hey my car will start up but wont idle when it idle it starts to die and gas just overflows thee carb and the motor slowly dies any help would be nice. its a 1966 pontiac starchief withe a rochester carb and 421 if tht helps. thanks alot.
Patrick
Might be a stuck float.
Yeah, the overflowing for sure is a stuck float shut off valve. You can bang it with a screwdriver to dislodge it. Or pull the top of the carb off and inspect the float and needle valve for proper operation. Also, check the float itself. The metal ones could leak and the plastic ones can absorb gas to make them too heavy. Might be time to just do a carb kit on it to clean it up and install new jets, etc.
I’d guess that the carb kit (rebuild) is in order.
The carb is a simple device. The float in the float bowl operates a needle valve (the shutoff valve that Bing referred to). The valve allows gas into the bowl only until the float hits the high point (the bowl is full) and the float closes the valve. Just like the ballcock assembly in your toilet. This keeps the fuel supply on the “fuel side of the venturi” at a constant basically ambient pressure.
As the low lateral wall pressure in the venturi pulls fuel through the orafice from the float bowl, the float drops, opens the needle valve, and the valve lets some more fuel in.
If the float or the valve is not closing, because the float isn’t floating or the valve is hanging up, the fuel pump will just keep pushing fuel right through the bowl and into the venturi. Like a tiny firehose.
You know, considering the age of the car, if it isn’t all-original you may just want to buy a rebuilt carb and replace the whole thing. That’ll give you a new needle valve, new float, new gaskets and seals, new everything.
There is one other possibility, these had phenolic floats that can get saturated with gas. This has the same affect as a stuck needle and seat.
Yeah, Bing mentioned that. Were they using these phenolic floats in '66? I keep envisioning pressed metal cans.
I had a 66 Catalina, 389 Quad. Yes, it had the plastic floats.
Try this. Disconnect the fuel line from the carb. Plug the line. Start the engine and let it run out of gas. Re-connect the fuel line. Start it up and see what happens. Sometimes a bit of dirt in the needle & seat will flush out…if it still floods out, a carb overhaul is in order…
Im having the same issue 65 starchief 389 2 jet…i just rebuilt my carb still just turns over… That needle valve does it look like a nail inside a spring? Mine sticks thinkin thats my problem …ima newbie btw
The valve I was referring to is attached to the float. It controls the amount of fuel in the float chamber…
Most carburetors have a little pump next to the float chamber. This pump is connected to the throttle linkage and provides a squirt of gas when the throttle is opened. Check its operation by looking down the carb throats with a flashlight and work the throttle. You should see the fuel squirting in. This fuel is usually enough to allow the engine to at least momentarily start…Carburetors are actually fairly simple devices…If the fuel level in the float chamber is correct and the metering passages are not plugged up or blocked, they will provide the correct amount of fuel to the engine…
The needle valve does look like a nail. It could also be gummed up with varnish or have a piece of crud preventing it from closing it all the way. With old cars, all is possible.
The carb is a simple device… in theory. In practice, the devil is in the details. And getting this most analog of devices to perfectly meter fuel in all conditions, and having the car start, drive, and idle perfectly in any temperature and weather with a carburetor is pretty much a lost lore these days.
If you do decide to rebuild the carb (good idea) make sure to replace the float as well. That’s where a lot of flooding problems can be eliminated. Sticking needle valves are also a very real possibility. The phenolic type floats were actually a downgrade from the earlier metal floats. They were cheaper and failed (gas saturation) constantly.
Needle valves dont come in most rebuild kits …im going to buy a replacement carb for now and im going to start playing with timming …my 65 has 49k on her soo my replaced carb will b temp