Racing Fuel Systems

I have a Quick Fuel 750 DP, brand new. Regular 10% ethanol gas, and hooked to a mechanical fuel pump on a sbc. After the engine sits for a day or 2, the level in both bowls seems to drop from the middle to the bottom of the sight glass. At first I thought this may be a fuel percolation issue happening after a hot shutdown, but after firing it up cold last night, letting the bowls fill to the halfway mark, shutting it off, and then letting it sit for 2 days, the bowls are back low. Any ideas where the fuel is going?

Pull the bowls and tighten the PV or PV plugs. And make sure the bowl screws are snug,

Thanks I'll give that a shot. Once the fuel level gets even with the bottom of the sight glass it stabilizes. Strange.

Did you let the engine warm up, then turn it off? Then, 5 minutes later open the hood, take the air cleaner lid off, and look inside the bores of the carb. Pretty much guarantee there will be fuel dripping out the boosters, accel pump nozzles, or from around the throttle shafts. A few years ago I started using the Holley 108-70 heat shield on every engine. Stops the heat soak which then causes raw fuel to dump into the engine after every drive.

Ok I took off both bowls. Metering block gaskets look great. Replaced power valve. Secondary side has no valve provision. Torqued fuel bowl screws to holley spec.
Fuel level still goes down, even filling the bowls on a cold engine. Could it be bleeding back down to the mechanical fuel pump? I noticed my gauge reads 6 psi after shut off, but a day later it bleeds down to zero. I also noticed when I pulled the carb off that the baseplate down by the throttle blades had some wetness. Also, when I pulled the carb off the manifold (with fuel bowls full) and tilting the carb to the side, I could make fuel leak out by the baseplate by the throttle shafts. Is this normal? This is a brand new quick fuel dp.

Tipping a carb even a little bit will let fuel dribble, this is normal. I've never really known the holley style catb to suffer from heat sink but that sounds like what you are experiencing - esp of it is in both bowls. Be different if it were in just the front or rear bowl. Get yourself one of the cheap RPC 1/2" open spacers and stick it under the carb, that will isolate it from the heat. After shutting the engine off and waiting for a little bit have a look at the boosters, see if they appear damp, all 4 corners, and maybe a whisp of white vapor curling up, those are sure signs that fuel is being pushed out the boosters, an extreme case you'll see adrip (that occurs when the float is too high but shuts itself off when the level drops a little).
Fuel can't siphon back out of the bowl thru the inlet fittings.
Let us know what you find, an interesting problem for a Holley style carb

I've run into something similar on a couple of friends' cars, neither of which are Holleys. One is a '71 Charger R/T with its original AVS, the other is a '73 Charger 340 with its OE ThermoQuad. Both guys are meticulous about these cars; the carburetors do not need attention. The 440 in the '71 is mildly modified, the 340 in the '73 is stone stock other than headers. As far as I'm aware, both cars have their intake heat crossovers open and functioning. OE mechanical fuel pumps on both cars. After sitting for any length of time (even as short as a 10-hour shift) the cars were hard to start. Cranking times of 10-20 seconds were typical, rather than the instant light-off one would expect. Both owners suspected fuel boil-off.

The owner of the 340 car just switched to ethanol-free premium, with no other changes, and has not had a problem since. Not once. I ride to/from work with him literally every day and am witness to the difference. The 440's owner tends to be a little more thrifty and didn't like the added expense at every fuel stop (never mind that the car's worth about $60K). On my recommendation--part of the reason for which was the car's enormous appetite for fuel pumps--he installed an inexpensive electric "booster" pump inline. Problem solved. In fact, it worked so well he did the same thing to his 100% stock 1981 Mirada (318-2V) which was exhibiting the same issue on E10. It no longer does.

The owner of the 340 has also mentioned repeatedly that the car's fuel economy improved enough that it costs about the same to run on E-free 91 as it did on the E10, despite the significant increase in price per gallon. No such luck on the R/T, of course, but his mechanical pump has lasted 4 years now. That's a new record.

Before you spend too much time chasing a "carb issue" that may not exist, run 'er to the "Walk" line on the fuel gauge and fill up with better gas.

Have you tried taking the carb off the engine and putting in a big pan, then fill carb up thru vent tubes? Have some white paper towels under the carb. See if the bowl levels drop.See if the paper towels are wet or show discolored fuel stains. If not, it would seem to be heat related.

Fill the carb with fuel and take it off, block it up to see if anything is leaking.