Hi Eivind,
You have a tuning problem.
What you describe is the classic wide idle gap scenario. This is why your motor is rich at a full tank and lean at the bottom of the tank.

The idle gap setting is too wide - allowing you to run the bottom end way too rich. The only way to get the motor to run with the bottom end this rich is to lean the top off.
It will run so -so like this when the tank is full, but as the tank level drops the lean top end (due to lower tank pressure) will cause lean bog etc because the HSN is choking off fuel supply.

Set the idle gap to .5 - .7mm or use the "cold pinch test" described in our tuning guide on the Tech Tips page of our web site at http://www.go-racing.co.nz

There is nothing wrong with the carb - trust me. You have just fallen into the age old trick of having too wide an idle gap with the long lsn carb.
Follow the tuning guide from scratch after resetting the needles to factory settings as described in the guide.

This may well be your problem with the lack of bottom end too. The GX5's are absoulute stump pullers, and have more bottom end than you will need in most cases.
You probably have sagging bottom end because it's too rich, and then plenty of top end because that's too lean.

Hope this helps. Give it a go and post back to let us know how you are getting on.

Pull that shimm out voodoo :)
Won't change the tune very much, but you will have a lot more zing in the motor without it :nod:

Hi, thanks for the reply.
When I was breaking in the engine I did the pinch thing and the bottom end was good, but then when I went to the track and ran the car about a bit the next week, the tune started to change during the day so I had one of the local experts help me out and he did mess with the idle gap setting, so you could be correct there.

I will go to a parking lot and play around with the settings and try to get everything correct in the bottom end, and see how I get on.

Hi Eivind - most local experts don't have experiance with the long needle LSN carbs, and invariably go straight to the HSN and lean it if the motor sounds a little rich. This is where your lean bog comes from. If he cranked up the idle gap, this is where your rich bottom has come from.
I sure the gentleman in question is very knowledgable and helpful - so I'm not meaning to shun his efforts to help.
But if he hasn't come across the long lsn type carb before, then he is 90% sure to tune it like a short needle carb, and it won't work out.

It is imperitive that the idle gap is set before the needles are tuned, then not adjusted again until all the needle setting has been done.
The relationship between the throttle slide (idle gap) and the LSN needle is most important. Raising the idle will richen the motor, and droping the idle will lean the motor - so compensating adjustments need to be made whenever the idle gap is changed as a rule.

Have a play and let us know how you get on. Be aware that pinch tests (particularly for setting the idle gap) only give the required results before the motor gets warm.
Once the crankcase is heated sufficiently, fuel vapourisation is such that pinching off the fuel line will result in a rapid rise in revs regardless of idle gap settings.

Will do Flanno. Cheers

hehehe the average woman tune raises her head yet again :lol:

Richen the top end an hour or two.

The pinch test when cold to warm is a guide only to get you close and is a 2 part test, 1- how high it revs just before it dies tells you where your idle gap is set and 2- how long it takes to rev up and die tells you how rich or lean it is.

When engine is up to temp or close to it, rev the engine a few short times in short bursts to clear the engine then hit the brakes, listen carefully to the idle, if straight away it is idling high and then drops after a second or more then it's an average woman tuned bottom end.

Idealy you should be able to clear the engine and hit the brakes and the engine should sit and idle with no drop in revs for at least 25 sec and even then it will only be a slight drop.

With very small adjustments each time,
!; lean the LSN and clear the engine and apply the brakes and start counting while listening closely to the revs to see what it does, if it drops quickly repeat step one untill it takes atleast 7 sec to drop.
2; then reduce the idle gap a little and clear it again and see what happens,

keep repeating step one and 2 untill it sits idle for more than 25 sec with no drop or raise, if idle is irrataic and raises then you are a little lean on the bottom and have gone too far.

I know what to listen for etc in person and what step to take, but it's hard to put into words on here, but I hope that helps.

The famous "nitro whisperer" strikes again, onya flanno :D

Too true mate, small adjustments, and a good ear is worth a thousand fingers.
A little off RC tuning, but same principals apply:
Taught chain saw and whipper snipper tuning for many years as a TAFE Teacher.
Would show classes how to tune same, asked them to take note of the engine sound, make sure they had it "right", then send them of to lunch.
During lunch, I would turn the low speed needle in 1/2 a turn and then would ask students to start their respective engines. After pulling on the cords and getting blisters, with no start, they would admit defeat.
The point here is, if needed, go back to original settings (or factory recommended) , return from there, SLOWLY, and listen to what the engine is trying to tell you.
Yes it doesn't happen overnite, but if you really concentrate on developing a good ear for engine tune, it certainly helps.

so im on my 9th tank now.. and from what i hear and feel.. the engine is altering its tune.. a bit.. staying at 89-94c .. i did adjust the screw on the throtle , turing it clockwise.. 1/4.. not trying to change it much at the moment...

with the MG66 ... do i leave it at the standard setting.. from the straight position its been been put 1/4 clockwise.. and the fuel adjustment screw ontop has not been changed much if anything. slight tickle in and then out to see.. how it responds.. spits and smokes nicely... white smoke.. so it looks to be well lubed.

so towards the end of the 9th tank... the engine switched of.. due to the windy weather it was pretty tricky to keep the engine at 100-110c.. and the annoying thing that happened.. keep inmind im still running it in.., the piston got stuck just when it switched of.. so i had to get the top of the glowplug wrench and placed it flat ontop of the piston.. and tapped it gently2-3 times.. and clunk.. it came loose.. and all was fine.. , phew!.. .. anyway im of to finish running the half a tank.. and then go onto the 10th tank..

Hey mate, craig here from over the ditch in NZ, I think i read a few posts back that your doing your runin on the MG66 by letting it idle through the tanks.
We have found its better to run the motor in on a runin bench with a prop at varied revs or in the car and driven around on the ground to get some load on the motor.
Doing the first tank on the starter box isn't bad for it, just up the idle a 1/2 a mm and set the lsn so it will keep running.
Also when doing tanks running in, heat cycle it by letting the motor cool down between tanks...then reheat it and go again, also if the motor jams just lever it free by the flywheel using a screwdriver off the chassis and make sure it never stops and cools with the piston stuck at the top, it will be very hard to unlock it then.

Eivind, what clutch are you running? i alway ran 1.1mm springs on a 3 shoe and 1mm on the 4 shoes, never been a fan of early clutches, with .21s even motors with strong bottom end engaging the clutch a few 1000 rpms higher than the std 1mm springs do will give you more take off power... also check you fuel tank isn't too tight on the rubber gromets where it screwed to the posts, that can cause tuning issues

if you do what guys like Shane and flanno have suggested your GO carb will be perfectly fine.
Something worth mentioning is the GO engines start rich from cold and even a well tuned one will be a bit rich for the first few minutes.

;)

I tried to reply to the 2 posts earlier, but for some reason every time It kept crashing, so I gave up trying to post... as I am at work ATM and can't keep trying, for ever leaving factory unattended.

thanks shane. just another question i'm running this engine in a buggy. a little less zing might help with drivability:nod:. if i leave the shim in will it cause any issues with the engine.

cheers

NO, actualy will be easier to tune too. (larger tuning window)

You will need to remove it at some point down the track thou, once you get a hold on keeping that buggy in line hehehe jks :lol: