb that is so screwed up only an expert can get it back in tune! Please note the above descriptions will apply to 99% of the modern car carbs being manufactured as of this writing with the exception of the Picco TORQUE carb that use’s no mid range needle at all.

It utilizes a fuel management ramp built into the slide; the carb also has two completely independent fuel delivery spray bars/jets. I wish I could tell every one exactly how to tune their engine but I cannot! My hope with this article is if I can help racers to more understand How your carb works, all the other instructions and guides will make more sense! Best Regards, Ron Paris Engine Tuning Tech Tips From Our Web Site: Should I seal the carb with silicone? Many racers find this a must! [Particularly in off road] Here are a few tips.

1** Be sure to use ONLY non-volatile silicone [it should say oxygen sensor safe on the tube!] you can find it at any auto parts store.

2** Use sparingly, apply a thin bead on the carb neck just below the main carb body [even if there is an o-ring present. Do not put it in the engine case bore or all around the carb neck. [That would glue the carb into the engine for good!] You just want a thin bead to squish out from between the engine case and carb body just as you fully insert the carb.

3** After positioning the carb were you want it tighten the pinch bolt assembly that holds the carb very snug! Then add a dab of silicone to both ends of the pinch bolt assembly.

4** Let fully cure before staring engine. Optimal carburetor settings: There is no such thing with any size or brand engine that has a predetermined optimal carb. Setting! If there were, all engines would come without adjustments. There are many variables that must be taken into consideration. Glow plug, fuel type and nitro, manifold type and length, pipe type and were the pressure filing is, clutch adjustment, gear ratios, tire size, ambient humidity, temperature and altitude etc, etc, etc.

The point I'm trying to make is no one can tell you were to “set the Carb” only some one they’re with you can fine-tune it! BUT IT IS NOT THAT HARD IT ONLY SOUNDS COMPLICATED. 95% of all problems are that the directions were not followed correctly. Ideal engine temperature: First let me say that there is no IDEAL temperature for any engine. There are many variables that affect it; Ambient temp, fuel type and nitro content, altitude & barometric pressure, Pipe/manifold type and setting, clutch setting, glow-plug, gear ratio, available traction, How hard your driving, on road or off road Etc. Etc.
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