Tuning Probs - Long needle carb

[Punisher]

Hi all,

I recently fitted a GO 21 GX 5R to my Losi 8ight Truggy.

I spent most of the last race meet trying to tune out a lean bog on acceleration - I had a bugger of a time tuning out the lean bog . . . in fact, the engine won, I never really got it running right all race meet - VERY frustrating!


After a bit of research I have learned that the GO engines are fitted with a long needle carb . . .

After some more research I found some info detailing how to tune the long needle carb - basically backwards to the short needle . .

- pinch test to set the idle gap and LSN
- tune the HSN
- fine tune the LSN

Apparently if it develops a rich bog on the LS, lean the LSN - easy.

But if it develops a lean bog (as I have been experiencing) it is recommended to add the fuel via the HSN NOT the LSN.


What is frustrating, is that the LSN and HSN on a long needle carb are very closely connected; any tiny adjustment to the HSN (and I do mean tiny) has a dramatic affect on the low speed tune.

I'm finding that I am chasing my tail.

The plug looks ok – HS performance is excellent – but it runs really cool – Temps are only 150F

To increase the running temps I try lean out the HSN . . . . and it develops a lean bog!

Also, when leaning the HSN, I am take the needle far away from the stock setting which I understand should not be required (ie considerably under level) - and of course any tiny adjustment to the HSN has a huge affect on the LSN . . .

- lean the HSN = develops a acceleration bog
- richen the LSN = lowers the idle speed – wont idle (does that mean my idle gap is too small?)
- increase the size of the idle gap – richens the LS!

** I just keep going around and around.

Does anyone have experience tuning these carbs? Can you provide some advice/useful tips?

I understand the long needle carbs are good on fuel economy and deliver a nice fuel curve – I would prefer how to learn how to tune it and make it work, but I can't really afford to wrestle with an engine and it's tune when racing.

If you can't help me with my tune, can you tell me which short needle carb will fit a Go .21 - I would entertain removing the long needle carb and keep it for another day when I want a challenge.

Thanks in advance for your help.

 

[Greywolf74]

Someone may correct me here but as far as I know if the engine is bogging then its to rich. If its to lean it flames out. I dont believe there is such a thing as a lean bog. Either way though use this tuning guide to tune your engine. The guy that made this tuning guide is an expert and its a great easy to follow guide.
https://www.rcnitrotalk.com/forum/showthread.php?t=102688

 

[Nitro4910]

The Idle Gap & LSN setting are not one in the same.

You set the Idle Gap with measured paper clip or I set mine with number size drill bits by using the smooth shank of the bit.

number size bit spec's, most hardware stores have them for $2-$3 each.
#70-0.0280in/0.71mm
#67-0.0320in/0.81mm
#64-0.0360in/0.91mm
#61-0.0390in/0.99mm
#60-0.0400in/1.02mm

Use the pinch test for the LSN.

The HSN should have very little effect on the LSN.

Lean = cutting out
Rich = bogging.

How to set the Idle Gap.
https://www.rcnitrotalk.com/forum/showthread.php?t=94326

The best tuning guide I've ever used, below.
https://www.rcnitrotalk.com/forum/showthread.php?t=76808

 

[Punisher]

Unfortunately for me there is certainally something such as a lean bog . . . rich kinda burbles and has lots of smoke, lean kinda "warrrts" without smoke.

Thanks for linking Robin's tuning guide - indeed it is a beauty, I use it for my short needle carbs, but it doesn't work with my long needle carb - it's like learning tuning all over again.


Thanks for posting - Nitro4910 . . I am familiar with the difference between idle gap and LSN, and as you say, the HSN should have very little effect on the LSN - and this is the case with a short needle carb - but unfortunately not the case with a long needle carb. With the long needle carb, I can change the HSN 1 hour, and it dramatically affects the LSN - it's very frustrating.


My understanding is that the pinch test can be used for LSN and idle gap . . .

- when pinched the revs should increase only marginally - say 500rpm, if the revs churp up dramatically then the idle gap is too large - reduce the gap until the idle only increases a little

- when pinched if the engine stalls (with only marginal increase in rp) after say 2 seconds the LSN is too lean . . . if the engine stalls after 10 seconds, the LSN is too rich.


What I find frustrating is that I tune the idle gap and LSN . . . moderate increase in rpm when pinched - about a 5-6 second stall . . . . so theoretically the low speed is set . . .

Then I move to the high speed . . . take fuel out so WOT is nice and crisp and it develops a lean bog . . . add LSN . . . idle drops . . . increase idle gap . . . goes rich . . . put LSN back were it was . . . lean bog . . . add fuel via HSN . . . bog gone, good WOT response, but temps only 150F . . .


The long needle carb is sooooo hard to tune . . . with a short needle carb the LSN and HSN are separate - start with a richish LSN, work back the HSN from rich, then adjust the LSN for throttle response . . . . with the long needle carb, move a needle either HS or LS and it throws everything off . . . .


It could be that my idle gap is too small and it would have a very high idle . . . but I am limiting my idle speed with a lean LS tune? Hence the bog . . . . and I am masking the bog with my HS tune, hence the low temps . . . ??


It's doing my head in!

 

[TK5310]

Hi Bill!
Thought 'punisher' may have been you.

Guys, we spent about 2 hrs trying to tune this f*cking thing last night.

Idle gap is bang on, pinch test has the rpm rise a bit but not excessively, engine cuts in 6-7 secs on a pinch test - as per the tuning guide for this engine.
Set up like that throttle response is good on the move, but poorish from a standing start.

Engine temp rises at idle - LSN is lean.
Richen up the LSN & bog goes away, throttle response becomes pretty good, but it won't idle.
Bump up the idle gap to increase the idle & everything goes pig rich, retune the LSN via pinch & fine tune on the road & you end back where you were with the smaller idle gap - won't idle if rich enough to get rid of the bog off the line, get it to idle & it has a lean bog (& yes, it is a lean bog).
Throw in the HSN affecting the LSN in a big way & this little pr|ck becomes a right PITA to tune.

We even moved to a hotter plug (T4) with minimal improvement.

Neither of us are gun tuners, but we can tune a nitro engine pretty good - got us both stumped.

Best we can manage is fair idle (won't cut out) & okish throttle response, neither great.

Get either the idle right or the throttle response right & the other goes to hell.

My suggestion is to ditch the carb & fit a regular carb, or maybe try a T5 plug.....

 

[Punisher]

Hi Bill!
Thought 'punisher' may have been you.

My suggestion is to ditch the carb & fit a regular carb, or maybe try a T5 plug.....

I'm thinking the plug should not have so much influence on the engine temp - I'm also thinking about ditching the carb. :whhooo:



(Yeah man - 'punisher' is me

 

[TK5310]

I can bring a mash hammer & 'fix' the carb good enough that the only solution is a regular carb if you like!

Would get rid of the internal debate as to whether to persist with the stupid bloody thing!

Who knows, maybe when the motor loosens up a bit it'll idle great & have good response with that carb - hell, my picco got roughly 1000% easier to tune as it loosened up.

But can you be bothered when the simplest solution is a non-retard carb?

 

[Punisher]

LOL

Perhaps bolt on a DP 4 barrel - easier to tune! - Reckon I could get one working better than that bl**dy long needle too!

 

[Nitro4910]

Here is a thread that explains Long needle carbs a little better.
http://www.rctech.net/forum/offroad-nitro-engine-forum/590663-long-vs-short-low-speed-needles.html

This is not something I have run into before, so I am limited in useful knowledge here.
Maybe give Robin at ERCM a shout, he most likely has more experience with this type of carb.
https://www.rcnitrotalk.com/forum/forumdisplay.php?f=142

 

[Extreme RC Mods]

The problem with long needle carbs is that the needle is still inside the jet from idle to the mid rpm transition choking back the fuel it probably needs. Most likely it takes almost 3/4 or more throttle to pull the needle fully from the jet so full fuel flow can occur and this will really mess with the tune. I would either replace the needle with a blunt tip needle or replace the carb entirely. It could be that the engine needs more fuel run through it but by the description, it doesn't sound like the problem will go away.

 

[Punisher]

The problem with long needle carbs is that the needle is still inside the jet from idle to the mid rpm transition choking back the fuel it probably needs. Most likely it takes almost 3/4 or more throttle to pull the needle fully from the jet so full fuel flow can occur and this will really mess with the tune. I would either replace the needle with a blunt tip needle or replace the carb entirely. It could be that the engine needs more fuel run through it but by the description, it doesn't sound like the problem will go away.

Thank you Robin - that's all I needed to hear - the carb is going in the bin!


Makes sense – the carb is only ever going to deliver a certain AFR . . . . Because the HSN has such an influence over the LSN, if the needle is the wrong shape or wrong length, all the stuffing around in the world is not gonna give the right curve from idle to WOT - to tailor a fuel curve the HSN and LSN need to operate 'independantly'.


I feel better about this now - at least I'll know not to keep persisting with the carb.

 

[Extreme RC Mods]

No problem, that's where the last long needle carb I came across ended up, in the bush.

 

[TK5310]

So a long needle carb is basically the RC equivalent of an SU carb!

It all makes sense now, if the LSN is governing the entire fuel curve other than peak rpm WOT then the only way to change the fuel curve is to change the shape of the needle, all you're doing by turning the screw is 'moving' the fuel curve up or down, not changing it.

SU's are a PITA to tune as well & often require a massive selection of needles & some time with linishing tape to get them right.

 

[2revo1maxx]

Maybe it needs an airflow restrictor.

 

[Greywolf74]

I learn something new every day

 

[2revo1maxx]

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.

 

[Punisher]

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.

Thanks for doing some research and putting up a post to help - I do appreciate it.

That tuning guide makes it sound easy doesn’t it?! A mate and I spent two hours trying to make it work. :whhooo:

Great in theory, but in the real world it isn’t so simple.


I'm sure you understand this, but for the sake of the forum;

In a short needle carb, most of the engine tune, from just off idle to wot is provided by the main circuit - HSN (high speed needle) . . .

The LSN (Low Speed Needle) is only short and controls AFR (Air Fuel Ratio) for idle and initial take off . . .

Sure, the engine may run a little rich in the middle, but this is not necessarily a bad thing.



In a long needle carb, from just off idle, to 75% of throttle, AFR tune is governed by the very long LSN . . . in fact, the Low Speed Needle in a long needle carb should be described as the 'main jet'.

The fuel available to the LSN is controlled by the HSN. (Perhaps the HSN in a long needle carb should be described as the 'fuel jet')



So what's happening, is fuel for the idle and 75% of the throttle is being tuned by the LSN . . .

The HSN controls the total volume of fuel to that needle and 15% (the last 15%) of the throttle = WOT.


Now, say having tuned the idle and hence 75% of the throttle, WOT is rich, very rich - so you reduce the HSN fuel - what happens? As the HSN controls the fuel to the LSN, idle and 75% throttle has been affected too.


The tuning guide above says "Come back to LSN for fine tune and consistent high idle."

Fine tune! In real life when you "Hit the track and tune the hsn." . . . it's not a fine tune adjustment required - a very small adjustment to the HSN dramatically affects the LSN. .5/12 on the HS will give you a lean bog! . . .

If your HS needs 16/12 taken out of it, the tuning process takes ages and becomes very frustrating.


The carb could be made to work, but I just dont have the time, energy or necessary mental disposition to do it. :\


As Robin and TK (TK5310) suggested - the place for this carb is the bin!


Thanks again for your input.

 

[2revo1maxx]

Trust me on this, that long needle carb is how all nitro engines should be, but they are a pita. If you can't get it, then it's time to swap it out. Personally, I hate to give up on stuff like that.

 

[chickenman242]

Hmm just trying to get my head around this
Basically a long needle carb needs to be tuned from the bottom first??
I had an argument at a hobby shop when I rebuilt my rb, this dude tried to tell me this was the way I needed to tune the rb which of course was not the case
Ultimately all fuel coming into the engine is governed by the hsn correct, so if thats the case then how can the engine be tuned successful from the lsn first it would be my understanding that you would constantly chasing your own tail
Stupid question but could you simply change the lsn

 

[2revo1maxx]

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.