ok I know I need help…lol…1978 dodge 440 with 4 barrel theromquad…I seem to never get the secondary valve to open…if I stomp on gas nothing wether parhed or driving…I am guessing this is my lack of power problem. I am not sure how the secondaires open? vacuum or linkage…any help would be great
I have not seen a Thermoquad in many years. My suggestion would be to look for lock-outs caused by the choke. If the choke is not fully opening it may be keeping you from getting 4 bbl action. Outside of that, you have to find someone that has more experience than me.
Pictures I’ve found seem to show linkage, not vacuum, but who knows? 34-year-old carbs can have all kinds of problems, the thermoquad body can warp over time, especially with the heat in a motorhome’s engine compartment. More info here:
Just happened to have a book lying nearby with some TQ info in it about the secondary lockout.
Operate the choke linkage to the full open position.
Look at the tang on the secondary linkage and the stop.
There should be a gap of approx. .075 inches.
If not, bend the tang to attain that gap.
wow tanks for such a great site and to all whom replied…I am working with the choke to see if that is what is holding back the secondaries…I think this is the right track…will keep all posted
As long as you baby it when it’s cold, you could probably just remove the troublesome choke lock out on the secondaries. I did exactly that with a quadrajunk (Quadrajet) on my first car.
The secondaries use both linkage and vacuum to open the secondaries. When you go past 75% throttle, the linkage moves a link on the secondaries that limits how far the secondaries can open, then the vacuum has to begin to build up in order for the secondaries to actually open.
It should only take 5 to 7 inches of vacuum to open the secondaries up to the % throttle that you have opened to.
Have you found that the secondary plates are opening to the full open position when the primarys do the same? You can test this with the engine warmed up, choke off, and engine off. Push open the air valve and advance the primarys to full open and look. I am assuming that this is in a motor home application where you can observe the carburator action while someone else is driving.
If you have a problem with the primary part of the carburator or any other power production problem, the secondary throttle plates can be open but the air valve will not drop. In fact, if you force the air valve open while pulling under power, the engine will probably stumble badly.
You might have a plugged jet on the primary side or ignition failure.
Keep us in the loop on this problem. Let us know what you find.
ok so here I am again…have looked at it carefully…please excuse me if I call a part wrong…On the passenger side of carb there is a device that is hookes to the secondaries…it has a vacuum line comming off bottom plate of carb…when in idle theres is plenty of vacuum which is keeping secondaries CLOSED tight…when I give gas and bring up the rpm it seems the vacuum increases which means they are even harder to open? so any help guys?
I think what you are referencing is the choke vacuum pulloff unit. It would have manifold vacuum on its hose to pull the choke open on cold startup to lean the mixture from starting mixture to cold choked running setting. The odd thing about the TQ is that the choke operating shaft is over the secondary air valves but is connected to the choke plate over the primarys. The opposite end of that shaft operates the fast idle cam and the secondary throttle lockout lock.
The secondary air valve is spring loaded i.e. you can push that down manually. The way the TQ works is that the secondary throttle plates open progressively with the primarys below the secondary air valve. As the engine can develop vacuum i.e. can handle the air/fuel mixture, the air valve drops down allowing air through those massive secondary throttle plates. If you notice the shape and location of the support of the secondary air valve, you can invision how vacuum would pull that open. The counter spring calibration is set according to the application.
let me ask a question here, when you look at the secondaries on this carburetor, are there two sets of throttle plates (aka butterflies), one at the bottom and one at the top or is there just the one set at the bottom? Most of the ones I’ve seen only have one set of butterflies. The primary has two, one for throttle and one for choke. There is no choke on the secondaries but it could have two sets of throttle plates, one mechanically operated and one set vacuum operated.
Yes Keith I have upper and lower secondary valves…lower seem to be spring held closed but might open with air flow…uppers are connected to a vacuum device that keeps them closed when there is vacuum…meaning with the engine on and running…I was thinking my vacuum would decrease when under hard throttle but it seems that it incresaes at least when sitting in park…today we will be driving it with engine compartment open and a vacuum gauge connected to check operation.
Vacuum should NOT be holding the uppers closed, just the opposite. When you are in gear and you floor the gas pedal at low rpm. the vacuum in your intake manifold goes to zero, or pretty close to that. At zero vacuum, there is very little airflow through the venturis so fuel is not drawn into the air stream and into the engine. The engine bogs down. This is why the secondaries should remain closed.
As the vehicle accelerates and rpm’s begin to rise, a vacuum begins to build up inside the intake manifold. As the vacuum builds up, it can support a larger throttle opening without bogging down the engine. At around 5-7 inches of mercury, the secondary plates begin to open. They should be fully open a little above that.
You may not see these opening at idle speed with a high vacuum because there is usually a part of the throttle linkage that prevents them from opening at anything less than 75% throttle.
You need to find somebody who knows car and buy them a six pack to look at yours, it should only take a couple of minutes for somebody who knows carbs to figure out if the basic vacuum and secondary operations are correct.
In researching at Google (thermoquad rebuilding) I came across a explanation of an auxillary application of the choke pull off on the TQ. It is linked to the secondary air valve (upper spring loaded door) and acts as a dashpot for that air valve. The principle is that if the secondary throttle plates are opened abrupt, the diaphragm slows the opening of the secondary air door causing a enrichment through the secondary that mimicks an acceleration pump. As the manifold vacuum heads toward zero, extra fuel is added to compensate for the delay of the secondary fuel flow keeping up with the air flow and to wet the intake manifold runners. In the video the narrator explains that the dashpot effect of the vacuum diaphragm prevents a lean bog upon sudden openning of the secondarys. He also advised to keep the choke pulloff/dashpot fresh so it responds quickly.
Learned something I didn’t know. I also found that the M4MC (quadrajet) uses the same technique to dashpot its air valve for the same purpose.
Let us know if you find out more
Assuming it’s an automatic transmission, does the transmission downshift to allow enough RPM so the secondaries have a REASON to open…??
Yes, Caddyman. And along that line, the ignition timing, vacuum and mechanical advance must be correct to enable the engine to operate correctly to produce the vacuum needed to open the secondaries. And in diagnosing the carburetor you must consider manifold vacuum, ported vacuum and venturi vacuum. Carburetors are fading from the scene and mechanics familiar with them are probably rare.