E-Bodies.org Cuda Challenger Forum

Hi All.  I'm a newbie to Carter Thermoquads, this is my first one to tinker with and rebuild.  As I understand this carb is from a '72 340 AT Cuda (on that same engine now).  The problem I was having was that fuel was leaking out of the top gasket (between the upper air horn and the phenolic body) at both rear corners.  I bought and installed a rebuild kit, with new gaskets, but unfortunately it is still leaking from these upper rear corners. 

During my rebuild I did address the following:  I installed the 2 O-rings into the phenolic bowl wells before installing the air horn.  I installed the needle tree with the dimples facing towards the air horn and with the spring, and it is operating correctly (moving up with accelerator).  The choke is installed and working properly.  The choke diaphragm is holding vacuum (and with the link installed is holding the secondary air butterfly closed), and the diaphragm opens and closes correctly when vacuum is applied or removed.  I replaced the float needles and seats, but I didn't have to readjust/bend the float levels because I confirmed they were already set at 1" from gasket surface to top.  Seems like that was all the tricky stuff, unless I missed something.

The rebuild kit came with 2 upper gaskets, looks like the second one has a few extra holes but otherwise is the same.  I am thinking about installing this gasket as well (2 gaskets), if that might help fix the leak.

Any suggestions?

One last dumb question.  How does the secondary airhorn butterfly open up?  By vacuum?  And at what point in vacuum drop will the choke diaphragm release it?  I had the air cleaner off with the car running (hot, primary choke butterfly already fully opened), and then I pulled on the accelerator lever by hand.  Even with high RPMs or RPM surges the choke diaphragm never released (always had vacuum), and so the secondary airhorn butterfly never opened during this testing.  I'm just not sure how that is supposed to work.

Sorry if some of these are dumb questions, any and all info appreciated.  Thanks!

- Wade

Paging @ScottSmith_Harms (https://forum.e-bodies.org/mlist/scottsmithharms_164)  :brainiac:

Quote from: Cuda Cody on May 09, 2017, 01:16:17 PM
If you rebuilt it and it's still leaking, then maybe it has a part that is warped?

Thanks for the suggestion, that was my thinking too.  In hindsight, I should have checked the flatness when I had it all apart, I was just thinking the new gasket would fix it and I'd be all done.  Bummer.  Anyway, I will check that when I get it all apart again, thanks for the suggestion.

- Wade

Warpage is normal and a quick fix with some emery paper over a piece of glass. That upper secondary door is vacuum activated and very easily tuned by a captured spring.

Quote from: RUNCHARGER on May 09, 2017, 02:01:46 PM
Warpage is normal and a quick fix with some emery paper over a piece of glass. That upper secondary door is vacuum activated and very easily tuned by a captured spring.

Excellent!  Thanks for the info, I'll try the emery-paper-on-glass trick.  And thanks for the insight about the upper secondary door operation.  I didn't readjust its torsion spring tension, pushing the door by finger seemed like it was working fine, I just wasn't sure how it was actually activated.  Makes sense, thanks!

- Wade

Make sure the float level is set properly & that the float do actually float often they will leak & fill with fuel  the fuel level should be below the level of the gasket so something is wrong.
It sounds like the carb is over filling which can be dirt in the needle &seats ,or old assys ,were they replaced & set properly when it was apart  floats or set incorrectly or sunken / full of fuel , to set the level invert the top & use  drill bit between the tip of the flot & the flat surface of the top plate of the carb , I don't have the spec handy sorry .
The secondary air door has an adjustable spring & it is normal it will not open just revving the engine in neutral ,you can lower the sprig pressure but if it bogs as it opens it is set too light .There is an outer lock ring & an inner screw to change the pressure ,there is a special tool but it can be done with a pair of small screwdrivers

Thanks for the reply.  When I rebuilt it I did install new float needles/seats, and cleaned everything with carb cleaner while apart.  The spec for the float adjustment was 1", and that's the height they were at already so I didn't have to readjust them.  The floats themselves are the originals.  They aren't the brass types, they are the solid plastic types.  I saw a video online that these types of solid plastic floats can actually start to absorb fuel over the years, but I'm not sure how to tell if that is happening, and the rebuild kit didn't come with new floats.  They felt very lightweight, and I could see a fuel line on the sides of them, so they appeared to have been working properly.  I did confirm (crudely, via blowing into the fuel line inlet) that both float seats were opening and closing.  So I may still have a mystery...

For the secondary air door adjustment, I think I'm fine with how it is now.  And I wasn't excited about pulling that plug out.  Mostly I just questioned how this door operated in general, especially since it seemed locked in place by the choke dashpot.  When I couldn't see the dashpot relaxing, or the secondary air door opening, when I pulled on the accelerator in neutral I thought something might be wrong.  But sounds like this is all normal operation in a garage-test environment.  Thanks for the great explanations guys!

- Wade

Hi Wade, Cody recently put a tech book here just for you https://www.e-bodies.org/resources/master-technician-service-conference-reference-books/

I have found some float tabs (the part that rides on the needle)  bent way outta speck so you could adjust the float to spec but not have enough travel to hold the needle closed..without a good known one to compare to I don't know how you would determine this.. Also it's easy for the pins the floats rotate on get out of position which screws it all up.. 
  As CP suggested some trash in the line can keep a needle open, a quick shot of compressed air will sometimes clear..   
Go through the manual above step by step measuring everything twice..  :alan2cents:


Good luck
Wade

Sounds good Bullitt, thanks.  One issue troubling me is that the carb is leaking out of both rear corners, which would have to mean that both brand new float needles/seats are contaminated or messed up somehow.  And this problem was happening before, which is why I took the carb all apart in the first place.  But it is an interesting thought about the float tab operation and purging with compressed air, I'll try all that.  Looks like I'll have to take this whole thing apart one more time....

Thanks again for all the input guys!

- Wade

One more thing comes to mind..I've discovered you really need to re-tighten the screws that hold it together after a heat cycle.. They will be loose. 

Quote from: Bullitt- on May 09, 2017, 04:56:49 PM
One more thing comes to mind..I've discovered you really need to re-tighten the screws that hold it together after a heat cycle.. They will be loose.

Thing is the screws hold the top in place on the body & the body in place on the baseplate... But the fuel level should be well below the upper gasket & there shouldn't be a path for fuel to reach the lower gasket.... The upper gasket may see some splash from fuel flowing in through the needle & seat....   I seem to recall the gasket that seals the seat to the upper body was designed to incorporate a splash shield.....  I'm gonna go look & se if my memory is correct...

Ok I was wrong, the splash shield was on a Motorcraft carb...

So Wade, how much fuel are we talking, is it constantly getting wet or just a slight wetness when the bowls were filling?   If it's constantly wet to the point of dripping, fuel is getting past the needle & seat, possibly sunk floats, possibly the seats aren't sealing to the lid... 

Sorry to chime in so late......

Diagnosing a carb long distance is REALLY difficult. That said, the single most common issue concerning fuel coming from the upper areas of any carburetor is a float (or floats) that are out of adjustment, too much fuel in the bowls will cause it to purge out wherever it can.

.02

while I quite agree that a float issue may be the culprit here, I've also got suspicions about those swine O-rings mentioned. While they were put in the bowl, were they in the correct position to seal as they're supposed to do? They were the issue when I rebuilt my TQ. Just my  :alan2cents:

Quote from: jimynick on May 09, 2017, 08:16:45 PM
while I quite agree that a float issue may be the culprit here, I've also got suspicions about those swine O-rings mentioned. While they were put in the bowl, were they in the correct position to seal as they're supposed to do? They were the issue when I rebuilt my TQ. Just my  :alan2cents:

Those O rings keep fuel from pouring into the engine through the lettering wells but don't effect fuel coming out the top...

OK, I found and fixed the culprit (with your guy's great help)!  It was indeed warpage of the phenolic, just as Sheldon and Cody suggested.

To diagnose it, first thing is I ran the car for a bit, then pulled the carb off, being sure to hold it flat.  I then removed the air horn, and found the bowls to be about 2/3 full.  More importantly, the walls of the top 1/4" of the bowls were bone dry, so I felt good about the float operation.  So then I took the phenolic center out and set it on a flat table.  Can you say U-shaped???  Especially right in those 2 back corners.  Ugh!  While the floats were fine, those corners are right where the needle valves are, and as Wild suggested I believe the fuel was splashing around the needles on entry, which then spewed fuel out of these practically unsealed corners.  Before this fix, driving the car 2-3 blocks would be enough to soak each corner halfway down the sides of the phenolic.

So, as suggested I sanded and sanded it on a flat surface, then sanded some more... For about a half hour, until that top surface was perfectly flat.  After reassembly and reinstallation I just got back from a 1 mile trip around the neighborhood, and no wetness whatsoever!  Thanks All!

- Wade

Nice job everyone!!!!   :clapping:   Glad to hear that you got it fixed.    :bradsthumb:

Back in the 70's & early 80's our local Chrysler dealer parts dept kept 2-3 Thermoquad bodies on the shelf....  Common problem... Still a great carb....

Good news Wade!

 :woohoo:   Great Work...   

All that being said I've had my hands inside of probably a dozen TQs & never found a warped phenolic body   :dunno:

 :bradsthumb:

Glad to hear you not only found the problem, but were able to fix it, good going!  :bigthumb:

Glad to hear you figured it out. For what its worth, contrary to popular belief warped bodies on TQs are quite rare. I probably restore 100 a year and havent had a warped one (enough to require repairs) in over 5 years. That said, its a standard procedure of mine to check them all on a surface plate for flatness. A small amount of warpage is normal and wont leak once the screws and gaskets are properly installed but excessive warpage can lead to leaks and other problems. .02
.

Thanks again for all the nice comments and great support.  Like I said, in hindsight I should have just checked the flatness while I had it all apart the first time.  Live and Learn!  Much appreciated guys, thanks!

- Wade