Tuning Probs - Long needle carb

[Punisher]

Here's my understanding of it, feel free to help me educate myself and others:

The hsn still controls the maximum amount of fuel the engine can get, the lsn is capable of delivering roughly 3/4 of that. The lsn is long and tapered in such a way that it can regulate that much fuel until the the slide opens up all the way and the hsn can make it's contribution.

Spot on.



It would be a lot easier to tune the long needle carb if you could start with the HSN and then go to the LSN . . . then you would know the range you had to play with - once in the ball park it would tune a little more like a short needle carb - the issue with this carb is that any tiny adjustment on the HS moves the LS.


I know what you mean when you say all carbs should have this metering system - whilst the long needle provides mid throttle metering (with the needle) which is a good thing, the needle also dictates the fuel curve which is a bad thing.


In 1:1 scale cars we have the SU carburettor - the fuel curve is dictated by the shape and length of a needle that pulls in and out of a jet - the amount the needle is pulled from the jet is dictated by manifold vacuum (not the accelerator pedal).

Much trial and error is involved to get an SU carb running nicely on a performance engine as the correct shape and length needle needs to be identified.

Once the needle size/fuel curve is correct they are a great little carb.


Another type of 1:1 carb is the down draft carb made by Holley . . . Rather than fuel mixture being dictated by one fuel circuit (controlled by a needle in a jet), Holley's down draft carb has individual fuel circuits - idle, transfer, cruise & power - each circuit is introduced in series as the throttle is opened - each delivers their extra portion of fuel on top of the previous circuit - each circuit can be tuned to deliver the correct amount of extra fuel required by the engine.


The short needle carb can be likened to the down draft Holley carb; but instead of having an idle, transition, cruise & power circuit, the short needle carb only has an idle and power circuit.


The issue with the RC long needle carb, is that it has the same problems as the 1:1 SU carb in as much as the length and shape of the needle needs to be correct in order to get the fuel curve correct . . .


What the 1:1 SU carb has working for it though, is the amount the needle is pulled from the jet (which controls fuel supply) is controlled by vacuum/engine demand, not the throttle - this makes the carb a little more forgiving.



I know a thing or two about 1:1 carbs, so yes, it is a little frustrating for me to be beaten by a basic (in comparison) RC carb - and like said, once set, I reckon the long needle carb would be a good little carb, but I just dont have the time, energy or necessary mental disposition to sort it out.

More significantly, I can’t afford to be making trial and error adjustments at a race meet.

A .5 hour or less adjustment on the HS will require the LS to be moved.

If any, are the performance benefits and/or fuel economy provided by a long needle carb worth all the stuffing around? In my humble opinion - no.


Thanks again for your comments - I hope this info can assist someone to understand and tune their long needle carb.

 

[nitrobrahhper]

I found this, and it makes sense to me.

Set HSN and LSN to factory settings.
Set idle gap to .5mm and fire up the engine.
Get engine to operating temp.
Tune LSN to get consistent, steady idle (20 sec or more).
Hit the track and tune the hsn.
Come back to LSN for fine tune and consistent high idle.
Adjust idle screw to drop idle.
Do a couple WOT passes and if idle briefly hangs ( 2-4 seconds) richen up the HSN.

What if you randomly get flame outs after wot when coming to idle?

 

[tudordewolf]

What if you randomly get flame outs after wot when coming to idle?

I found myself in that situation, felt like I was going in circles getting a good idle, good takeoff, and good WOT, but then after a few passes it'd stall, though the engine wasn't too hot and it would start right back up on the first pull.

I ultimately found that upping the idle gap a tiny bit and richening the LSN just a little solved the issue. I think what was happening is that carb would close slightly ahead of the RPMs decreasing, and while the engine was still spooling down it would "starve" itself.

As rewarding / tempting it is to bring the idle all the way down to a "purr", a slightly "robust" idle just about that is a lot more reliable.

Edit: I did not realize this was nearly a decade old thread, hopefully not an issue.

 

[nitrobrahhper]

I found myself in that situation, felt like I was going in circles getting a good idle, good takeoff, and good WOT, but then after a few passes it'd stall, though the engine wasn't too hot and it would start right back up on the first pull.

I ultimately found that upping the idle gap a tiny bit and richening the LSN just a little solved the issue. I think what was happening is that carb would close slightly ahead of the RPMs decreasing, and while the engine was still spooling down it would "starve" itself.

As rewarding / tempting it is to bring the idle all the way down to a "purr", a slightly "robust" idle just about that is a lot more reliable.

Edit: I did not realize this was nearly a decade old thread, hopefully not an issue.

The engine is at the end of its break in so it may still be a little tight i will try to richen the low and increase the idle gap slightly I don’t mind a semi high idle as long as it’s single stage
And consistent for 10-15 seconds before really loading up

 

[tudordewolf]

The engine is at the end of its break in so it may still be a little tight i will try to richen the low and increase the idle gap slightly I don’t mind a semi high idle as long as it’s single stage
And consistent for 10-15 seconds before really loading up

By the sound of it, the low might not even need to be touched, I noticed the break-in keeps smoothing things out even after the 5th or even 10th tank in some cases, after which my stalling issues basically vanished.

The 5-tank break in is probably a compromise between practicality and impatience designed to at least establish a baseline of care for the engine, but there's potential for more.

 

[nitrobrahhper]

By the sound of it, the low might not even need to be touched, I noticed the break-in keeps smoothing things out even after the 5th or even 10th tank in some cases, after which my stalling issues basically vanished.

The 5-tank break in is probably a compromise between practicality and impatience designed to at least establish a baseline of care for the engine, but there's potential for more.

I usually run 3-5 just idle really rich keeping the engine above 200f then each tank after I start leaning a little bit at a time but letting it cool completely between each tank I’m probably a half gallon or more through it my last engine was a gallon and a half before I got it dialed but I think the engine being fully broken in was why I finally got it right maybe I just need more patience with this one new plugs come in tomorrow I’ll throw one in since this is still the original plug it seemed pretty good I can’t tell if it’s a little distorted at the tip of the filament but it looks like it was running pretty primo

 

[gandalfnz]

I found myself in that situation, felt like I was going in circles getting a good idle, good takeoff, and good WOT, but then after a few passes it'd stall, though the engine wasn't too hot and it would start right back up on the first pull.

I ultimately found that upping the idle gap a tiny bit and richening the LSN just a little solved the issue. I think what was happening is that carb would close slightly ahead of the RPMs decreasing, and while the engine was still spooling down it would "starve" itself.

As rewarding / tempting it is to bring the idle all the way down to a "purr", a slightly "robust" idle just about that is a lot more reliable.

Edit: I did not realize this was nearly a decade old thread, hopefully not an issue.

I have a this problem with all 3 of my savages, and only with them.
Must be the way the carb is made.

Top end is fine, take off is good, but idle goes way down after a few blips and then shuts off.
Increase the idle, then its too high.....honestly so annoying, I can figure out what I am doing wrong.

 

[nitrobrahhper]

I have a this problem with all 3 of my savages, and only with them.
Must be the way the carb is made.

Top end is fine, take off is good, but idle goes way down after a few blips and then shuts off.
Increase the idle, then its too high.....honestly so annoying, I can figure out what I am doing wrong.

Although the hsn seems set I think it’s to lean and the bottom is to rich the other day I went much richer on the high needle then leaned the low and closed my idle gap because I tried opening it before and it was alright but the idle was kinda up and down and if I made the bottom fatter I lost performance