AdamR
Big Member
I have 2 TQ, a 73-340 TQ and a 77-440 carb. Both carbs need a rebuild, the 73 carb need a secondary throttle shaft (broken screw). Will the secondary throttle shaft from the 77-440 carb work in the 73-340 carb ?
Thermoquad interchange
- Thread starter AdamR
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805moparkid
Slant and AFX Guy
no way to turn out out from the underside??
Yes.
AdamR
Big Member
No luck. Its flush on both side.
AdamR
Big Member
Thanks, Thats what I was hoping for.
AdamR
Big Member
Will the entire base plate swap ?
I think the bore size is different. 340 1 3/8" 440 1 1/2". Maybe Rob knows fer sure.
I just looked at my chart and they are different sizes.
I just looked at my chart and they are different sizes.
AdamR
Big Member
The plastic bowls look to be the same. I know the 440 carbs have the primaries and the later base plate has more vacuum lines going to them.
Measure the openings in the bottom. The 440 one should be a 1/8" bigger.
matchlight
New Member
One thing to watch for if you interchange base plates. There's a tiny hole on the front side of one of the secondary bores that's covered by the edge of the throttle plate when it's closed. As soon as the secondary plate starts to open, air flows through this hole, up a well, then along a small groove around the bore, through a calibrated restriction, and gradually spoils the manifold vacuum in the passage and rubber hose leading to the vacuum break diaphragm.
This serves two purposes. One is that it lets the vacuum in the diaphragm that's holding the air valve closed bleed off over a couple seconds. Without this damping effect, the moment the air flow against the valve overcame the force of the spring holding it closed, the valve would snap open and create a momentary lean flat spot.
The other purpose is to prevent too rich a mixture if the throttle linkage is opened just enough to crack the secondary throttle plates. That wouldn't create enough air flow to open the air valve, and it would act like a choke plate. There would be a very high vacuum in the secondary venturis, sucking way too much gas out of the secondary nozzles.
Well, I found out the hard way that if you get the wrong combination of bowl and base plate, the little air passages won't meet up and form a continuous pathway. Then the bleed air won't reach the vacuum diaphragm, and it will put too much closing force on the air valve. So when the secondary plates start to open, the air valve doesn't.
The mixture will be so rich the engine will actually lose power--like being on the freeway with the choke most of the way closed. The car will just keep stumbling until you open the throttle all the way. That lowers the manifold vacuum enough that even without the bleed air, the diaphragm finally lets the air valve open, and the car takes off.
This serves two purposes. One is that it lets the vacuum in the diaphragm that's holding the air valve closed bleed off over a couple seconds. Without this damping effect, the moment the air flow against the valve overcame the force of the spring holding it closed, the valve would snap open and create a momentary lean flat spot.
The other purpose is to prevent too rich a mixture if the throttle linkage is opened just enough to crack the secondary throttle plates. That wouldn't create enough air flow to open the air valve, and it would act like a choke plate. There would be a very high vacuum in the secondary venturis, sucking way too much gas out of the secondary nozzles.
Well, I found out the hard way that if you get the wrong combination of bowl and base plate, the little air passages won't meet up and form a continuous pathway. Then the bleed air won't reach the vacuum diaphragm, and it will put too much closing force on the air valve. So when the secondary plates start to open, the air valve doesn't.
The mixture will be so rich the engine will actually lose power--like being on the freeway with the choke most of the way closed. The car will just keep stumbling until you open the throttle all the way. That lowers the manifold vacuum enough that even without the bleed air, the diaphragm finally lets the air valve open, and the car takes off.
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