Thermo quad bogging badly when stomped on

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dancinwulf

dancinwulf

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My 340 Thermo quad seems to just go flat when you stomp on it, is that fuel
or timing. runs fine until you really put the peddle to the metal.
 
Secondary air door may need tightening.
 
Double check to see how far open the throttle blade are.
Re adjust the pumpshot.

Tells us about the drivetrain and engine modifications.
Be detailed.
 
rebuilt 340 727 trans it was running fine then suddenly when you stomp on it
it just goes flat. How do I adjust the pump shot my knowledge is more on engine repair not carburetors
 
dancinwulf said:
Not sure how to tighten the air door any help would be appreciated
Click to expand...

There's a special tool sold to do it, but you can do it carefully with a small and medium screw driver. If you look on the driver's side of the carburetor, there's a ring adjuster with two slots that locks down on a slotted adjuster. Find a flat screwdriver that fits the lock ring on both slots and carefully break it loose just a tiny bit....but not loose enough to unload the inner slotted adjuster. That adjuster isn't threaded and if you loosen the lock ring a lot quickly, the center adjuster will unload quick and sometimes makes the spring come off the inside. Then you'll have to remove the carburetor top to fix it. Not the end of the world, but being careful will keep that from happening.

So just barely loosen the lock ring just a little. Now, turn the inside adjuster counter clockwise with the smaller screwdriver to tighten the air door. You can take your finger and feel the tension on the door as it tightens up.
 
Here's the tool. It's a T handle with a hollow handle. It had a ring on the end with two tabs to engage the lock ring. You insert a flat screwdriver into the hollow handle to hold the inside adjuster to keep it from unloading while you loosen the lock ring with the tool. It's a nice tool, but I never have needed one.

Thermoquad Air Door Adjusting Tool
 
The pump shot is adjusted externally by moving the linkage rod located at the bottom of the throttle arm into the hole closer to the carb body up top on the arm that see saw’s back and forth located up top, drivers side, up front. Readjust the linkage arm (bend it) to get the same height on the other side of the fulcrum point for the plunger it is connected to.

Upper right side of the picture below.

94CBD648-CB96-46AD-9A0E-9AED4F53F4FA.jpeg
 
Or the air door is not opening I've seen that happen will also cause the engine to bog tap the air door with your finger it should have a light resistance and spring back quickly
 
Dancin,
Because this happened 'suddenly', it is a back to basics job.
Check that the choke blade is fully off & linkages across the top of the carb are free & working.
I doubt it is acc pump shot, but could be. Easy to check. Run the engine & switch it off. Remove A/C & while looking into the pri bores, actuate the throttle lever. You should see a squirt of fuel from the acc pump nozzle into the pri bores.
No squirt means an acc pump problem. Plunger worn out, valve problem, link hose off. Needs a new kit.

Secondary operation. Any binding here will cause a very rich condition & a bog. A rich bog usually has a blubbery feel about it, hard to explain, but different than a lean bog. When these carbs get old, the sec air valve [ right rear of carb ] rubber diaphragm in the AV goes hard. This prevents the AV from opening, acts like a choke & you get a rich bog. To test: while watching the diaphragm, start the engine. Plunger should pull in. If not, dia is faulty, or rubber hose is leaking, etc.
If dia pulls in ok, switch off the engine. Gently push in the dia actuator with a flat blade screwdriver & then let go. The actuator should release smartly. If it is sluggish, dia rubber has gone hard.

If all of the above checks out ok, I would get a rebuild kit for the carb & rebuild it, especially it has a lot or unknown miles on it.
 
Suddenly after rebuild or suddenly while driving?
 
Yep AJ, completely agree. They do pretty good in drag racing too!
 
If the carburetor was running fine as you say in your second post and then suddenly started bogging when opening the secondaries, then either the secondary spring has come unattached or broken, or the choke pulloff has gone bad.
 
rumblefish360 said:
Suddenly after rebuild or suddenly while driving?
Click to expand...
No I replaced the gasket between the manifold and the carb with a phenolic one which is slightly thicker so I must have messed something up. If I disconnect the vacuum hose from the chock it all works fine so not sure what I did. With the hose connected it holds it closed even when warm.
 
RustyRatRod said:
If the carburetor was running fine as you say in your second post and then suddenly started bogging when opening the secondaries, then either the secondary spring has come unattached or broken, or the choke pulloff has gone bad.
Click to expand...
You are right its the choke pullout if I disconnect the vacuum line to the chock it works fine but the chock is new so not sure what happened
 
Looks like a nice car, why do you want to " Stomp on it" ? Is this from a dead stop or a roll ? What are you trying to have happen ?
 
RustyRatRod said:
There's a special tool sold to do it, but you can do it carefully with a small and medium screw driver. If you look on the driver's side of the carburetor, there's a ring adjuster with two slots that locks down on a slotted adjuster. Find a flat screwdriver that fits the lock ring on both slots and carefully break it loose just a tiny bit....but not loose enough to unload the inner slotted adjuster. That adjuster isn't threaded and if you loosen the lock ring a lot quickly, the center adjuster will unload quick and sometimes makes the spring come off the inside. Then you'll have to remove the carburetor top to fix it. Not the end of the world, but being careful will keep that from happening.

So just barely loosen the lock ring just a little. Now, turn the inside adjuster counter clockwise with the smaller screwdriver to tighten the air door. You can take your finger and feel the tension on the door as it tightens up.
Click to expand...

Awesome advice. How could you discover this yourself!
 
dancinwulf said:
You are right its the choke pullout if I disconnect the vacuum line to the chock it works fine but the chock is new so not sure what happened
Click to expand...

It shouldn't work "fine" with the choke pulloff disconnected, because the choke pulloff doubles as the vacuum brake for the secondary air door. Disconnect the hose from the choke pulloff at the carburetor. Now depress the choke pulloff into the diaphragm and put your finger over the hose where you disconnected the it at the carburetor base and let the choke pulloff go where you depressed it. If it stays put and releases when you take your finger off the hose, it's good. If it releases before you remove your finger, it's bad.
 
dancinwulf said:
No I replaced the gasket between the manifold and the carb with a phenolic one which is slightly thicker so I must have messed something up. If I disconnect the vacuum hose from the chock it all works fine so not sure what I did. With the hose connected it holds it closed even when warm.
Click to expand...

Where do you have the choke pulloff connected? It should be at the base of the carburetor in the rear.
 
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