Why is my 1950s Standard New Compton flush so sluggish?

Nine times out of ten on those old Comptons, the jet channel running from the tank spud down behind the trap is still choked with calcified scale. Just pouring diluted acid down the rim doesn't always build enough head pressure to push through the internal passage feeding that bottom siphon hole. Grab an inspection mirror and a stiff piece of copper wire or an old speedometer cable, poke it up into that lower jet opening at the bottom of the well, and see if you hit hard crust. You often have to physically ream that passage out while feeding acid directly into the flush valve opening. Check your standing water level in the bowl before you even pull the handle, too. If your fill valve's refill tube isn't sending enough water down the overflow pipe while the tank fills, the bowl water sits an inch or two below the weir of the trapway. When you go to flush, the first half of your tank volume gets wasted just priming the bowl up to the spillover point instead of triggering the siphon. That completley ruins the velocity and leaves you with that lazy swirl.

The other common culprit is modern replacement hardware. Standard engineered these beasts to drop four to five gallons in roughly three seconds. If you put in a modern flapper, it's likely calibrated to drop shut fast for water conservation, starving the bowl of the head pressure it desperately needs. You want an old-style Douglas-type brass valve with a proper rubber tank ball, or at least a fully adjustable heavy flapper set to stay open until the tank is practically empty.

A quick way to isolate it: dump a five-gallon bucket of water straight into the bowl as fast as you can. If it violently siphons and gulps, your trapway is clear and your problem is strictly water delivery from the tank.

Take a look at the cross-sectional aperture of the tank-to-bowl gasket you used, because plenty of modern "universal" sponge gaskets possess an inner diameter slightly shy of two inches, which readily compresses inward under bolt torque and creates a significant hydraulic restriction right at the bowl inlet throat. If you choke that transfer orifice, you completely lose the instantaneous volumetric flow rate required to initiate a proper flush.

Also check the standing height of your overflow tube; if you replaced the original brass column with a shorter off-the-shelf assembly, you are forfeiting several inches of hydrostatic head pressure that the original casting strictly depended on to drive water through the manifold.

Dollars to donuts you set it on a wax ring with one of those plastic funnels built into it. Those old Standards have a deep ceramic horn out the bottom. When you mash a plastic-sleeved wax ring down onto an old cast flange or lead bend, that plastic sleeve pinches crooked and throttles the discharge right at the floor. Fought that exact same lazy swirl on a '52 Compton thirty years back. Scraped off the modern ring, bedded it on a plain old-fashioned wax gasket with zero plastic, and it imediately flushed like a freight train. Pull the bowl and take a look.

Check where the water's actually sitting in the tank before you flush. Those old tanks almost always have a water line stamped right into the back porcelain, and it's way higher than modern rigs—usually just an inch below the top of the overflow tube. If you threw a standard Fluidmaster 400A in there and didn't crank the float adjustment way up, you're probaly missing a gallon or two of head pressure. Won't pull a clean siphon without that full weight behind it.

Update: Raising the water level up to the stamped line made a noticible difference with tank volume, so thanks for that tip. But it's still doing a lazy swirl instead of that aggressive siphon kick. Pretty sure that lower jet channel is still choked with scale. What's the best trick to snake or work acid directly into that bottom jet hole without pulling the whole bowl back off the flange?