Carb size question

krtrcn

Member
I was talking with my brother, yesterday, about the smaller size carbs that are used in Europe.
We've both heard that it could be beneficial, mainly better performance, with a 100cc motor over the bigger carbs that are used in the U.S..
Just wondering, because either one of us has used one or know anything about them, and what is the purpose of the 3 rd jet on a Ibea.
Thanks Alan
 
I was talking with my brother, yesterday, about the smaller size carbs that are used in Europe.
We've both heard that it could be beneficial, mainly better performance, with a 100cc motor over the bigger carbs that are used in the U.S..
Just wondering, because either one of us has used one or know anything about them, and what is the purpose of the 3 rd jet on a Ibea.
Thanks Alan
Depends on track, fuel, clutch, pipe, etc. Stock at asphalt sprint small tracks when run direct drive on gas you can be faster with a small carb giving you a better response. The 3rd jet is closed when the engine is still cold like the warm up lap, and open once warm for extra fuel, especially full throttle by the end of the straights.
 
Just my opinion, I think the 3rd jet was designed for use on direct drive engines, as the rpm's increase ( way past clutch engines speeds) the engine fuel flow must be decreased to match the decreasing air flow, so good tuners will set high speed jet LEAN very lean ( low speed just the opposite, very rich for good fuel flow at peak torque ) then tell the driver to choke the carb at the end of all straightaways cooling the piston. That is it.
 
A lot of it has to do with the different exhaust chambers used between direct drive and clutch engines. The two have very different fuel/air demands throughout the RPM range.
 
Just my opinion, I think the 3rd jet was designed for use on direct drive engines, as the rpm's increase ( way past clutch engines speeds) the engine fuel flow must be decreased to match the decreasing air flow, so good tuners will set high speed jet LEAN very lean ( low speed just the opposite, very rich for good fuel flow at peak torque ) then tell the driver to choke the carb at the end of all straightaways cooling the piston. That is it.
You are right, we used to choke the air box at the end of the straights in ICA, and we also played a lot with the carburation through he race, especially the high needle. I have been unable to make my 3rd jet work with a clutch on dirt, it won't work if opened, and with that one closed I also found it very difficult to tune it for a dirt oval on gas.
 
Third jet with a clutched engine probably won't ever get enough signal to do much because your not gonna turn enough rpm . Like Magmo said it came about for direct drives insane rpm numbers. It's strictly a high speed enrichment with its nozzle way out front it lacks signal at all but very high rpm. Low signal always makes for difficult tuning.
 
The smaller carbs for ICA/DD 100cc air cooled in Europe is a spec deal and makes for great throttle response when there’s no clutch. There’s more to be found on them which is why we convert them to 116’s. That said, in stock form direct drive 100cc’s I don’t think I’d run something different than the norm stock smaller carb because you’d just load it up on tight corners. Those guys and FIA knew what they were doing in the homologation process and it was to create parity between parilla/italsistem/atomik/sirio/rotax/CRG/PCR etc. on our dirt oval applications those small spec carbs couldn’t hang because they don’t offer enough fuel and air to match the pipes and clutch engagements we run. The 3rd needle IBEA and Baroni carbs are made for that one thing and are just a very fine adjustment for formula A/ICA/DD where the airbox plugging is a must to keep it together (every long straight on euro road/sprint requires plugging the airbox to flood the cylinder to keep it from sticking)
 
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Third jet with a clutched engine probably won't ever get enough signal to do much because your not gonna turn enough rpm . Like Magmo said it came about for direct drives insane rpm numbers. It's strictly a high speed enrichment with its nozzle way out front it lacks signal at all but very high rpm. Low signal always makes for difficult tuning.
Good point, John. I wonder if you know if a small carb (like Baronis, HHCs, IBEAS, etc) would work on gas closing the 3rd jet on a dirt oval with an axle clutch. I have had plenty of issues trying to make a TSL98 ICA stock on gas, tried several pipes, played with clutch engagement, etc, but it would die, so I finished using a bigger carb on methanol and it worked fine. Now I don't agree with those stating that an engine built into a 116 is necessarily faster, at least on a sprint asphalt track on direct drive configuration. Most TAGs are still slower than my old DD 100cc ICAs on small sprint tracks. Even my KZ shifter is not faster at some of this tracks. The response of these engines was huge on gas as they came out. Yes, they have tight specs by FIA/CIK rules but those were real racing engines with a lot of work done to develop them compared to other engines used in karting. Obviously they are not ideal for dirt ovals or RR as they came. That's correct.
 
I built lots of IBEA's years ago for FA. The only changes we ever made to the drill pattern was to add a fourth jet. They are set up to be rich off the bottom , they have the shutters notched right at the low side ports to enhance signal when the butterfly shuts. When some racers I worked with tried to run a 24 mm Ibea on a JICA, the needles could only be barely cracked, and was not tunable. That's the only issue you may face with it.
 
I built lots of IBEA's years ago for FA. The only changes we ever made to the drill pattern was to add a fourth jet. They are set up to be rich off the bottom , they have the shutters notched right at the low side ports to enhance signal when the butterfly shuts. When some racers I worked with tried to run a 24 mm Ibea on a JICA, the needles could only be barely cracked, and was not tunable. That's the only issue you may face with it.
I guess it will be better then to keep the bigger carbs on methanol for dirt ovals. I just loved not having to flush the engine every race.
 
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