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Old 03-06-2007, 06:58 PM   #1
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ron paris tuning guide part 1

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Tuning tech: carb Needle balance

We get dozens of tuning questions from around the world at Paris racing daily, The number one questions is, were do I set my needles on the carb? Unfortunately there is no such thing as a definitive universal setting for any engine!

Every application will have it's own unique requirements, even two IDENTICAL set ups can and most likely will have at least slightly different settings.
Please see tech tips following the article below [from our web site]
I would like to address what seems to be the # 1 mistake we encounter in engine tuning:

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 opining 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 reving 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!


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 DRAMATICL Y. {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 Y2 and % 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!
0ne 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,

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 fullyinsert the carbo

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 carbo 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 COMPLlCATED. 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.
All information is based on the Paris/Exergin probe; it is the only probe designed for model engines and has worldwide patented Emisivity compensation. We tested dozens of devices before we started working with Exergin and introduced infrared tuning to the model industry.
All of the following conditions MUST be met before any temperature can be considered correct, the engine is assumed to be in good shape.
I} There must be a visible trail of smoke when accelerating from EVERY comer.
2} The idle is stable.
3} The glow plug wire stays somewhat shiny.and the coil stays round [UN-distorted]
4} The performance is good.
Picco on road average conditions 20 to 40% nitro 205 to 230 degrees f
Picco off road average conditions 20 to 30% nitro 215 to 250 degrees f
Nova on road average conditions 20 to 40% nitro 220 to 260 degrees f
Nova off road average conditions 20 to 30% nitro 230 to 270 degrees f
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Old 03-06-2007, 07:00 PM   #2
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ron paris tuning guide part 2

How to "READ" your glow plug:

OS and Turbo glow plugs go gray sooner [easier] than McCoy plugs; this is not necessarily bad. Actually when A new plug wire just goes slightly gray after a 5 or 10 minute hard
run it means your very close to an optimal horsepower tune, but be careful the next step is TOO LEAN!
My rule of thumb.
I} Wire and surrounding bottom of plug wet, with like new shiny wire = rich side of optimum power 85%
2} Wire and surrounding bottom of plug starting to dry and wire starting to gray = Very close to optimum power 95%
3} Wire and surrounding bottom of plug dry, wire totally gray but not distorted optimum power 100%
4} Wire and surrounding bottom of plug dry, wire distorted = slightly lean DANGER!
5} Wire and surrounding bottom of plug dry, wire broken and distorted or burnt up = extremely lean possible engine damage!
Note: You can only "Read" your plug in a nearly new state [Wire like new and shiny] A gray plug can still operate well. After it has totally gone gray, performance can start to fall off. To test just put in a new plug and if there is no difference in performance save the gray one or put it back in. If your engine does not feel or run right tray a new plug before making major tune changes

Effects of changing manifold to pipe length:

Rule of thumb:
Longer manifold = more bottom end torque. [At the expense of some rpm]
Shorter manifold = more top end rpm [at the expense of some torque]
BUT you can go to far with either direction causing the engine to run erratic or HOT! You must keep it in the "sweet spot" Bottom line: there is no set length for every engine or pipe, experiment with the length to suit your driving style or track needs.

Does it mater were you put the pressure fitting in your pipe:
If you're talking about a single chamber pipe like the Associated or Paris AL 12T no big deal.
But if you're talking a two-chamber pipe, and you go from one chamber to the other you will have to re tune the engine.

Even then it is not a big deal once it is tuned for that location there is little if any performance difference. [It is all relative to available pressure Vs needle setting] I recommend tapping into the first chamber on two chamber pipes [the intake side] were the pipe is at its largest diameter. The second chamber has more constant pressure but can be more difficult to tune in high altitude or fast changing weather conditions

How to shut off you engine and stop runaways:

If the engine is out of control and at high RPM DO NOT USE YOUR HAND on the flywheel!
Some guy's flip the car over and stick the toe of their shoe on the flywheel! I have seen it work but 50% of the time the engine lets go before you can stop it! The method I like is as follows:
Try and hold the car down hard so the wheels can't spin [this may waste the clutch shoes but it is cheaper than an engine!] Yank the body if you can't get to the air cleaner, then yank the air cleaner and put your finger on the carb intake. It is kinda like handling a wild animal but it is a good method.
Do not just push your finger down on the foam air cleaner, which will suck all the dirt into the engine! Holding a rag on the exhaust works too [if you happen to have one handy] but, Be sure there is several layers of rag it will take more than a few seconds to choke it And it will be HOT HOT HOT!
If the engine is idling nonnally, Ijust touch the flywheel with the handle of an old screwdriver!

Ron's tips for healthy Fuel:

I always shake my fuel because some oil separation is common especially with higher nitro but I mostly shake it to re oxygenate the nitro [old habit from topfuel/funny car days] Water absorption Is the #1 reason for "Bad" fuel! And is little affected by any nitro or oil/additive package. The methanol is what's so hydroscopic [Water magnet] My advice as always.
I Never leave the bottle open any longer than it takes to fill your fuel bulb.
2 never leave the FUEL BULB uncapped any longer than it takes to fill the car.
3 always rinse out you fuel bulb at the start of each day with a little fresh fuel.
4 NEVER draw fuel back into your bulb or put it back in the jug.
5 keep your fuel out of direct sunlight and off the ground.
6 store your fuel in a stable temperature.
7 I like to squeeze the extra air out of the bottle before tightening the lid.
8 Some times it is handy to save some 1/2 gallon or quart bottles to transfer to as you the Gallon remember to rinse them out with a little fresh fuel first. If you follow these simple steps your day will never be ruined by bad fuel.

Will it hurt my engine to run it out offuel:

My opinion is if your engine was tuned correctly, to start with it will not harm it running out of fuel!
I have done this hundreds of times with no damage even in our killer engines.
If just before running out of fuel the car was tuned:
1 } Tuned Slightly rich, it will speed up for a brief moment just before it dies as you pass through the "Ideal performance" mixture stage.
2} Tuned for ideal performance, it will fall off in power just before it dies as it passes through the too lean stage.
3} Tuned too lean to start with, it will bog down noticeably and may blow the plug as it pass though the "way to lean you just screwed up stage".
This can cause some damage to the piston and sleeve if the element wire breaks and falls in to the engine.
However, the brief lean moment should not hurt the bearings, bushings, rod, crank etc. if you're using high quality fuel [especially a high castor blend like Bluethunder race formula]

Is it ok to switch nitro content or brands of fuel?
There are many opinions on this subject; I feel there are no problems doing either!
The most important thing is to use a good quality fuel!
Personally, I break our engines in on the same nitro we race with, but have had equally good results either way!
The only recommendation I would make is, if you do switch brands it would not hurt to:
I} Let the engine idle a few minutes before you go out on the track.
2} Run Y2 to 1 tank of fuel on the rich side.
3} Check your glow plug and if it looks good fine tune and go for it.
The reason I say to check the plug is that different fuels have different oil additives and detergent packages.
This may wash away some of the built up oil deposits and platelets or bonding agents [As the oil companies call them] and foul the plug. After a few runs your new fuel will leave deposits of there own, some brands you can see the residue because of its blue or red color. Other brands you can't see because they're a light yellow or amber color but rest assured it's there!
It is noruml to have these build-ups and it cause's no harm!

What is the best Fuel?

Blue Thunder Race Formula [Paris/Blend] Has a High castor to synthetic ratio. Castor oil is simply the best lube available for model engines [Alberto Picco and Mario Rossi agree] but not all castors are the same! We use the finest ultra Pure Triple A refined castor. Then we add a synthetic supplement package, which includes anti foaming and rust properties. There is also a wetting agent to aid in Lubricity and reduce storage dry out. The fuel is manufactured with the latest high tech computer controlled mixing equipment; using the freshest lab tested and certified USA made nitro and methanol by the largest manufacture of model fuel in the world.

Should I use after run oil:

Race Formula Blue Thunder is a high castor blend that includes 3% Synthetic with anti gumming agents.
It is always a good idea to use some after run oiL
First let the tank run completely empty at idle until it runs out of fuel [try re-starting it several times]
Next put a few drops down the carb [open] and a few drops down the glow Plug hole [leave the plug out and put a rag over the top of the engine]
And spin it over for 5 seconds on the starter box.
Last put a few more drops down the carb re install the glow pug and spin The engine over 5 more seconds.
That will do it!

We May be releasing a after run oil but in the meantime Marvel Mystery oil works great and can be had at any automotive or hardware store.
Other substitutes would be automatic Trans fluid, 3inl oil or Mineral oil from the drug store! NEVER USE ANY OILS THAT CONT AINE SILICONES!!!
Cleaning your Engine:

First, follow the after run oil instructions above!
Remove the engine from the car, you can then wash it off just be careful not to wash dirt into the front bearing [even if it has a rubber sealed type]
I brush it off behind the flywheel then rap a strip of cloth around the crank, "wedging" it between the front of the engine and back of the flywheel.
I then plug the carb and exhaust [be sure there is a glow plug installed] then squirt it off with methanol or denatured alcohol
[Not Isopropyl it is 30% water]
I do not recommend brake cleaner or motor spray.
Then blow of off with compressed air if you do not have compressed air shake the engine vigorously and blow it off until dry with a hair dryer, being careful not to get it HOT just warm to the touch.
Last I remove the strip of cloth let the engine come to ambient temp [so there is no condensation formed inside] and put it in a zip lock bag or re-install in your car!
You can see many other useful tech tips at the Paris web site [go to tech tips on third page]


Once you've found a good setting for the high-speed needle that allows the engine to make good power yet still push plenty of blue smoke from the exhaust
(especially when the car exits a turn), it's time to set the low-end needle and the idle-stop screw.

Most engine manufacturers recommend a specific setting for the idle-stop screw, and they're usually well within the ballpark. For now, set the idle-stop screw so
that the engine will idle at a moderate rpm without stalling.
Bring the engine up to operating temperature by driving it for a few minutes. Now stop the car and listen to the engine's idle speed. If the engine idles fast but then slows down in just a few seconds, the low-speed needle is probably set too rich. Lean the low-speed needle (by turning it clockwise in one-hour increments) until, after running a few more laps, the idle stays high for about twenty seconds or so when you stop the car. Once you've done this, use the idle-stop screw to make the final adjustment of the idle speed.

For sport engines which lack a low-speed needle, the idle-stop screw is the only method of adjusting the engine's idle speed. For these engines, simply turn the idle-screw clockwise to increase idle speed, and counterclockwise to reduce the idle speed.

Setting your engine's idle speed isn't a contest to see how low you can get it without stalling the engine! Your goal when setting the idle should be to allow the engine to run at moderate rpm without the clutch being engaged whatsoever. Your car should be able to sit at a standstill when idling. If you have to hold the brake, the idle is too high. If you have to blip the throttle to prevent stalling the engine, the idle it too low.


Nitro powered RIC vehicles can be tons of fun, or they can cause tons of frustration. The difference between success or failure lies with the break-in process. If you follow these steps, have patience, and use your noggin, you're assured of success. Rushing through the break-in procedure or worse, forgettting it altogether, is a recipe for disaster.

Tuning your engine is the hardest part of nitro racing. But once you've learned how the carburetor works, and which screw does what, it will all become second
nature. So when you're running your car, you'll always know exactly what to adjust to gain the highest level of performance possible.

1. Rich is good. Blue smoke should always be coming from the exhaust.
2. Always set the high-speed needle first
3. Never try to tune a cold engine-wait for it to get up to operating temperature.
4. Always begin your engine tuning from a rich high-speed needle setting. Never start out with a lean setting.

Tuninq all carburetors and Enqines for the experienced racer:
when starting a new engine follow the set up instructions first! Start the engine and let idle at a moderate speed for two or three minutes. Then shut it down and let it cool for a few minutes. Be sure the piston is not at the top. Repeat this procedure three or four times.

2) Now run the engine on the track, making sure the engine is rich. You should hear a "four cycle" sound. Be sure to let off the throttle going down the straightaway once or twice so the engine is not straining in its rich condition. Repeat this procedure for three or four tanks of fuel, letting the engine cool several minutes between tanks. Also, check the glow plug often. It should look new (shiny wire that is not distorted).

3) You can now start leaning [turn clockwise] the maximum needle a little at a time in one hour increments [imagine the needle like the hour hand of a clock] checking the glow plug often. [I use a glow igniter with a meter on it] I recommend running a few more tanks a little on the rich side before "Race tuning" When tuning for maximum performance [Race tune] adjust until engine will reach Good RPM then open needle 1/8 to 1/4 turn.

set the low end or idle:

4) After getting top end set close, bring model in, listen to the idle. If it is idling fast then it slows down in a few seconds, it is probably too RICH on the bottom (minimum spray bar). In that case, make it more lean (turn clockwise % turn), then run another lap and bring the car in again. Repeat this procedure until the idle stays high for at least 10 to 20 seconds; at this point readjust the aircrew so the idle is at a moderate speed. The wheels should not turn, and the clutch should be fully disengaged.
If you had to turn the minimum spray bar in more than two or three more turns recheck the slide adjustment! [Picco Torque carburetor only]

5) you may need to repeat steps 3 and 4 several times to achieve the "perfect tune"
Be patient. Tuning is the hardest part of gas racing, and is the first secret to race winning results. you ask your hobby shop or an experienced modeler for help.
Three last tips
1) Be sure engine is warmed up before tuning

If you are a beginner, it is highly recommended that

2) always tune from rich to lean. When in doubt, richen up the maximum needle (top end)

3) If you have to turn minimum spray bar (low end) in more than three to four turns from flush, it is a indication the slide adjustment is too loose and needs resetting (Picco Torque carburetor only).

Good Racing,
Ron Paris
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Old 03-06-2007, 07:04 PM   #3
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Thanks Chuck...
This should help alot of people
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