I don’t know… Fast lap in medium at Quincy was ran out of the draft at heavy weight, so there’s that…
I think the ultimate answer is that you can’t overcome physics. A heavier kart/ driver will be slower.
The question that is more interesting is “but by how much?” This is where it seems we are all unclear, as a lot of it depends on factors beyond this, namely things that try as we might, we can only make somewhat consistent - the track conditions, parity among engines, and of course ‘skill’ of a driver.
To really understand anything meaningful about weight vs. pace, you’d ideally want the largest data set possible. A couple people have reference relative pace between different categories, but is this enough to make a full determination? Perhaps at one track the larger set of data gives us one conclusion, at another it yields a different result. Another factor to consider is the disparity between two different engine package situations - would time difference be the same gap between a light group and heavy group of 206’ers, and similarly weight difference Rotax racers or Shifter kart racers? Just looking at your local series’ lap times could give you a hint, but it’s still a smaller data set as there are so many variables that are hard to eliminate.
It’s a frustrating question to really nail down, but one id be very curious if with a nationwide data set a more meaningful conclusion could be met.
Generally a light driver will beat a heavier driver.
I am ahem several kilos heavier than most of my competitors when I race and I regularly beat those who are lighter - some even have had to weigh up. I am not fighting for wins, far from it, but a decent karter who can adapt to the kart that is under them does add a lot.
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Within certain limits, of course, I’d consider weight balance/distribution to be more important than total weight.
Its not scientific but one of the TAG snrs moved to TAG heavy at the final rounds of the ICP Cup. So same track (he moved Sat to Sun), same tires, same engine, just 35lbs of lead (assuming he met the weight in TAG). 2 tenths slower from the final the day before to qualifying the day after.
Comparing to someone doing times with a tenth of his on the Saturday, he was 4 tenths slower then he’d be expected to be (compared to the TAG snr) in the pre-final.
The local track is ~600m and reasonably technical. The difference between Rotax Light and Rotax Heavy is about 1second. 20kg MAW difference is all.
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There’s no blanket answer across the board. I think the closest you’ll get is comparing the same track, tires, track condition (grip level) and kart weight.
Naturally 20lbs on a 360lb kart is a larger percentage vs a 400lb kart too.
Average speed comes into it too with karts because the aero is so poor. In road racing for example weight.
The true answer is “it depends” 
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A follow up which I think might satisfy a lot of people. Based on this video from “Col”
The calculations he came up with there for a national level “TAG restricted Heavy” vs “TAG restricted Light” there is roughly .07 seconds of difference in lap times for every 5 pounds. He does add that these amounts could change between different classes, but it’s at least an honest look at how real karts might respond to weight differences.
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Add one other variable to the ‘weight vs lap times’ equation; where was the weight added and what did it do to the balance/driveability of the kart. Thinking back I remember when I added 20’ish pounds to my kart (HPV powered at the time), and my times actually improved vs when it was lighter. 75% of the weight I added was behind the seat (e.g., heavily rearward bias), so clearly I had my setup off in the first place (e.g., too much weight forward vs back when it was lighter). Nonetheless, where weight is added and how it affects the setup/driveability would definitely affect the outcome of lap time differences between lighter vs heavier.
Edit: even though what I say above is true and possibly worth some mental exercises, I also think there’s a lot of value in comparing times of top light vs heavy classes since it can be fairly reliably assumed that those guys have their weight distribution figured out well.
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So I am in kind of a weird situation here and I wanted to try and get some input on it.
Next year i’ll be 15 so right on that line between Jr and Sr. Originally I was planning on just sticking with Jr for another year because that is where all my friends are racing but it is looking like i’m gonna end up being close to 10ish pounds over the minimum weight for the class. How much time do you think this will actual relate to and would it be better to just try and move to Sr now and put some ballast on the kart?
I’m wondering, how many in your junior class have the same problem, or, maybe they’re racing at 4, 5, 6 pounds over. I know I never went racing right on the class limit.
My rule of thumb is that 5 lbs doesn’t really hurt you, 10 lbs starts to.
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Do you know if with the KA100 engines you can change them back and forth between restricted and non restricted for junior and senior or is however you get the engine how it stays?
The only difference is the header, so you should be able to just swap the headers from junior to senior I believe. I know people who have done it, just not sure if there is a different way engine builders build a junior vs. senior engine to optimize performance.
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Do you feel comfortable moving to seniors? Don’t base it on your friends in the class, base it on your comfort level and ability.
I understand that weight is the enemy of longitudinal acceleration but (up to a point) the friend of lateral acceleration.
First, am I right?
second, how can we use the excess weight we’re sometimes stuck with, to our advantage?
Thanks 
If weight was perfectly in the center of the kart or the COG of the kart then this would be easier to generate an answer for…the issue of course is that is not true.
For longitudinal acceleration, where the weight is at maximum is also a factor - same with (-)acceleration (braking). This is why adjusting the rake of the chassis, seat position, or height of driver can affect this parameter. However, especially in a kart, this effect is considerably (at least to the driver) less than the effect on lateral acceleration.
No two drivers in the same kart, with the same seat position will progress through a corner exactly the same. This is because no person is built exactly the same as any other person - no suit is the same, or helmet, or tire even, lap to lap. Sure, minor differences don’t matter to most of us, but significant ones will - how tall a driver is, their body type, and overall weight of course. Even if drivers weigh the same, where their mass is concentrated can vary significantly.
Some drivers could move the seat forward or back, but you can’t change the driver in weight or body type (think rower vs marathon runner). Generally karts that have a higher percentage of front weight tend to be more aggressive in steering than those with weight further back. To similar extent, although displayed in a different outcome, drivers that sit at different heights affect the handling drastically- all these affects agnostic of the drivers actual weight.
Ultimately, a driver that weighs more transfers more weight in the kart while cornering or while accelerating or braking. That might not be a positive necessarily though, as we can’t chop up a driver into 5lb increments and move it around the kart. No matter how you slice it -heh- a lighter driver does have a fundamental advantage at all times, even in a regulated min weight category because more mass is available to shift about the kart to optimize performance compared to a heavier compatriot.
Excess weight in a driver is tricky to view as an advantage - again you can’t move it around where you want. But, lots of heavier drivers make it work. A heavier guy higher up might have an advantage cornering in the rain for example, or set way low it could theoretically help in a gripped up situation if your competitors are taller and bulky chested (or have fat heads) - shades of gray.
In a concession kart you can’t move the seat around, aside from forward or backward, or perhaps by adding seat padding. In these cases heavier drivers may want to try and shift weight forward to help keep excess weight off the rear axle and tires to minimize chassis hop.
I wouldn’t say that I feel uncomfortable but it is a little daunting. To this point I have only ever raced against juniors but I am definitely build more like the seniors. I’m about 30 pounds heavier and 6 inches taller than everyone else in the class.
At the end of the day I just want to race in the class that I can be most competitive in, so that is what I am trying to decide now.
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Thanks for your thoughts Eric.
“… because more mass is available to shift about the kart to optimize performance compared to a heavier compatriot.”
You’re referring to ballast here?
You’ve talked a bit about the height of the driver in the seat. I wonder if I should try a seat cushion again to test the effect of raising the COG on unloading the inside rear.
My answer to a weight disadvantage is to use the slight grip advantage it confers to do less accelerating than the other guys.
Appreciate your thoughtful answers.
Lee
Yes, referring to ballast such as Lead weight. I’ve seen people use batteries on a non-tag chassis before though and get away with that as well…which in retrospect not sure is ok…but anyway…
Yes! A seat pad or cushion is a great way to try a COG change to see the effect, either below or behind the driver we’ve done both before prior to moving a seat…
I like your thought on doing less accelerating than the other guys, but the question would be exactly where is that disparity occurring. Especially on a rubbered-in track, more grip may not be ideal!