Rich, here is an article I wrote for a local forum, and posted on here a while back.
It covers everything re idle gap and long low speed needle carbs on the GO motors.
It should help anyone struggling with tuning these carbs.


"I have cut and pasted an article I placed on our local RC forum here to help customers with the idle gap / LSN senario. Hope this helps to explain things for those out there who are struggling with this most common of tuning problems -

"THE AVERAGE WOMAN TUNE" - A COMMON PROBLEM
I have been asked by a customer to post something up on the most common tuning fault out there, both with the "long taper" LSN style carbs (GO and a couple of other brands), and most "short needle" carbs as well.
Over 90% of basic tuning faults revolve around having an idle gap that is too big.
This results in what we affectionaltly call "the average woman tune"

ie: A fat bottom, lean on top, with a wide gap (my sincere apologies to any of the fairer sex who may be reading this).

THE PROBLEM
The problem is setting the idle gap too wide and then trying to tune the motor.
The only way you can get the motor to run and idle "normally" with a big idle gap is to have the bottom end so rich that it semi floods the motor and makes it sound like the idle gap is correct.
Because this over rich "false idle" situation exists, now the only way to have the motor run half decent at higher revs is to cancel out the over rich bottom end by running the top end way too lean.
This will see the motor run "sort of ok" while the tank level is high and you are getting good tank pressure to push the fuel through the overly lean and closed off high speed needle.
When the tank level drops (usually below half), the tank pressure cannot build up enough after off throttle periods (cornering etc) to push enough fuel through the high speed needle resulting in lean bog and flat spots.

SYMPTOMS
If the situations below sound like your motor, then the average woman tune is most likely going to be your problem.

Your motor starts off rich and sluggish, but comes in at the end of a tank "machine gunning" at idle and running very hot.
Power sags towards the bottom of the tank. Motor runs hot.
Revs hold up when you go over jumps, despite backing right off the throttle.
Your motor comes in after a WOT burst and sits there idling really high for a few seconds before dropping down to a real low idle all of a sudden.

HOW TO TEST FOR THE RIGHT IDLE GAP
The correct idle gap for most motors with a good race tune is around .5 - .7mm in most cases.
If you were to remove the venturi and look down the throat of a well tuned carb, running a race tune for max performance and run time, a lot of people would be surprised just how small the idle gap actually is.
All motors will vary a little, so rather than to physically try and measure the idle gap, it is best to set the idle gap using one of the two methods below (or a combination of both).

COLD PINCH TEST - This is used when the motor is cold only.
Start the motor and let it run for 30 seconds or so on the starter box.
Pinch the fuel line off about 3cm back from the fuel nipple.
The motor will rev up and then die - this tells you two things:

The amount the motor revs up and how quickly it revs up is telling you about the idle gap. The length of time the motor runs before it dies is telling you how rich or lean the LSN is.
We are more interested in the idle gap at this point. What you should hear if the idle gap is set correct, is a slight, slow rise in revs (about 500rpm or so). The revs should hold at this slightly increased speed until the motor starves of fuel and dies.
If the revs chirp up very quickly and quite high - the idle gap is too big.
Adjust the idle gap down and keep re testing until the slow and slight rise in revs is attained. Your idle gap is now set ready for tuning the needles.

FREE REV TEST - If the motor is already hot, a good way to check for idle gap is to free rev the motor on the starter box several times to clear it out, then give it a decent rev and quickly apply the brakes.
If the revs hang up and the motor takes a few seconds to retuen to a normal idle - the idle gap is too big.
With the idle gap set correctly, when you rev the motor and apply the brakes, the motor should come back to idle straight away and sit there unchanged for some time.

WHY IT'S IMPORTANT
Because the low speed needle is part of the throttle slide assembly, and moves with the slide, it is very important to have the relationship between the slide and the needle correct before attempting to tune, by having the right idle gap.

THE LONG TAPERED NEEDLE
GO use the long tapered low speed needle becasue it gives a very smooth power band with no fuel delivery "transition" period between the low speed and high speed needles.
With the conventional short needle system, the low speed needle controls the flow of fuel into the motor (fuel it receives via the high speed needle) from idle up to about 1/3 to 1/2 throttle. Once the short needle pulls out of the fuel delivery tube, the amount of fuel the motor receives from this point on is directly controlled by the high speed needle setting.

This is where the long tapered low speed needle carbs differ greatly from the shorter needle carbs described above.
In the long needle carb, the high speed needle is basically just a fuel valve.
The long low speed needle never actually comes completely out of the fuel delivery tube, so it directly controls the amount of fuel the motor receives from idle right up to around 95% of WOT. At WOT the tapered tip of the low speed needle is still just inside the fuel delivery tube and more fuel can pass around the fine tapered tip, and it is only at this last 5% of WOT that the high speed needle plays a very small part.
The secret to the long needle carb is having the high speed needle set rich enough to allow sufficient fuel to get past the low speed needle in the fuel delivery tube at all times.

THE GARDEN HOSE COMPARISON
Going on what we have just said above, you can use your common garden hose to see just how the long needle carb works.
Effectively the high speed needle on your carb is just a tap - just like the tap on the wall that your garden hose is connected to.
The low speed needle is the adjustable spray nozzle on the end of your hose, delivering the water depending on how much you pull the trigger.
If the tap (high speed needle) is turned on enough, everytime you pull the trigger (open the low speed needle setting by pulling the throttle open) be it a small amount or a big amount, you will get the water (fuel) you require.
However, if you go and turn the tap nearly off (lean the high speed needle too much), then when you pull the trigger you will only get a trickle of water (fuel) instead of a spurt - your dreaded lean bog out of the corner etc.

That's why when you get lean bog with the long low speed needle carb, you need to RICHEN THE TOP END - NOT THE BOTTOM END.
By richening the bottom end you will start the downward spiral of the average woman tune. You richen the bottom, but it still lean bogs, so you richen it a little more. Now the motor starts to flood at idle and the idle drops, so you turn up the idle and raise the idle gap, moving the low speed needle as well and further richening the bottom end. Soon you will have a motor that is basically untunable unless you reset the idle gap and start from scratch.

TIPS
Set the idle gap as above before doing any other tuning.

If the motor won't idle at this gap - lean the bottom end 1/8 of a turn at a time until it will idle - DO NOT ADJUST THE IDLE GAP.
This will get the bottom end closer to where it needs to be, as it's generally the rich bottom end flooding the motor, not the idle gap that is causing the idle to be too low.

Tune the bottom end first, then the top end, then revisit the bottom end, then and only then tweak the idle gap if required.

If you change the high speed needle setting, you WILL change the low speed needle setting to some degree as well.

Any change to idle gap or high speed setting generally requires a corresponding change in the low speed needle setting to maintain a balanced tune.

Well that's it. Apologies for the sermon, but I was asked especially to clarify the things above by a customer for the benefit of others who have approached him with mis-tuned motors that fit exactly into this catagory.
Hope it all makes some sense and helps a few people to enjoy their motors a little more."