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MEDIA - Nitro engine tuning guide

Tuning a Nitro Engine

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THE TUNING BIBLE!!! 

My name is Neal Smith, I am the owner of www.thenitroshop.com , I was formerly with Clockwork Racing Engines... I go by several names on the forums, Nitrojedi being one of them, sometimes Maximo.... Anyways I have been running nitro since 1984 and over the years become very proficient at tuning and have developed somewhat of of my system that works extremely well for short needle carbs like the LRP has... I promote tuning by sound over tuning by temperature and often times I don't even bother to use a temp gun... Once you learn what to listen for there is no better way to tune then by sound, it is without question the most accurate and consistent method.... When train yourself to listen to the engine you will find that the sounds it makes never lie, a lean engine will always sound like a lean engine no matter what the temp says..and if a engine doesn't sound lean then it isn't lean.......doesn't matter if your temp gun is telling you the engine is 400 degrees, if the engine sounds proper then it is proper.... a overheating or over lean engine will have telltale sounds that always will be present no matter what...there is no way to make a lean engine sound rich and no way to make a rich engine sound lean
So first and foremost is you need to set your idle gap size as well as make sure your linkage is functioning correctly........When I say functioning correctly I mean that when your radio is at neutral your throttle slide is closed 100%, with no further possible movement....It then has to pass the throttle down test..that where you hold full throttle then let go of the trigger, the throttle slide needs to snap shut 100%...it cannot hang open even the tiniest amount...this is non negotiable, if your linkage is not closing 100% during this test you will never be able to properly tune your engine.. So this means hold full throttle, let off the trigger....then check your slide and try to push to closed further, if you can closer it further then your linkage is hanging open.... i CANNOT STRESS TO YOU ENOUGH THE IMPORTANCE OF A PROPERLY FUNCTIONING THROTTLE LINKAGE !!!! YOUR LINKAGE MUST BE CLOSING 100% WHEN YOUR TRIGGER IS AT NEUTRAL !!!!! Do not rely on using your brakes to close the throttle slide, it has to do it at neutral with no further movement when you hit the brakes..... Also its a good idea to have some deadband on your throttle slides, meaning your throttle servo has some free play before it opens the carb slide...If your dont have any free play your throttle can get feathered opened by the servo twitching... SO always be sure there is free play in the linkage when at neutral...
So once we know your linkage is working and snapping itself shut we want to set a 0.5 mm idle gap...0.5 mm is the thickness of a exacto blade... if you have calipers measure a piece of wire or a needle or pin and use that as a feeler gauge to set the gap..if not look at the thickness of the X-Acto blade and use it as a guide to set your idle gap.... Even use a feeler gauge to test to make sure your linkage closes correctly...this here is without question the #1 reason most guys fail at tuning a nitro..their throttle slides are not operating perfectly and they have to large of a idle gap..... these 2 critical things are the #1 overlooked issue and the source of 1000's of phantom air leaks.......I cannot stress enought, make sure you have a 0.5 mm idle gap and that your throttle is operating correctly ! till these 2 things are absolutely perfect we cannot properly tune your engine, no ifs ands or butts about it ! your wasting your time adjusting the engines needles till you know for sure these 2 areas are correct !
So lets talk about tune shifting and how heat affects the engines tune...Many tuning guides make no mention of this and its also a huge source of tuning issues.... These engines go thru a massive tune shift as they warm up, and the hotter they get the leaner they get, as well not only that but the harder you run them the leaner they get.. A good race engine will take 10 minutes of hard driving on a track to become thermally stable and for the tune to stabilize, up until that point the tune continually shifts leaner and leaner... When first fired a properly tuned engine will be sputter rich often with a very high idle and often want to stall easily... However as you continue running the enegine the idle will start to stabilize and as you get the engine on the ground and under load you will see the tune slowly start to clear up, and the longer you run it the better and better the tune becomes, till after roughly 10 hard minutes you will have the tune right back to where you left it the day before when you tuned it.... As i say the tune shift these engines will go thru is massive, from initial fire up to final hot tune you are talking a huge shift in engine mixture....l see soo many guys tuning their engines too soon and end up with a overly lean engine once everything heat saturates...... On a race track how hard a engine is run determines the ideal needle settings... Meaning I can set the needles perfectly for a average racer and he could race all day with a perfect tune..but if i gave the same car to a faster driver the tune would very quickly become too lean as the engine is being worked harder with the faster racer.......... this holds true for your MT's...meaning the perfect tune may vary depending on what your doing with the truck and how hard you are running it..... Wide open top end runs in heavy grass is going to want a much richer needle position then popping short wheelies in your front yard...So understand that the tune is going to move around on you and thats its perfectly normal....... what your going to need to do is learn to listen to the engine and make the appropriate adjustments..... And truly once you know what to listen for its super simple to tune these engines !!!! I mean it, its really really really simple !
Idle speed ! that's right, that's what we use to tune the engine, we listen to its idle speed both when we let off the throttle as well as its idle speed after sitting still for several specific time tests...... A lean engine will always, and I mean always want to high idle ... you will do a top end pass and when you let off the trigger the engine will carry RPM before it drops down, in severe cases it will make Chewbacka sounds...What you want to happen is for the engine to drop down to idle the instant you let off the trigger, then sit at a steady idle for 1-2 seconds then to drop a slower slightly lopey idle, relaxed and almost like a Harley Davidson...A lean idle will sound like a buzzing bee, it will be steady and buzzy and have no lope to it, very not sound relaxed..... Now if your linkage is hanging open or your idle gap is too big it can simulate the lean buzzy sound....this si why soo many guys get their needles all mixxed up, as their linkage and idle gap issues are creating a false lean ( well it is actually lean due to too much air ) ................. So As I say listen to what the engine does when you let off trigger, if it carries RPM your HSN is getting too lean ( or your idle gap is is still too open ).............
On these carbs preset a 0.5 mm idle gap..use the LSN solely as a idle mixture control..meaning do not use the LSN to adjust the engines low speed performance...the LSN is to be treated only as a way to adjust the engines fuel mixture while at idle....nothing else, All performance tuning is to be done using the HSN.... if you have a rich bog out of the corner lean the HSN, if you have a lean bog off the line fatten the HSN.....do not whatever you do use the LSN for anything but idle mixture control !
Now when at idle you want the engine to have a lope to to it, not almost stalling, but with a definite and audible miss ... you want it to slowly load up the longer it idles , meaning the longer it idles the lower and lower the idle gets, eventually tot he point it will stall out, however the longer it can sit low and slow ans still stay running the better...a lean LSN Will never allow this to happen..if the LSN is lean the engine will always have a steady buzzy idle and will not load up just sitting there idling.... when I tune a engine I continually check the idle, how it idles, how it idles down and how long it takes to load up........ Always listening for a fast idle down to a slow steady idle then a quick transition down to a slower lopey idle............... A lean engine will never be able to properly idle down..if the HSN is too lean it will carry RPM and if the LSN is too lean the idle will stay high and buzzy............ most videos I see online have a high buzzy lean idle......
So when I tune I baseline a 0.5 mm idle gap, I fire the engine, adjust the LSN to get a reasonable starting point for idle speed, then I fatten the top end till she sputters and then run by run slowly lean it down..little by little I lean down the top end always double checking my idle down then idle speed.... as I approach the engines sweet spot I make finer and finer adjustments, till the point I am only moving the needle a hair, like 1/48th of a turn.... when your near the sweet spot a 1 hour movement is too much..... as the closer you are the sweet spot the more sensitive the needles become.... So I basically lean my HSN till I start to notice the idle hanging when I let off, then I slowly fatten it back up till it goes away.... I do run after run after run always checking to make sure the engine idles down correctly and steps down to the correct idle stages..... it never lies, if the engine idles down cleanly then steps to a low slow idle you are not lean, no matter what the temp gun says..and visa versa, if your idle hangs and your engine does not step down you are lean, no matter what your temp gun says.....

Once you hae your tune set to where you think its close start doing some tests...first test is to do a wide open pass, bring the car to a abrupt stop..let the engine idle down to its second stage loping idle, then as soon is you hear it step down crack full throttle....If your are lean the engine will give you a lean bog right here... So if you get a lean bog fatten the HSN....retry the test till you can do a rapid throttle pull from a fast top off wide open............. then do this same test but doing a 3 count, then the same test with a 6 count, then a 10 count........ Once you are fully heated up you would like to see a slight loadup when you allow for a 6-10 second stop... not anywhere near enough to stall the engine, but a slight gurgle with extra smoke cloud is what we are after..........
So this is really it..listen to how the engine idles down..a properly tuned engine will idle down instantly, then quickly step from a smooth slow idle down to a slightly slower lopey idle...a lean HSN engine will always carry RPM and will want to idle high and idle will stay buzzy if the LSN is too lean....
Anyways coffee break and time to watch some TV..hopefully i dont have to many spelling issues
30% tunes easier then 20% runs cooler
lower oil tunes easier then higher oil, runs cooler and prefers a richer mixture..
I run 30/7 and it has a huge tuning window and can be run much richer then a fuel that is 20/14 , this is why the 30/7 fuel ends up performing much better and still protecting better then the 20/14...to get the 20/14 to run proper and crispy it needs to be run much leaner with a much hotter burn then what the 30/7 needs......I know this is backwards to common knowledge but is indeed accurate...