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Old 03-21-2010, 02:34 AM   #16
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OK think where does the fuel go thru first???
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Old 03-21-2010, 09:23 AM   #17
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OK think where does the fuel go thru first???
I know you're not asking for yourself Chris, but I figured it is a good post to add to.
HSN is the first regulation point, The LSN is a secondary restriction to the supply provided by the HSN when the throttle slide is closed. There is a fixed orifice and a tapered needle for each set, as the screw is turned inward (lean) the cross section of the needle is a larger diameter and blocks off more area of the orifice and results in a lower fuel flow. when the throttle is opened, the LSN is pulled out of the spraybar and allows the flow from it to be controlled by the HSN only.
This is a decent drawing to show it, it is not showing the LSN in the throttle slide though, so keep in mind that the LSN moves with the slide.

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Old 03-21-2010, 10:49 AM   #18
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i am going to assume that a 3 needle carb with a mixture control valve (mid) would act in the same way.

Or does all the fuel go through the mid???

BTW, great post.

this site could definitely benefit from more of these kind of facts that give actual answers and not opinions.
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Old 03-21-2010, 10:53 AM   #19
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it really does show you how the lsn only has control over idle and take off. You hear it all the time, but a diagram like this helps you really understand.

I am sure this thread will help my tuning no doubt.
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Old 03-21-2010, 12:48 PM   #20
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i am going to assume that a 3 needle carb with a mixture control valve (mid) would act in the same way.

Or does all the fuel go through the mid???

BTW, great post.

this site could definitely benefit from more of these kind of facts that give actual answers and not opinions.
With a midrange adjustment added, the LSN is much longer with a slow taper and functions as a midrange modifier as well. The midrange screw will affect positioning of the spraybar jet. If you look at that diagram above, it's the gold part protruding into the carb throat on the side of the HSN. The profile of the needle will determine how progressive the midrange behaves. If the midrange screw is moved in .5mm, the LSN would have to be moved out the same amount to maintain the same relative position of the two at closed idle position.
It's really more of an art than a science to adjust the carbs. Having a good understanding of the feedback you get from the engine and which part of the adjustments will alter it is the most important thing I've found to focus on and to do that you need to see how the parts function. Reference all the needle positions and rip a 3 needle apart, that will be the best way to see for yourself.
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Old 05-04-2010, 09:22 AM   #21
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When setting the idle gap do you take the venturie out first ?
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Old 05-04-2010, 09:52 AM   #22
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When setting the idle gap do you take the venturie out first ?
Madbrad, you read my mind! I was wondering the same thing to, but I figured that the inserts would block off that section (like if you went from a 9mm to a 7mm) of the carb and you have to redo the idle gap (.5-1mm) for the insert.

I hope one of the long time nitro guys can shed some light.
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Old 05-04-2010, 10:04 AM   #23
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Always set your idle gap WITHOUT the venturi.
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Old 05-04-2010, 10:38 AM   #24
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At the track is others making the high speed needle too lean then making the bottom too fat. This causes way too many issues. I always fatten the top then lean the bottom then check to see if the idle drops after being warmed up and sitting at idle. If it drops then lean it more on the bottom. when it gets steady then adjust the idle a bit, only a bit. Then go drive it on the track and if it's slow then lean the top in small steps until it hauls butt. If you go past that then you will shorten the life of the engine.... Finally recheck the bottom end to make sure it has enough fuel at idle after driving it at race speed. If it idles fast it may need more fuel on the bottom. If it races then drops then your idle is off and you need to lower idle and lean the bottom to get it steady....The top is usually fine....Maybe a little lean but this is only one hour adjustment at this time. Hope this helps.
this is some really good quality advice.
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Old 05-04-2010, 11:27 AM   #25
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At the track is others making the high speed needle too lean then making the bottom too fat. This causes way too many issues. I always fatten the top then lean the bottom then check to see if the idle drops after being warmed up and sitting at idle. If it drops then lean it more on the bottom. when it gets steady then adjust the idle a bit, only a bit. Then go drive it on the track and if it's slow then lean the top in small steps until it hauls butt. If you go past that then you will shorten the life of the engine.... Finally recheck the bottom end to make sure it has enough fuel at idle after driving it at race speed. If it idles fast it may need more fuel on the bottom. If it races then drops then your idle is off and you need to lower idle and lean the bottom to get it steady....The top is usually fine....Maybe a little lean but this is only one hour adjustment at this time. Hope this helps.
I thought if you were to get the engine to its temp and tapped the throttle to rev the engine up a little, the IDLE should stay HIgh then drop a few seconds later. Am I wrong? Or do I want a steady Constant Idle? Like how good of a revving idle does one want? And the idle gap should never be more than 1mm open correct? Thanks guys... Great advice and information here... Sticky!!!!


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Old 05-04-2010, 12:17 PM   #26
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NOVAROSSI ENGINE TUNING
hsn is the control valve, set the control valve first when final race tuning . after doing so go to the lsn and lean until the engine keeps a consistent idle but does not ting when idling , that is caused from lack of fuel on lsn . lower the idle until it is as low as you can get it and it will maintain a good safe idle during on track decel and on the bench for at the least 20 sec.
after you do the low end you will usually have to richen the hsn about 1-2 hours

there will be a few things that will need to be tweaked slightly but that will get you in the ball park .

OTHER ENGINES TUNE DIFFERENTLY !!!

[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.


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!
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!
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Old 05-04-2010, 12:31 PM   #27
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Houston.

Lets say I think I got my Engine to where I think she is Dialed. How could you tell if she is what or what? I know People that say they can tell by just listening. I think She is great. I have her tuned to 205 F. Do you think I could go more? Or have a wrong tune. I would say I get about 6-7 min run time. On 1 tank.Im not sure never really timed it. I know you shouldnt use a temp gun, but what do you think? I have about 1.5 gallon on her, and same plug when bought.
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Old 05-04-2010, 12:57 PM   #28
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Houston.

Lets say I think I got my Engine to where I think she is Dialed. How could you tell if she is what or what? I know People that say they can tell by just listening. I think She is great. I have her tuned to 205 F. Do you think I could go more? Or have a wrong tune. I would say I get about 6-7 min run time. On 1 tank.Im not sure never really timed it. I know you shouldnt use a temp gun, but what do you think? I have about 1.5 gallon on her, and same plug when bought.
CHANGE THE PLUG!!

what engine is it? some run very well at the low temps and some don't, 1.5 gallons it should be broke in fully by now . if you are happy with the power delivery why mess with it ? the runtimes tell me your tune is a little off though
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Old 05-04-2010, 01:09 PM   #29
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CHANGE THE PLUG!!

what engine is it? some run very well at the low temps and some don't, 1.5 gallons it should be broke in fully by now . if you are happy with the power delivery why mess with it ? the runtimes tell me your tune is a little off though
I was told to change it, LOL I am going to..

It is a OD SS .21 // JP4 pipe setup thats all i have.

And I know I could tune or Fine tiune it a little more I just dont know which way to go or start with. I dont wanna lean her out too much n suffer life.
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Old 05-04-2010, 01:36 PM   #30
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I was told to change it, LOL I am going to..

It is a OD SS .21 // JP4 pipe setup thats all i have.

And I know I could tune or Fine tiune it a little more I just dont know which way to go or start with. I dont wanna lean her out too much n suffer life.
i was thinking on this last weekend,my jx21 was 189f,i tried to lean it & i end-up 204f (empty tank) it will idle all day long ,i know my hsn is set ,is it fine to lean it more cause i remember the hsn is 2 out turns from close??already
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