This won’t help but is interesting: I googled alternatives and decided that depleted uranium was not feasible:
Ecomass injection moldable high gravity compounds are an ideal lead replacement material. Ecomass’ line of density modified high gravity compounds combine the design and production advantages of plastics with the desired density of metals.
I’ve had to add a lot of weight my whole life… Diving weights will work of course, or you can mold your own weights if you’re cool melting down lead. I’ve got a bunch of weights that I molded to fit the seat better and have been using them for years.
Seat should take 4-6 weights on the back, 1-2 on either side, 1 under each front tab of the seat, one under the crotch of the seat and then you can put weight on the floorpan in front of the fuel tank and stack some on the front fork. That should get you pretty close.
Also, make sure you’re running a decal kit, those can weight a few pounds.
We had 45 lbs on the seat of our first cadet kart. Thanfully cadet weight is lighter here in Canada. You can also add one in front of where the steering shaft meets the frame. Its not ideal but its the cost of doing business when there small. The seat can take if no problem if you are careful with how you mount it. Get the weights as flat to the seat as possible so it doesn’t flex the seat when you tighten the bolts. One thing to keep in mind is the weight wil change hiw the kart handles. If you have the room deciding where to mount it can be helpful.
First race here and was worried about weight. I weigh 125 on a wet day so with my kart I was about 280-290. Well I needed to be 360 so I found some absolute cinderblocks of lead. Each 40 lbs on eBay and bought 6 custom 1/2in countersunk bolts from a metal place and bam 360 on half a tank of gas. Definitely looking to go smaller pieces so if the scales are feeling one way or the other I can adjust less than just 40lbs. Overall they actually worked pretty well though and seemed to be alright with the seat.
For what it’s worth, I would rather have several smaller blocks than one huge hunk of lead. One of the main issues with lots of lead is keeping the seat flexing the way it’s supposed to, and when you’ve got massive bricks of lead on the seat, it doesn’t do that as well.
As TJ said, cupcake pans work good. Just remember to wear a respirator. The fumes from melting lead are pretty nasty, and if you get some zinc in there, it’s even worse. We sell lead weights, marked for weight and predrilled for $3/lb at our local track. We have full safety gear, leather pants, jacket, face shield, forging gloves. When you drill through your “cake” take small bites, lead grabs badly. Also check your local rules, some sanctioning bodies have limits on what each weight can weigh.
This thought has crossed my mind several times. Working in the automotive repair industry, we often have buckets full of discarded lead wheel weights from balancing tires. Usually they are collected by our battery supplier to be recycled into car batteries. We don’t pay them and they don’t pay us for the used lead, so I don’t think it would be an issue to take it.
You could probably visit your local tire shop and get as much as you want. Just have to smelt it and screen out the unwanted steal bits and slag. There are several youtube videos on smelting kart lead.