Author Topic: Lets talk about leaky bottom plugs.  (Read 160 times)

Offline Cliff Ruggles

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Lets talk about leaky bottom plugs.
« on: June 04, 2021, 03:09:29 AM »
I get all sorts of calls to the shop, so many these days I can barely get anything else done.

Most of those calls are for parts and/or tech support.  I also get a LOT of calls about the fuel bowl draining down from "leaking bottom plugs" and folks having to pour gas into the carb/intake get their engines to fire up after the vehicles has sat more than a week or so.

Well right to start with a BIG hand should be slapping you in the side of the head IF you have to dump fuel into the carb to get the engine to fire up and you think it's a leaking bottom plug issue!  This is NOT related to leaking bottom plugs.  Even if the carb was bone dry from a fresh rebuild it should fill quickly while cranking the engine and a few pats on the throttle should have the accl pump supplying enough fuel so it roars to life instantly.

If you have to crank and crank and crank the engine plus pump the throttle dozens of times without any fuel getting to the engine the bottom plugs have NOTHING to do with your problems.

This doesn't mean they aren't leaking and need attention, it just means that you either have a fuel drain/back issue, fuel delivery issue, or the accl pump just isn't making it (most likely cause).  This is why I will ALWAYS recommend installing one of my accl pumps when you call for tech help about this issue.  Those "blue" seals showing up in other kits are NOT making it.  Mine will not fail in this fuel and has a lifetime warranty.

I'd also add here that despite all the videos on Youtube and long-lengthy responses from "gurus" on the Forums telling you that ALL Q-jets leak at the bottom plugs and they need a ton of "monkey-chit" dapped over them during the rebuild that in most cases they are NOT leaking.

Leaky bottom plugs are actually quite rare after 1968.  From 1965 to 1968 if the casting has the pressed in brass cups for rear plugs 100 percent will be leaking.  I've never once to date found a set that wasn't.  On those same early units leaks are somewhat common at the front pressed in lead plugs as well.

Rochester knew about this issue and started pressing in aluminum plugs and swaging over them by 1969 which for the most part corrected that issue.  A few years later they got rid of the lead front plugs and started pressing/swaging in aluminum plugs there as well.  They kept getting better at it and leaking plugs in the later carbs are pretty rare.

Even so every single one should get pressure tested and repaired if/as needed.  I still see a few leakers in later units.  Just had a 1980 Turbo T/A carb in here with one leaking rear plug.  The leak was so tiny that it took 140psi of compressed air applied to the rear jet holes for 30 seconds to find it, but a leak is a leak and it got repaired as needed.

This pretty much covers leaking plugs, so keep in mind that the less intrusive you can be to your carburetor the better.  I would NOT start pulling plugs out and tapping for screw in plugs without verifying with compressed air that you have a leak right to start with.  I'd also add that 9 out of 10 times if you find a small leak at a rear plug a few gentle taps with a small machinist hammer will usually swage the material back out and stop the leak for another 30 years or so.  I'd so all this before attacking the casting with a drill, tap, screw in plugs and fuel type epoxy......hope this helps some........Cliff
« Last Edit: June 10, 2021, 03:29:21 AM by Cliff Ruggles »

Offline Mr.Pontiac

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Re: Lets talk about leaky bottom plugs.
« Reply #1 on: June 09, 2021, 08:33:47 PM »
You are probably the smartest guy I ever know, forgive me but 140 psi air to find a leak?excessive to me. Just thinking out loud. I usually have my reg. Set at 75 and think it’s high.

Offline Cliff Ruggles

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Re: Lets talk about leaky bottom plugs.
« Reply #2 on: June 10, 2021, 03:27:31 AM »
That just happens to be where my compressor is set at, 125-140psi.

75psi would be enough....

Offline Greasy Harley

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Re: Lets talk about leaky bottom plugs.
« Reply #3 on: June 11, 2021, 09:55:26 PM »
Good write up, Cliff.
My truck has this problem.  It has your rebuild kit and the accelerator pump works perfectly. I did check for leaks when I rebuilt the carb, it was fine. Sounds like it must be a  "fuel drain/back issue, fuel delivery issue". It's mostly a mild annoyance, but I would like to figure it out.
Thanks for sharing this info.