Steve O'Hara's Easy Idle KT-100 Setup (WB3A carb)

Ted Hamilton

Helmet Painter / Racer
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The article you posted has some good information but you do not need to modify the butterfly to make your KT100 start easy, idle and run well on the track through the whole range.
I have been helping a new karter with his KT100 sprinter for a couple months and he has made several trips to the track by himself to practice and this is is routine.... he plugs in the starter and hits the button and the motor starts and sits there idling without touching the throttle pedal while he puts on his helmet and driving apparel. The he pushes the kart over to the grid area and gets in and drives his 10 to 20 laps without touching the carb. The kart is equipped with a 2 disk L&T wet clutch that does not get hot even though he does not lift the rear and let the wheels turn while getting dressed.... the idle is low enough to allow the kart to sit still without a problem.
I set the carb up for him as follows:
Arm height is set with the diaphram end fork approximately .020" above flush with the circuit plate. 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.
You should see the people at the track stare at my guy when he walks across the pits pushing his KT100 powered kart while it is idling away like a motorcycle... they never see that but only because the majority of the guys running the Walbro carbs set them up wrong and conclude that they won't or can't idle.
Start with those settings and fine tune... you will probably end up a little leaner on the high speed jet since you are not running long stints.
Good luck
Steve O'Hara
and a reply:

I've seen you post this time and time again and it seems like nobody hears you. I was one that set up my carb with low pop-off and needle setting as most run and questioned your set-up listed above.

So I tried your set up for a new karter and it worked perfect! No messing with the carb, no messing with the clutch, the kid was able to go out and learn to drive without worring about nothing more then staying on the track. The motor ran clean, it pulled good, no bogs, easy to start..

So I thought, well it works for the new guys how about someone that has a few years under their belt. I changed my carb over to setting you listed above, tweeked the high speed just a bit once out on the track and went just as fast with your set up as I did with the other.

And this was after listening to those saying, you'll run rich, you'll get left behind, you'll foul a plug, it don't work, Diz told me to do this way, Joe Bob said to do this way, I've know someone that said it don't work, your engine can't idle and be fast, etc etc..

It will work perfect for the new karter, and an experenced karter can make it work just as well to run up front.

Stoney Creek Motorsports
Burris Racing
Is this setup for a stock KT with pipe or visually stock with internal mods? Or it doesnt matter? I will try this setup when I get my stock KT back together.

Thanks for the post Ted.
The setup works great with most exhaust and clutch combinations for new drivers, auto crossers, and recreational drivers that don't run their engine package right on the ragged edge. For very experienced drivers that can be on the throttle nearly 100% of the time running a pipe and slipping the clutch to 10K or more the setup will tend to begin running a bit lean in the mid range after 5+ laps of heat buildup. It really comes down to temp control vs. ease of tuning and consistency. I find that all but the most experienced drivers prefer the setup that idles and runs clean with stable jet settings. As they get better and their lap times fall to within a second or so of the fastest guys they will begin to accept the trade off of a more difficult carb tuning process for the small gains that can be found by dumping more fuel on the engine in the low to mid rpm band. Unfortunately the only way to get the extra fuel to the motor for hard pulls at 10 to 11K is to lower the popoff and raise the arm and once that change is made the carb gets much harder to manage, won't idle, hard to start and the jet settings get way out on the low and way in on the high.
I have had many auto crossers report that the changes to the carb made dramatic improvements in their performance as they need to be able to fire up on command and have clean crisp performance instantly. Others that really benefit are the weekend recreational drivers that often share a kart between drivers with widely differing driving skills. One jet setting fits all if the carb is set up as I have recommended. With the more common setup, the whole group is bummed as the newbie driver fouls plugs and ruins the clutch trying to deal with a carb that wants to flood the motor unless the throttle is held wide open from the moment the engine fires.
It's definitely very much a driver preference decision.
Steve O'Hara
I run my twins where they work like this however they are opens and the carbs are set different.. if I set them like one engine where they load when you lift then one will stall and the other will continue to rotate it till it fouls so I came up with a set up where when I lift they will come down clean if I lift when I come off they are both still running at about 3500 and very clean...the set up I have relys on high needle just dumping all the hot mix that the ignitions will fire..with twins there is allway..allways enough power off the turns
Steve -- with this setup can you still get a cooling burst by choking the engine manually like an ICA?
Ya, if you run an airbox. Any time you can choke a carb it's gonna give the engine a cooling rush of fuel. Saying that you can set a carb at a particular setting and forget it and it will work great is an over simplification for a new tuner, he should realize that it won't be best for all senarios he may encounter and needs to be tuned for conditions. I learned a long time ago that what worked great in middle Tn. wasn't so good in Tallahasee, Fl. After running week after week in Tn. and getting my tune just right, then showing up at Hymen Meyers speedway a fast 1100 ft high bank track and proceeding to stick my KT on the backstretch at full song with the same settings. If you stay in your area and it worked this week , chances are it will continue to work, and air temp will be the biggest factor affecting the tune. Jon
Jon makes a good point about moving from one track to another especially if the elevations change or the weather conditions differ a lot. Just to be clear, the carb settings I advocate for easier tuning do not mean there is NO tuning. The carb will still need to be dialed in for the kind of air you'll be running in on any given day. If tuned properly the first session the engine will idle and start easy, run clean out of the pits and on parade laps and tolerate different level drivers without having to change the settings over and over through the day or during a session.
There are certain fundamental tuning procedures that every driver should follow every time they go to the track and one of the most important is to make sure the engine can be made to 4 cycle at or near top end before putting your head down and driving hard continuous laps in the first session on the track. No matter what I run or where I run, I never run a two stroke hard until I have run it rich enough to 4 cycle for at least one lap. Once you know it can deliver enough fuel then you can fine tune by leaning the mixture just enough to eliminate the 4 cycle and then go ahead and run hard.
I always teach my new guys to follow that process, then run 6 to 10 laps and then do a plug cut, come in and read the plug. Nearly always the plug will be dark and wet. If the driver is very consistent I will suggest a small tweak to the leaner side and have them run another session and repeat the plug cut/read until they get the setting that gets them to a medium/dark brown center.
If the driver is one who I expect to improve as the day goes on I have them leave the engine rich and most of the time the plug read will move in the right direction as they get more confidence and increase the time they are at WOT.
One other note, the less experienced drivers who benefit from the easy tune setup always do better running pump gas as opposed to higher octane race gas. The pump gas will run clean at a richer setting than the race gas. In fact, some of the race gas blends don't run worth a darn in a KT100 or HPV unless the engine is tuned so lean the plug is light brown or gray and very dry. I hate to see engines run like that for recreational driving, club racing outings etc. The performance gain is tiny, if any but the chance of engine damage is much higher.
Again guys, these discussions are directed to the folks that have a bad experience with the KTs and HPVs when the carbs are set up to run with the jets way open on the low and nearly closed on the high. There is a place for carbs set up that way and by the time the drivers are good enough to benefit from the difference in performance that those settings offer they are also good enough to figure out how to get out of the pits, do parade laps etc.
I recently spent some time with some of my buddies that race the HPVs in a very competitive series and I ran a carb set up to idle and tune at 5/8ths on the HS and 1.25 on the Low, 18psi pop off and a pretty low arm setting when running with them. At the Perris track in So Cal I was right on pace and the package ran great. At the Adams track the same carb setup just could not keep up at race pace as it would just get too lean in a couple spots right around 10K pulling off tight corners and it was costing me a few tenths each lap.
For a twin I would go with the easy tune carb setup every time. Clean crisp performance means helps a twin a lot and they accelerate so fast they never get the low speed heat buildup that the singles suffer.
Steve O'Hara
Very good, reliable information right there! I race my twins at the safe settings (with a little of my own spin) and never have to touch them, always with plenty of torque and power.

Brian #89
Great article

I got my engine back from builder/dyno. He called out for h 1/2turn L 2-1/2turn. I couldn't get around the first turn without stalling. I was frustrated. I read this article and gave it a try. Perfect, didn't quite idle but I could get some laps down without stalling. Thanks for the info.
wb3a carby idle issue, air leak

i recently brought a kart and had some issues with it not wanting to idle and very hard to start.
so i searched the net for similar a issue, and seen this. thought i would try these settings.
after testing, it still didn't want to start/idle, it would take allot of throttle to make it fire. then very laggy down low.
i suspected an air leak so i brought a throttle shaft. while apart i thought i will pressure test motor, crank seals etc.
when i tried to seal intake manifold, i noticed some serious warpage on the aluminum spacer between carby and barrel.
i lapped all the faces flat including the carby front face.
it fired first time (electric starter), no throttle etc. only had to increase idle screw a slight bit and it idled.. smooth :D

hope this helps some 1 else with similar issue

thanks Steve for the settings.
should make for a fun day next time i take it out. not a frustrating one.
I tried Steve's settings on a customer who runs auto cross where you have to wait with the engine running for your turn to go. Worked out great.

Very good advice here. I remember buying a Yamaha down in the Spanish East coast, sea level , more humid too, set carb by a very experienced young guy who later became a Formula One test driver, and when I tried that engine up North at my home track, higher altitude and not humid at all, it took me some time to find the right carb settings for that track compared to the other one.
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When I started karting back in 1980 the way I was told to set up my carb was what is called running the HIGH SIDE, neddles at H 3/4 - L 7/8. These are the settings suggested in the book "Racing the Yamaha KT100-S. Of course I tuned on the track but something I also did, running a engine clutch, was setting the kart on the ground with engine running and turning the low side down while bumping the gas until I got a boog then turning it back up, usually to 5/8. Afterwards I would just tune with the high side. My plug was always a medium chocolate brown. This worked for me with great success till the mid 90`s when I stopped racing. When I started up again in 2000 everyone was running the LOW SIDE with the settings that Steve suggested. What`s the difference, is one way better than the other? Also, what does it mean to run rich enough to 4 cycle your engine? What does it feel or sound like? I don`t think I`ve ever done it.

I`d like to add that when I started racing karts I ran some sprint and street courses then went to dirt only.
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Staggerdog , your method was probably as good as any. I did it a little differently, I'd turn that low out until it tried to load and wouldn't rev , then brought it in a little at a time until it cleaned up and would pull revs off the bottom. We probably ended up pretty close to the same setting. Then get it up on top end and set the high side to get my head temps and plug color where I wanted them. 4 cycleing is when it breaks up or stutters on top end, if it's just a little rich it will do it at your highest rpm. EC used to call it " shooting ducks ". Jon
I got my engine back from builder/. He called out for h 1/2turn L 2-1/2turn. I couldn't get around the first turn without stalling. I was frustrated. I read this article and gave it a try. Perfect, didn't quite idle but I could get some laps down without stalling. Thanks for the info.
I stopped racing in 1979, but I built a dyno in 1985. I raced the KT 100 myself for three years. During those three years, I, like most everybody else, tuned my carb pretty much like Steve suggests. But then the rules changed in about 1981. IKF came out with new rules for combustion chamber CC's and port Heights. This really change the game. Where as we used to slip the clutch at about 9,200, we now had to slip the clutch at 10,200.

One of the first things I noticed when I got the dyno working was, that a KT 100, with the carb set at 1.5 L and 0.25 H, the engine ran really good. I found that by setting the low speed open even more, the low end torque increased.

At a race in San Diego, with the air density at about 103, and the ambient temp at about 82°, we had to open the low speed to 2 1/2 turns, and even that was not quite enough. 2 1/2 turns is wide open on low speed. More than 2 1/2 does no good. Adding another race, where the track was up in the mountains, (foothills really) in the afternoon, when the air density was down quite a bit, we actually had the high speed needle closed. And just to let you, know we were walking away with that race.

Now with these settings, the engine will not sit and idle, they're only good for racing real hard. Steve's settings are real good for a novice just trying to make some laps, just be aware when you get into a real race, there are better ways to set the carb.

If you want a lesson on why this is so, just ask.

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)
I wonder about the varying load conditions on LTO dirt vs. asphalt sprint... I'm going to try these settings, mostly so I can worry about other issues with my duals. We'll see what happens.


I think you'll find the so called Steve O'Hara settings will work very well on your twin.
Duals accelerate much faster than a single so the engines will spend less time at or close to the peak torque speed of the engine and that is where the trouble with heat control is always most severe. The common settings of way out on the low and way in on the high evolved as a way to feed the engines as much fuel as possible right at clutch engagement speed and then have the engine lean out to produce maximum revs way up high in the power band.
Those setting were particularly important when running some the highly developed pipes that came along in the 90s.
With your dual, good throttle response is a primary concern and the "O'Hara" settings produce good throttle response even when the driver is off the throttle quite a bit as you often are when running a dual on a tight sprint track. The settings are also much more driver friendly when running pace laps or formation laps for rolling starts... you can leave the settings alone and don't have to be twisting needles shortly after the green flag is shown as you would with the LS dominated settings.
For the record, I have never been able to make a KT100 run worth a darn with a low speed needle setting under 1.25 turns on a sprint track. We used to see those 7/8 low/3/4 high settings work once in a while in the enduro racing days but that was on tracks where we fired the engine with three seconds to go before the green flag and went full throttle for a cold standing start and once we were up to speed we ran 90% of the race at or close to max revs. We were using slippy pipes that allowed us to alter the shape of the pipe to maximize top end performance so naturally we needed to control the temp way up high by dumping more fuel on the engine. The road race track gearing kept us at around the mid 13K range for top rpm so the setup was quite a bit different than the carb settings that work well on sprint tracks.
Steve O'Hara