[Jim Derrig]

The 39mm  carb is the “correct” size for a 125 shifter.  I use quote marks because in IMHO this means it is wildly oversized unless you know what you’re doing and are running at or near the kart’s maximum potential.  Otherwise, the engine will seem sluggish and too rich.  This is one of many reasons it’s usually not a good idea to start out with a shifter kart–the carb is not forgiving for those at the beginning end of the learning curve.  The cure for what seems to be a carb problem often is to stick your foot in it and to drive faster, which may mean driving over your head.

The pump around system is one approach to curing a perceived issue with the float sticking in high-g corners–the kart sees g forces that are unobtainable on a bike.  A lot of very fast shifter drivers that don’t believe in pump arounds and do fine with floats.

[John Starkweather]

Yeah that sounds right then because it’s fouling out almost immediately unless your full throttle. What’s a good size carb that is going to be relatively forgiving and can be adapted to the pump around system?

 

[Jim Derrig]

Where are you located and are you running sprint or road race?  Honestly, it’s been several years since I ran a shifter and then it was a modified Honda not a YZ, but you look to be seriously oversized on your jets for a sprint track, even at sea level, especially if you aren’t an experienced driver.  Rejetting might cure a big part of your problem.

[John Starkweather]

The end goal is a sprint track but we’ve just been testing on my street before we take it out to the track. Located in houston tx and it was bought used so we’re unsure of what is correct. What do you think is a good jet size to try? The jet formulas are a bit above my head. Do you think I should get a smaller un-mutalated carb?

[Jim White]

Throttle response will be greatly improved with a smaller carb.

[John Starkweather]

What’s size would be good? And is there an easy formula for finding the correct jet size?

[Jim White]

Find a 34mm Dellorto. Get a 175 inlet needle and a Dellorto fuel pump. As for jet size I don’t know.

[Jim Derrig]

While the Delorto is a better carb, I think that’s a bit too expensive a path for you to run down right now, since you’re clearly just getting your feet wet.  If you are just puttering down the street you’re going to be overcarbed even with the Delorto.  You need to take it out on a real track.

Switch to a 52 low jet and a 172 main.  That will lean you out quite a bit but still should be plenty safe.  Check the plug after a few laps–black is rich but at least it won’t explode, brown is what you want, and white/chalky means too lean.  Move up or down from this baseline setting as required.  If you meet other drivers at your local track, they can probably help too.

Incidentally, forgive my preaching but driving a shifter kart down the street is a real bad idea.  Maybe its okay to make sure the brakes actually work, no parts are falling off, etc., but in addition to being dangerous and illegal, it will give you almost no useful data.  My Skip Barber instructor said the next step up from a shifter is an Indy car.  How much data would we get puttering a Dallera-Chevy around in the local cul-de-sac ?  You’ve got a real race car and it needs a real race track.

BTW, based on the photo, the vent  below the carb looks like it is tapped into the crankcase and is a vent for running a fuel pump off the pulse.  If your pump is running off another vent then plug it.

 

[John Starkweather]

[quote quote=57089]While the Delorto is a better carb, I think that’s a bit too expensive a path for you to run down right now, since you’re clearly just getting your feet wet. If you are just puttering down the street you’re going to be overcarbed even with the Delorto. You need to take it out on a real track. Switch to a 52 low jet and a 172 main. That will lean you out quite a bit but still should be plenty safe. Check the plug after a few laps–black is rich but at least it won’t explode, brown is what you want, and white/chalky means too lean. Move up or down from this baseline setting as required. If you meet other drivers at your local track, they can probably help too. Incidentally, forgive my preaching but driving a shifter kart down the street is a real bad idea. Maybe its okay to make sure the brakes actually work, no parts are falling off, etc., but in addition to being dangerous and illegal, it will give you almost no useful data. My Skip Barber instructor said the next step up from a shifter is an Indy car. How much data would we get puttering a Dallera-Chevy around in the local cul-de-sac ? You’ve got a real race car and it needs a real race track. BTW, based on the photo, the vent below the carb looks like it is tapped into the crankcase and is a vent for running a fuel pump off the pulse. If your pump is running off another vent then plug it. [/quote]

 

This seems like sound advice. We have another pulse line so I will plug the vent in question. I will switch over to those jets and take it to the track to see how she does. The only question I have if we keep this carb for now is in regards to the to ports that have been filled in.. can you give some information on what they are and why they may not be needed in this setup. Also everyone talks about what position the slide is in but I don’t seem to have an adjustment on mine.

[Jim Derrig]

Sorry, I can’t help you about the ports.

The slide itself doesn’t adjust, but the slide needle can be moved up and down using the clip that holds it in place.  The needle itself also can be changed.  This is a minor/subtle adjustment and not something that should concern you at this point.  If I were you I’d just take it to the track, put my foot in it, and see what happens.  Bring a friend and full safety gear please.

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