Yamaha KT100s Carbu settings

P

Pota

New member
Hello guys,

Im new here, from Norway :)
I just bought a Gokart with Yamaha KT100s.
Ive been told that i can adjust the HP of the Yamaha by adjusting the Carb.
Im not gonna race any classes, so I just want as much HP as possible!
How do I do this? And how much HP can I gain?
Does it excists any tutorial on this :)

Thanks alot for all replies!


Best Regards

Thomas
 
I just got this from my builder:"With the pipe (SR-Y), slip the clutch about 10000 to 10200 and turn the engine approx. 15000. Carb should be close to 1 3/4 to 2 on the low and 1/4 to 3/8 on the high"

You should be somewhere around here, and watch head temp if you can. I have mine set to alert me when I get to 400 degrees so I can keep an eye on it.

Oh and welcome to "here" from Norway
 
Some good advice! Still, there's more to it. It makes a difference what exhaust you have on the engine. It makes a difference how high the fulcrum arm is set. It makes a difference how many CC's you have in the combustion chamber. It makes a difference what the compression pressure is. It makes a difference what the air density is. The advice you've been given could be correct, but it's better to know than to just go off without knowing.

When you get the carbs right, obviously the horsepower will be at its best. If you get too rich, the horsepower will be down, but if you get too lean you can hurt the engine.

Fill out your profile so we know where you're at. Maybe somebody in your area could help you who's closer than we are.

From the desk of Al Nunley
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory.(Al Nunley)
 
jamiespop said:
I just got this from my builder:"With the pipe (SR-Y), slip the clutch about 10000 to 10200 and turn the engine approx. 15000. Carb should be close to 1 3/4 to 2 on the low and 1/4 to 3/8 on the high"

You should be somewhere around here, and watch head temp if you can. I have mine set to alert me when I get to 400 degrees so I can keep an eye on it.

Oh and welcome to "here" from Norway
Click to expand...

This is good advice. will keep you in the safe zone for all conditions. There's not a huge amount of power to be gained by having the carb perfect so what you have been told is a good setting. Especially if you are just getting a feel for the carb/engine. Just keep an eye on the low when you start running caution laps. It may be to rich if you run to long under idle or caution lap speed. How long is too long, IDK, when I go to caution lap speed or below I always close the low to 1 turn. But the KT carb thread is supposed to be very good with a set and forget type setting. You will need to run the pop-off pressure at the stock setting of 18psi. A lot of engine builders in the dd setups hate the stock pop-off pressure but it works well with less trouble.
 
Hello guys,

Since im new to this, I need to do this slow to learn :)

From Steve's post i read this:
"The pop off pressure is set at 18 pounds (stock spring)
Low speed needle is set and left at 1.25 turns and the High speed needle is set at one half turn. The idle adjustment screw is backed off until the butterfly is fully closed and then brought back until it just touchese the lever on the end of the throttle shaft and then is screwed in 1 full turn."

Does this mean that the LOW speed needle is turned one turn and then 0,25? and the High speed one and a half turn (1,5)?

And what way, clockwise or anticlockwise? And where do you begin? If you turn it clockwise, do you first screw the needle all the way anticlockwise till it dosent go any further befor you turn it 1,25 and 1,5?
Who is High and who is low, is it marked? Or is the blue needle as linked the HIGH?
KT100-New-2.jpg


Should I do anything to the pop off pressure, and what is it?

What does he mean with:
"The idle adjustment screw is backed off until the butterfly is fully closed and then brought back until it just touchese the lever on the end of the throttle shaft and then is screwed in 1 full turn."
I know its the butterfly in the carb. But what does he mean, "and then is screweed in 1 full turn"?

Should the engine be running while i do all this?

Thanks for all replies, and sorry for my english and loads of questions :)
 
If the picture in your latest thread is a picture of your engine, the carb should be setup to Swedtech Racing Engines specifications with respect to lever height and pop-off. Since these two items have items have a direct effect on desired needle settings I suggest you contact them at:

https://www.google.com/search?q=swe...&rls=com.microsoft:en-us:IE-SearchBox&ie=&oe=

The blue needle is the low, yes carb body us marked L and H where the needles enter the carb body.

To adjust a given needle gently turn it in clockwise until it is seated on the carb body the turn counter-clockwise to the desired setting.

"and then screweed in one full turn" means to turn the idle adjustment screw clockwise one full turn thus slightly opening the butterfly.
 
No, its not my engine. I think my is stock with walboro (something) carb :)
So the only thing i need to do is screw my low to 0,25 and the high to 1,5?
Or do i need to adjust idle adjustment screw?

And do I need to do something before setting the carb other than turning of the engine? Unplugg the spark or ?
 
The engine should not be running when setting the initial settings.

Pop-off is the required fuel pressure required to overcome the spring tension thus allowing the needle to open and fuel to enter the carb.

If you have a stock Walbro WB3A carb, the lever height and spring tension, hence pop-off, should be a good starting point for Steve's needle settings and idle speed adjustment. LOW SPEED NEEDLE 1.25 TURNS, HIGH SPEED NEEDLE 0.5 turn THEN ADJUST THE IDLE ADJUSTMENT SCREW.

Good luck,

Paul
 
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You need to know that starting it with the low end at 1.5 turns out will usually foul plugs, so it might be better to use around 1 turn for starting and then remember to open up some after getting on track, and closing when stopping, just remember to always bring a new plug and wrench with you to the grid.
 
Thanks guys... But how should the idle speed be adjusted? Cant it stay the way it is?
And what kind of Sparkplug is best for the Yamaha KT100s?

Jack, should both needle be turned of after race or only the low?
 
If your carb is set up like the one in the picture, you can see that you can't close the high-speed needle without first removing the low-speed needle, the one with the blue plastic on it. That low-speed needle is an aftermarket addition. The WB3 originally came with a much shorter screw that you adjusted with a screwdriver. With the original screw, the high-speed could be turned, in or out, as much as you wanted.

When you close the needles, be careful not to force them. Too much pressure and you might damage the hole that the needle goes into.

You're going to hear a lot of different theories on where the needles should be set. Steve's are good for those that are not all that experienced. They're not bad in any way, but you might find more experienced drivers, (not that Steve is not experienced) might have different ideas.

The WB3 is a really simple device, but does take a little training, on your part, to get it functioning right. First off, and this is just in my opinion, there is never a reason for the engine to just sit and idle. You can set the carb so it will idle, but it's never going to idle when you're on the track racing, not if you're planning on being competitive anyway.

Be sure to always have a filter in-line between the tank and the carburetor. The smallest piece of debris in the carb can hurt its function.

I found, early on, that having a rebuild kit in my toolbox, at all times, was a good idea.

From the desk of Al Nunley
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory.(Al Nunley)
 
Paul Ahrens said:
The engine should not be running when setting the initial settings.

Pop-off is the required fuel pressure required to overcome the spring tension thus allowing the needle to open and fuel to enter the carb.

If you have a stock Walbro WB3A carb, the lever height and spring tension, hence pop-off, should be a good starting point for Steve's needle settings and idle speed adjustment. LOW SPEED NEEDLE 1.25 TURNS, HIGH SPEED NEEDLE 0.5 turn THEN ADJUST THE IDLE ADJUSTMENT SCREW.

Good luck,

Paul
Click to expand...

Hi Paul, just so you know the pop-off pressure is not lifted by fuel pressure. The diaphragm is lifted by the vac that is generated in the venturi. Small vacuum over a large surface area like the diaphragm has enough pressure or negative pressure to lift the needle off the seat and allow fuel through. Fuel pressure generated by the carb pump is very low, around 3 psi. I used to think the fuel pressure lifted the needle to but that's not the case. But the pop-off gauge does use that principle to set the seat pressure. It confuses most people as it did me as well. Hope I helped.
 
Like Al says, the idle speed is relatively unimportant, recommended needle settings are basicly for starting, usually they will need more adjustment on track, except low end, (Tall end needle), that will usually need to be reduced from running setting back to starting setting, 3/4 to 1 turn. We run NGK BR10EIX plugs.
 
Hey again guys...
I started my gokart today. I use some sort of Denson sparkplug, similar to NKG b9ES.
What I experienced is that when going full throttle (100%) it kinda powered down... if i went back to 90% it went full throttle again.
What could be wrong?
And how should i warm the engine up?
 
I sure hope that running wasn't on the stand? If it was while running on the ground, It would seem to me that you need to learn to adjust the top end, (short) needle while running. Too rich or too lean can give that result. You need to have a temp/ tach guage combo.
 
Carbs

Pota,

from your description it sounds like the engine is starving at wide open throttle and when you back off a little it gets better. The simple way to see if that is the case is to open the high speed needle a little to see if it gets better. Here's another tip that will help you... when a 2 cycle engine is running rich it will produce a large amount of white smoke from the exhaust, when lean it will produce a little or none. If you start your engine and it tends to die when you crack it to full throttle quickly open the high speed needle. If it runs very rough and blows out a lot of smoke you want to lean it on the high speed needle.
Steve O'Hara
 
I see. I was running on it a stand, is that a problem?
When you mean alittle, do you mean 1/4 of a turn?
Should i do this while its not running?
 
Running an engine full throttle on a stand without any load is not a good idea. Very easy to seize or break any part.
 
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