We have a 49 chevy with a carburetorated 2.8L V-6-new aluminum radiator, over heats and dies after about 20 minutes of driving-new intake manifold gasket, coollant recovery system, new thermostat-what

A thermostat should only allow flow after reaching its desired temp, 195, however, if this is just idling and you have a brand new aluminum 4 core radiator then it may really not be reaching temp and reading low because of no load and a great radiator. I assume top of the intake manifold is where your temp sender is? Did you use a laser infra red temp reader and point it at the block in several spots? You may see175, 185, etc all over the side of the eng. The neck of the thermostat housing should be the closest to 195 but the lower hose might read 140 / 150. Alum radiators work awesome so when that thermostat does open it's a rush of cooler coolant. What is it doing when you drive steadily @ 40 - 50 mph? If the gauge reads say 140 and just fluctuates slightly from that then I would say yes, replace the thermostat. If your gauge shows it now reaching temperature then it's just that your radiator is doing a helluva job when it idles. BTW, electric fans? On a switch or temp sender? Or mechanical fan always pulling air even at idle?
Hope this helps

Find the location where the upper radiator hose connects to the intake manifold of the engine by tracing it from the radiator to thermostat intake housing bolted to the intake.

2 Remove the radiator hose from the thermostat intake housing by loosening the band clamp that secures it into place. With the clamp loosened, pull the hose off the housing.

3 Remove the two bolts that secure the housing to the intake manifold and then lift the housing off the intake.

4 Lift the thermostat out of the intake manifold and place the new thermostat into the hole. The end of the thermostat with the pin protruding from it sticks out of the intake manifold.

5 Lift the thermostat intake housing gasket off the intake manifold and place the new one onto the intake manifold.

6 Place the thermostat intake housing onto the intake manifold and bolt it into place. Reconnect the hose using the band clamp.

Six-Cylinder Engines

1 Drive the Dodge onto a set of front-end ramps and secure the parking break. Allow adequate time for the radiator fluid to cool down. Thirty minutes is usually adequate.

2 Loosen the petcock on the lower right hand side of the radiator and drain all of the radiator fluid into a drain pan. Close the petcock once drained.

3 Trace the lower radiator hose to the location where the hose connects to the engine block. The metal housing the hose connects to is the lower thermostat outlet. Remove the hose from the outlet by loosening the band clamp and then by pulling the hose off the outlet. Be careful--some fluid will drain from the hose.

4 Remove the two bolts that secure the thermostat outlet to the engine block and pull the thermostat outlet off the block.

5 Discard the O-ring inside the bottom edge of the thermostat outlet and place a new one into the outlet.

6 Pull the thermostat out of the engine block and replace it with a new one. The end with the pin protruding from it sticks out of the engine block.

7 Place the thermostat outlet onto the engine block and secure it into place. Reattach the lower radiator hose with the band clamp.

8 Open the radiator filler cap located on top of the radiator and fill the radiator with the fluid previously drained from the system.

please dont forget to give good rateing thank you

SOUND LIKE FUEL PUMP.OR VACUUM LEAK AT INTAKE MANIFOLD.CHECK MAKE SURE CARBURETOR MOUNTING SCREWS IS TIGHT.

is electric radiator fan coming on? if not the relay or fuse is bad

if car run 10 minutes then cut off.it could bad ignition coil or intake manifold leak.when engine warm up in which aluminum intake manifold expands by intake gasket worn already leaking.the intake manifold expanding causing more air enter the engine.causing it to shut off.

Here are the common cause of surges, cold stalls, stalls at stops, slow idle speed, erratic idle speed, rough idle and engine hesitation (and other problems), it is in most cases the idle speed control air-bypass valve and or throttle valve and upper intake, these area's get full of gunk and combustion residue over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.

Idles so low it dies and you have to keep your foot on the gas or it runs fast and won't return to idle?

valve problem most likely...

Here is the common cause of surges, stalls at stops, slow idle speed, erratic idle speed, rough idle and engine hesitation (and other problems), it is in most cases the idle speed control air-bypass valve and or throttle valve and upper intake, these area's get full of gunk and combustion residue over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.

Here is the common cause of surges, stalls at stops, slow idle speed, erratic idle speed, rough idle and engine hesitation (and other problems), it is in most cases the idle speed control air-bypass valve and or throttle valve and upper intake, these area's get full of gunk and combustion residue over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.