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

[Todd Giles]

Thanks SteveO and Al! Exactly the info I need.

 

[dvsu12]

Hi all,

My son and I are new to karting, have yet to take him out in his Cadet kart with the KT100J. First outing I just want him to get out there, turn some laps, have fun and get used to driving it.

I'm trying to set it up as close to how it has to be for Cadet class here in oz, which means running a 13mm restrictor at the exhaust flange on the motor.

Any thoughts on Steve's carb settings for a restricted J?

Just finished cleaning and putting a rebuild kit through the carb, popoff is approximately 8psi, lever height is set at 1mm (0.039").

Cheers!

 

[Steve O'Hara]

I'll start by saying I have no hands on experience tuning a KT100 with the restrictor plate installed so my remarks will be directed more toward theory than actual practice. The first thought that comes to mind when considering a restricted engine is that it will make less power so it should consume less fuel. Accordingly, I would start with the inlet needle lift, aka arm height, lower than what works best for an unrestricted engine. That means I would be setting the inboard end of the arm close to flush with the circuit plate or perhaps .010" above.
The restricted engine will flow less air so that means the drop in pressure at the venturi in the carb will not be as great as an unrestricted motor thus the pressure delta that moves the metering diaphragm will be less. On the other side of the inlet needle, the pump output pressure will also fall due to the smaller charge entering the engine on each cycle. Combine those two factors and the conclusion would be that the popoff pressure should be reduced a little. I would still start up around 15, otherwise you'll end up with the HS needle almost closed.
Let us know what you find with your testing.
Steve O'Hara

 

[dvsu12]

I'll start by saying I have no hands on experience tuning a KT100 with the restrictor plate installed so my remarks will be directed more toward theory than actual practice. The first thought that comes to mind when considering a restricted engine is that it will make less power so it should consume less fuel. Accordingly, I would start with the inlet needle lift, aka arm height, lower than what works best for an unrestricted engine. That means I would be setting the inboard end of the arm close to flush with the circuit plate or perhaps .010" above.
The restricted engine will flow less air so that means the drop in pressure at the venturi in the carb will not be as great as an unrestricted motor thus the pressure delta that moves the metering diaphragm will be less. On the other side of the inlet needle, the pump output pressure will also fall due to the smaller charge entering the engine on each cycle. Combine those two factors and the conclusion would be that the popoff pressure should be reduced a little. I would still start up around 15, otherwise you'll end up with the HS needle almost closed.
Let us know what you find with your testing.
Steve O'Hara

Update: We have been out quite a few times since my initial post, first outing was disastrous as I later found out that we were running far too lean, and caused low end flatspots / stalling.

Made a few changes to the carb to align with Steve's Easy Idle settings (I got the fulcrum height wrong originally ), and with the high jet opened approximately 1/8 of a turn, and the low jet at approximately 2 turns, we have not had any issue with stalling or flatspots.

Engine seems to pull cleanly (as well as can be with a restricted J!), and idles between 2000-2500rpm. It actually idles so well that my son has had a few offs at the track, got out, pulled it back on track, and drove off without needing any assistance (he is almost 9).

I think it's more of a compression issue (engine hours unknown when we bought the kart), but it takes a little longer than our other unrestricted J (3 hours since last rebuild) to get started when cold (maybe 5 secs longer?), but once warm, 1 second on the starter gets it going with ease.

I also applied these settings to our unrestricted J in our other kart, needed a little more low jet, but idles at 2200-2400 rpm when warm.

Exceptionally happy so far, so thankyou for sharing Steve.

 

[mtbikerbob]

Since this thread has a lot of good info already I am hoping someone can add to it....In rebuilding the WB3A carb with a rebuild kit you are given 3 half moon shaped gaskets for under the plate on the metering side. It appears that the one removed uses the gasket with the most openings in it. However, pegasusautoracing.com website says use the gasket with kind of gear shifter cutout and to discard the one with more material missing. Any thoughts on this? I also see someone is offering a "trick" gasket set, that appears to have thicker gaskets. What benefit would that offer?

 

[GoFast]

Thicker gaskets offer increased fuel flow.

I don't buy anything from Pegasus, as I've always been disappointed in their quality of goods they have offered for cars, like fake AN bolts.
I wrote a large post on one of my car forums, showing their counterfeit junk, compared to real aircraft bolts.
Have a suspension fastener fail on a 180 mph car, it can kill you !

Now you say thy are selling kart parts ?
I wouldn't......

 

[PD Engines]

Unbrako are the bolts you want to use. Ask Roger Penske for a backup opinion.

In recent years there have been plenty of aircraft bolt failures. Unbrako bolts do not break. They are made in USA with full heat treat technology.

 

[rainman]

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

I have always been terrible carburating the KT100s. I love my reeds and I have even been able to do good with the Walbro on my Comer P51 on sprint tracks or even Yamahas on sprint tracks, but had many problems to find the right carburation on pipe KT100s when running enduros. Last time I was at Roebling Road was in fact the 1st time I was able to make one run OK by the time I run the main, and I couldn't get it running good setting the HS needle as close as most do. I finish getting the speed I needed some way between 3/4 and 1 turn on the HS just as Steve suggests here. Thanks for sharing your unbelievable experience and knowledge.

 

[PukinDog]

Mr. O'hara, although the OP is nearly 10 years old(!) many thanks for taking the time to inform all KT operators of an excellent starting point for carb setup.. The information you provided is exactly what I needed to dial in my KT (which is run within the parameters you stated) I am a newbe to karting but 30 yrs + in the motorcycle restoration business. Prior to finding your post, I was fumbling around in the fog of overly rich blendzall and fouled plugs in my matching number (!) 1978 RC100s which I have just completed a full restoration. My motor is stock except for 52.5 bore. I cannot determine the model or brand of pipe on the kart, but it didn't matter, 7.5 inch flex pipe and I am at 2800 feet. The kart will be hung on a wall regardless. Finally, perhaps the most enjoyable part of the tune, from WOT the engine completely shuts down when I release the throttle, (I am thinking it will stall for sure) all the way to a very low idle (must be less than 2000) and just sits there and runs, just like your description. Easy starts (a nightmare prior to your setup) and great throttle response. Truly amazing. Hope you stop by this post and know how much time, energy and money you continue to save all KT operators.

 

[mc shelly]

Mr. O'hara, although the OP is nearly 10 years old(!) many thanks for taking the time to inform all KT operators of an excellent starting point for carb setup.. The information you provided is exactly what I needed to dial in my KT (which is run within the parameters you stated) I am a newbe to karting but 30 yrs + in the motorcycle restoration business. Prior to finding your post, I was fumbling around in the fog of overly rich blendzall and fouled plugs in my matching number (!) 1978 RC100s which I have just completed a full restoration. My motor is stock except for 52.5 bore. I cannot determine the model or brand of pipe on the kart, but it didn't matter, 7.5 inch flex pipe and I am at 2800 feet. The kart will be hung on a wall regardless. Finally, perhaps the most enjoyable part of the tune, from WOT the engine completely shuts down when I release the throttle, (I am thinking it will stall for sure) all the way to a very low idle (must be less than 2000) and just sits there and runs, just like your description. Easy starts (a nightmare prior to your setup) and great throttle response. Truly amazing. Hope you stop by this post and know how much time, energy and money you continue to save all KT operators.

hi, im thinking mayko bullet on the pipe. chuck