Let's back up for just a minute and talk about "leaking bottom plugs".
For the most parts it is a "myth" and tens of thousand of Quadrajets get needlessly "butchered" by folks who follow the advice of others (who should for sure keep their day-jobs) that the bottom plugs were leaking in the first place.
So let's skip rumor, myth and drama and get to the facts.
From 1965-to 1968 the leak rate for the rear pressed in brass "cup" plugs is right at 100 percent. Yes, even on NOS carbs right out of the GM boxes I can put soap/water over those plugs and blow compressed air into the secondary disc holes and get a stream of bubbles from them. On used units most leak like Niagra Falls.
In 1969 due to a LOT of problems and warranty issues in that area they made the castings thicker in that area, then pressed in solid aluminum plugs and swaged over them. Problem solved.
The leak rate after 1969 is next to zero, but in any and ALL cases the rear plugs should be pressure tested. If you find a leak, 99.9 percent of the time you can simply re-swage the material around those plugs with a small machinist hammer and good to go for another 50 years or so.
The factory continued after 1969 to do a better and better job of sealing up bottom plugs and most later carbs just aren't leaking.
Let's talk for a minute about the front plugs. Early carbs also had front plug leaking issues, but at not near the rate of the rear plugs. Initially the factory just used lead plugs driven into those holes and you'll find a few leaking here and there.
A leaking front plug is DEVASTATING as it's in the vacuum path of the intake and not only allows raw fuel to be sucked into the engine when it's in use they leak after shut-down causing a flooding condition after the vehicle sits for a short period of time.
So, in any and all cases part of a good rebuild should include leak testing ALL the bottom plugs and only making repairs if/as needed.
Methods of repair vary some, but as Kenth mentioned the "O" ring repairs are absolutely USELESS and always have been. Carbs that have came in here with that repair have a 100 percent leak rate right past that crap.
Repairs of the 1968 and older rear cup plugs is more difficult than later models. Here I "spin" out the cup plugs with a tight fitting drill bit, bead blast the plugs and casting, degrease, then put them back in with Marine Tex. That has proven to be a 100 percent successful repair. If the cup plugs are long gone I do over to the lathe and machine a pair that very carefully drive in place (the castings are thin and split easily) and glue them in and over with Marine Tex, same end result.
For ALL front plugs, whether lead or the later swaged aluminum I very carefully drill them out avoiding pushing them deep into the casting and tap the holes for 10-32 steel set screws 1/4" long. The castings are warmed up, degreased and the threads of the 10-32 set screws lightly coated with Marine Tex and torqued in place. This a 100 percent effective repair for that issue.
I offer a bottom plug repair kit on the website, but urge folks to leak test your carb BEFORE making any type of repair.
I'd also add here that "dabbing" any sort of epoxy over leaking bottom plugs is a complete waste of time/funds and about as effective as cutting your arm off and putting a band aid on it!.....hope this helps some....
