1973 340 Thermquad Leaks after Shutting Off, Help!

340 TQ started leaking fuel into the intake after I shut the car off. Then almost impossible to start without holding peddle wide open while cranking, followed by that rich, fuel smelling, smokey, start. I'm hoping someone here can get me close to where and what the problem is inside the carb. After shutting down the engine, I immediately checked inside the the carb and found that there was NO leaking above throttle plates. A little wet, but no leaks. When I cracked the throttle plates, and looked down past the butterflies, the intake is soaking wet with fuel, So the leak must be coming somewhere below the butterflies, right? Does that mean my floats are sealing and adjusted adjust ok, and its as base plate gasket leak or something else down there? No too familiar with TQs, but I hear they are prone to leaking for a bunch of reasons.

Probably the well's on the bottom of the bowl under the main jets unless a leaking needle/seat,a cracked bowl also but not common.There are no other ways for a leak other than mentioned.

62max - When you say "Probably the well's on the bottom of the bowl under the main jets" is this a gasket or an o-ring change or something more serious? I read somewhere that there are parts that are epoxy glued to the bottom of the bowl.

Yes, pull the carb & you will see if it is leaking out of those wells

There are seals under the long primary jets that can go bad too, but it's most likely the expoxied wells that start leaking overtime.

Those wells have some sort of glue to hold them in place. That's what was leaking when I had problems years ago. But it was a rebuilt carb so I sent it back for another one.

Scott Smith Harms at Harm's Automotive is someone you should speak to. He restores Thermoquads and repairs the leak being described.

Your TQ will come back looking like a piece of art!

Mike.

Mike,
I'm sure Scott does a great job but just looking for a quick fix to get me through the season.

The other guys posting have likely found the answer, the fuel well plugs could be leaking or an O ring is either degraded and leaking or installed incorrectly.

Had to stick in the O rings I forgot didn't ya !!

Scott, would I be able to see the fuel leaking for either condition by looking down into the venturies? The carb was rebuilt 3 years ago and has been trouble free until now.

Is the carb subjected to a lot of heat ?

In my experience- I have never seen bowl issues - I’m not saying it doesn’t happen- jsta my experience.

The first thing I would have looked for is sunken floats.

But if there’s no appreciable leakage above the butterflies.., bowl issues are to be looked at.

You would not normally see it looking down the venturis if the wells were leaking. Also, what Doc said about the floats would be correct, if you see fuel up high running out of the vents or shafts, etc. it's nearly always a float related issue.

It sounds like the glue used to put the fuel well plugs in has failed causing a leak but that's just my "1000 mile away" guess

scott, what do you use to glue the fuel well plugs ? i have used JB before with good results, and now there are a bunch of JB formulas to choose from.
is the loctite better, or do you prefer something else ?

At the bottom of the fuel bowl under the primary jets is a elongated cap that directs fuel from the jets to the feed tubes to the primary venturis. It is epoxied to the bottom of the bowl.

The feed tubes are cast into the top aluminum body and between the bowl and the feed tubes are "X" rings. Orings with a cross section of an X.

I have found that most often the o-ring (x-ring) goes bad. However the epoxy is known to leak too.

If you remove the top and the o-rings are shot, a simple rebuild will fix it. If the o-rings look good then rebuild it and put fresh epoxy around the elongated cap under the bowl. It will need to fit in the well of the base so don't go crazy. I do not know of a good type of epoxy but there are many fuel tank epoxies that may work. Hopefully someone will chime in with a good brand.

The first two Thermoquads I have rebuild I over tightened the top and crushed the X-rings. DO NOT OVER TIGHTEN THE TOP. Gentle gentle gentle. You can always snug it a little more later if it leaks out the sides of the top when cornering but you can not buy those X-rings with out the $35 kit. If anyone knows where to get them individually I would love to know.

Or you can send it to Scott and have a perfect one returned.

I have used several types of epoxy, as long as it is spec out as a fuel resistant formula, and all directions for prep & curing time are followed, any of them should work fine (JB fuel resistant, Loctite 401, etc.). Loctite 401 works very well and is my current choice.,

G902 Primary Well Seal, $3.00

There are conventional o-rings that work just as well as long as the diameter and thickness are the same and resistant to fuel as most are. I have used them periodically at times since the advent of the T/Q when I had no immediate access to originals and "never" had an issue.

Take it apart and fill the plastic main body with enough water to see if the wells seep. If they seep, carefully remove the wells and scrape off the old glue and glue them back on using SEAL ALL.

Thank You That rocks

Well,
carefully took the carb apart being careful not to spill any fuel so i can examine the fuel in the bowls. After removing the lid, the right side was half full and the left side was about 1/8 full, just about to the top of the jet. I filled both bowls up with fuel, but there was no leaks coming from the plastic transfer wells and the epoxy looked good. I even tried to twist them apart but they didn't budge. Floats were adjusted properly and held compressed air when the top was inverted. O-rings may not have sealed perfectly as one looked a little crooked sitting in the body. I will replace the o-rings and see what happens.

While your in there take a good look at (or replace) the needle and seat. Trash may have gotten on them.

Sometimes it takes a bit of time for the leak to show up on the wells. Put the main body on a flat surface and put some kind of liquid in it and let it sit for a few hours or overnight. When you put it back together make sure the main body fits smooth with the base. There are dowels on the bottom of the main body that sometimes need a bit of filing for a good fit.

Thanks for the additional input. Needle and seats hold air pressure from my compressor so they should hold fuel. Also, when I looked down into the carb past the butterflies, the top of the intake was soaked with gas, not just a couple of drops. I was definately expecting a crack or something more obvious like a dislodged well but everything looks fine. It ran fine for 2 years then something happened overnight. If it were a Holley, I would have said a power valve but not the case here.

I have 3 cars with thermoquads and the alcohol in the fuel used now is a solvent and will eat through the factory epoxy, jb weld, moroso epoxy, seal all and probably most other glues and epoxies. Believe me I know. If you put the bowl over a 2 lb metal coffee can fill it with fuel and leave it for a couple of hours you will see gas leaking out. The only thing I have found that is mostly resistant is marine tex grey epoxy. I have repaired at least 6 bowls for myself and others that were told that the thermoquad carb is a lost cause. All my cars are running ok and have had no leaks for at least 2 years.

Surfman- I might be having seepage after a few hours, but gas is dripping into the intake and soaking the manifold after a few seconds of shutting off the motor. My problem is not the epoxy holding the well cap on.

I'd use same gas to check for leaks. Gas, or similar oil products ( thinner for example ) goes easier by tiny cracks than water.

THIS IS SERIOUS? DO THEY HAVE PHENOLIC BOWLS!!! thats AMAZING

wondering what kind of them... here are several dozen of them depening on carb PN

Do a search on Ebay for Thermoquad you will find all kinds of small stuff like that.