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

Steve,
I am new to karting. My boys and I built a dirt track in our backyard. We bought a go kart with a KT100 in it, which I just rebuilt the carb using your setup idle. Have not been able to drive it yet, but it idles. What I am wanting to know now is can I use this setup on my son's 60cc 2 stroke IAME with a Tillotson carb?

Thanks,
Jim
 
Jim, your gonna find that what works on a 100cc engine with a WB3A is not gonna directly relate to your Gazelle 60 with a Tillotson carb, the fuel circuitry is very different. Similar settings would be purely coincidental although it's fairly typical with a Tilly to run more low side needle than the high side. I would also add that 18 lbs pop off would be a little high for the carb on your Gazelle because of the fairly large venturi size for only a 60 cc engine. Because of that factor it tends to be one of the more difficult setups to tune correctly, and presents it's own particular challenges to setting needles for the best overall track performance. Jon
 
Jim,

Although I have no first hand experience with the 60cc Gazelle I did spend a little time working with the 80cc Comers back when my son was around 12 years old and I would have to agree with Jon that the settings discussed in this thread I recommended for making the Walbro/KT100 package more user friendly would not be what I would expect to work on the 60cc motor with the Tilly carb.
Based on my work with the Comer/Tilly I would expect your setup will need a much lower pop off pressure... most likely around 8 psi otherwise it will just starve or run too lean at low rpms no matter where you set the jets.
Steve O'Hara
 
i have this setup on my dual yamahas now it idles and gos!!! and don't have to tune on it made the duals very nice . now just need to get clutches dialed in.........
 
If it were me, I'd set the clutch on those duals about 500 rpm low, and rely on the torque and reduce wear on the clutches... But that's just me...Glad to hear some confirmation that it worked for duals...
 
steve is the man on most any thing KART EXPERENCE and smarts 8 time IKF champion doing most all the work himself ! he has helped me with all my vintage mccullochs jon was ec's right hand man i am allmost sure ec built me a HL355 for my big cubic inch open flatheads best carb i ever had !!!! jon will that hl355 work on my 2 cycles ec called it a stage one and the carb has ecs number on it , i am sure he kept records on all his good carbs steve built me a BDC-16 mac carb it is my best mac carb (alky) i want to try a hr-191 on a mac it came on my copperhead us820 engine new it is still set for gas now but i just been useing gas lateley anyway what is difference in a hr-181 and a 191 one experenced mac guy told me the 191 hr would not work as good as a hr181 what do you'all say ( jon &steve) i seen a video of a racer runnin a hr181 gas on a stock 101 enduro kart race it was runnin strong pullin hard and he won that day is there alot of difference in a 181 and a 191 HR tilly they seem to be close but i am not experenced on 2 cycles ?
 
Steve, what's the lowest pop off and highest arm height you can get the KT100 to idle at with reasonable needle settings for a fairly tight track? It just has to idle long enough for me to jump in the seat.

Thanks,
Sundog
 
Does anyone know if Steve's settings work well for direct drive kt100 sprint? I have an older kart that will be used recreationally and I would like to be able to bump start it solo.
 
Guys, the settings I recommend work very well with a direct drive setup. Might want to go another half turn in on the idle speed adjustment screw to make it easier to push start but you want to be careful that you don't go too far and end up watching the kart drive away from you before you can jump in the seat. When I ran direct drive karts I still used a plug in starter to start the engine on a kart stand and warm it up and get all the air out of the fuel system. Before shutting it off I would throttle it up pretty high a couple short bursts to clean it out and then let it idle to see what the idle speed was with the engine warm and clean. You will want it to idle up around 3K... definitely a little faster than a chain saw or two cycle motorcycle idle but not so high it wants to run away with no load on it. Once the idle adjustment is made stall it with the brakes... not by choking it out. Going through that routine will usually result in very quick starts when push starting.
Sundog... you can make the engine idle with popoff as low as 6 psi and the arm can be set as high as the carb will allow and still get the inlet needle to seal. With those settings you will have to virtually close the HS jet and the LS jet will have to be under 1 turn. You won't like that setup as it will starve on initial throttle application. To eliminate the transitional starve you'll have to open the LS jet a bunch and then in the range of peak power the pump output pressure will overpower the fulcrum arm spring and the engine will be excessively rich. Depending on the pipe you use that may be a good thing as long as you know how to manage the carb on the fly. If you want to set the carb and forget it my recommended settings work great for most racers. If you don't mind the constant adjusting for different conditions then you can get away with all kinds of combinations. My impression from your posts is that you have the experience to handle managing the carb while driving so you can try different settings and see what works best for you. On the other hand, you may want to just take the carb tuning out of the equation and focus on the other things that produce good lap times like chassis settings, clutch tuning, driving technique and so on.
Steve O'Hara
 
Thanks Steve you got me thinking on carbs. You control the idle with a high pop off and low arm so you can run a fairly open low jet for on track.

Just for starting convenience, if I run my usual 10 psi and .050" below the gasket I could still get it to idle if I simply crank the low jet way in until it idles. Then once I'm in the seat I can open the low jet where ever I want it and go.

Sundog
 
Steve, what's the lowest pop off and highest arm height you can get the KT100 to idle at with reasonable needle settings for a fairly tight track? It just has to idle long enough for me to jump in the seat.

Thanks,
Sundog
Strange, I can't remember ever wanting my KT100 to idle. Maybe, if you spun out, it might be nice, but there's no chance you're going to win after you spun out, so who cares.

I've had some success tuning for people running the KT 100. I always raise the arm level with the back of the carb with 10 pounds Popoff. You need some Popoff, and 10 pounds is some. I've tried, on advice from somebody who wins a lot of races, four pounds, and I've tried 15+, and saw no difference. My opinion of that carburetor is; the spring is only there to keep the needle closed while the chainsaw is sitting on the shelf, maybe upside down or on its end, and you don't want gas draining into the engine. I believe, I have no way to prove it, that that needle never closes while the engine is running. An engine needs fuel to run, how can it run with that needle shut.
 
Well, if you can imagine a race track with no one there except yourself and you have to start the engine then jump in the seat, you want it to keep running for about 5-10 seconds with no throttle. Or maybe you're the last guy off the grid after you helped everyone else get started then you have to start yourself. The inlet needle doesn't close when it's running but it can change the fuel signal to the low jet in certain modes.

I had one track with a long straight long braking zone to a very tight turn then a short shoot to another tight turn. After the first turn you go full throttle to the next turn where you get off the throttle for a second then you open the throttle again and nothing, like some one pulled the plug wire. Figure that one out.

Sundog
 
SteveO-
Would anything change for an enduro running an axle clutch?
And for big tracks like Mid-Ohio or Daytona? Still check for 4cycle at end of straight?
 
Todd,
The so called "Steve O'Hara Easy Idle Setup" is definitely not the way to go for the enduro racing setup. Enduro or road racing as it is more commonly called these days on long high speed tracks places a much different burden on the carb than your typical sprint track or even the small dirt oval. When gearing for 90 to 100+ mph and using an axle clutch the motor will be forced to run at or very near peak power for relatively long intervals with slow rates of acceleration. Those demands on the engine create a lot of heat and the carb has to deliver all the fuel you can get out of it. For that reason the preferred setup for road racing will call for much lower pop off pressure in the 6 to 8 psi range and a fulcrum arm setting as high as the arm can be set and still get a seal on the inlet needle when the cover plate is installed. With the carb set up that way the engines are very hard to manage at low speed so the typical starting procedure will cause the engine to be started only seconds before the green flag is waived. I used to start my KT100s with 4-5 seconds left before the flag was expected to fly and the engine was not allowed to drop below 6 or 7 K while waiting for the flag to drop.... then full throttle for the long hard pull off the standing start. Depending on the setup and track the initial starts could cause the engine to run right at clutch stall speed for 20 to 30 seconds before the kart was up to a speed that allowed the clutch to lock in. The driver must be conscious of the engine speed in the longest braking zones leading to the tight corners as the engine will die with those carb settings unless the driver blips the throttle frequently while slowing the kart. Lot's of new guys learn the hard way early on if they have the carb set up for the best performance as they stall the engine slowing for a tight corner and end up needing a tow back to the pits.
Jet settings for the long road circuits are also much different as the demands for more fuel will often lead to the HS needle setting as far out as one full turn... something never seen on a sprint track. You will never really experience a "four cycle" as we know it on a sprint track. When the carb is set too rich the temp gauge will show it by indicating a large drop in the CHT as the engine speed climbs after the clutch has fully locked and the motor is working its way up to the 12 to 14K range. Typically the drop in temp will be in the 10 to 20 degree range when the motor is tuned for best performance and should stabilize around 375 with the old style head and 425 with the newer lighter version of the head.
Road racing is a whole different world.
Steve O'Hara
 
It just amazes me how two people, both very successful, Steve being more successful than me, can be so different with their carb settings.

The only point we agree on is the height of the fulcrum arm. For Enduro racing that is, that's where I always put my fulcrum arm, regardless of the type of racing. It always intrigues me when people talk about Popoff pressure. You need some, 10 pounds is some, I've never seen any difference with anything else. Needle settings?? We always set the high at about a quarter and the low-speed about 2- 2 1/2. We used the EGT, so I absolutely know that those settings were the best for us. Even those settings need adjusting with changes in the air density. On a real low air density day, we have run with the high-speed closed. Or very near closed. Oh, I should mention, this is with a pipe. Still, with a muffler they don't change a lot. We have gone from a pipe to a muffler, and after readjusting the clutch, I can't say that there was much change in the carburetor settings. Nothing major, the driver may have done some minor adjusting.

Tuning with a CHT is a waste of time in my opinion. The EGT tells you what's going on inside the engine. The EGT responds much quicker when tuning. The great thing about the EGT, if the engine starts to detonate a little, even a little, the EGT will go down. It gets a little tricky, because the EGT will go down if you're too rich also. But, if it goes down, the first thing you do is richen the engine up a little, if it goes down further, go the other way. It's totally different from a CHT. And much more precise.
 
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