I’m going to have to disagree with James on this one and say you should be able to blow fuel into the carb. Blow into the tank’s fuel overflow and fill the line all the way to the carb. If the line doesn’t fill, check the diaphragms in the carb or install a new carb kit. Any chance of borrowing a known working carb?
Definitely doesn’t fill when blowing. I sprayed everything with carb cleaner when I took it apart. I didn’t put in a new carb kit (probably should have for piece of mind), but nothing looked obviously wrong.
If it sounds like the problem is the carb, I’ll just order a new one and a new manifold gasket. I only just noticed I have the Tag Jr 23mm carb, and I’d be racing Sr anyways which requires the 27mm. I bought it from an adult so I assumed without checking.
Follow up steps to consider.
When moving to the larger carb, consider installing the larger radiator. Put the original carb and radiator up for sale as there could be someone going from a senior motor to a junior.
Do you have maintenance records for the motor? There are two groups of 2-cycle maintenance, top end, and bottom end. Speak to an engine builder for his recommended service intervals. Top ends are more frequent and consist of a piston, pin, ring and resurfacing the cylinder wall. Bottom end will be a rod, lower bearing cage, crank bearings, and seals.
Purchase a pop-off gauge and learn how to use it. You’ll need for future carb maint and troubleshooting.
Remember the inlet needle valve is normally closed in the pumper carbs. With it being closed there no path for the air to escape from the carb and line, therefore there’s no way for the fuel to displace it.
That is unless\until you exceed pop off pressure to open the valve. Probably don’t want anywhere between 6 and 12 PSI in your fuel tank though 
A float chamber carb (Rotax, KZ, 206) will allow fuel to flow into it as long as the float chamber is not full.
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Sounds like it’s carb time no matter what. While it’s off, inspect the carbon reeds for cracking too.
Ordered a popoff tester, new carb, manifold gasket, and new reed block. The whole reed block ($110) is only a bit more than petals ($75), so I figured it would save me some effort and one less thing to screw up.
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If it fires off starting fluid and then stalls, it’s not pumping fuel. Check for a seized needle, broken lever, seized pivot or broken return spring by removing the pumper assembly (both of the aluminum rounds above the carb) and pushing on the valve that’s opened by the regulating diaphragm.
Best way to fill the carburetor is to loosen the screw at the top of the carburetor and blow into the tank by the breather until fuel leaks around the edge.
I would recommend physically inspecting the reeds before replacing them. Take the reed block off, hold it up to your eye, look towards the sun with only that eye open, and if you can see daylight the reeds are worn out.
One last tip - stale gasoline will seriously degrade starting performance. If it’s in the tank overnight it’s stale. If it’s in a plastic jug for a week it’s stale. Fresh premium gasoline has often fired “unstartable” engines for me.
I’m guessing when you put the diaphragm back together something got flipped. It sounds like it is not pumping which is usually pointing to a diaphragm issue
Welp, I put in the new carb and it fired right up! I don’t know what’s wrong with the old one, yes probably something was flipped, but I’m not going to put it all back at this point to figure it out. Can’t wait to get it out onto the track!
I also appreciate that the larger carb uses M6 nuts that mount from the front. Finding a tiny wrench for the nuts on the smaller carb, with no clearance and only turning 1/8 at a time, was quite a chore.
Finally, there was nothing obviously wrong with the reeds (no gap), but I replaced it anyways since I was already in there.
Thanks everyone for all your help!
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