Carburetor swap kills engine

  • 178,000 MILES
Hi, I need some help. I have a 1973 350 Chevrolet engine which had an old Edelbrock four barrel carburetor that hesitated and backfired a lot. I found a rebuilt 1973 Rodchester Quadra-jet for a good price, so I swapped out carburetors. The Q-jet starts her and idles her smooth and strong, but the second I try to put it into gear she kills, even when allowed to fully warm up. The other carburetor would let it run in gear once warm. Plus, I get a double whammy when she dies hot. Hedder heat retards my starter even with a shield wrap. Then it wont start until she is cool again. There are no idiot lights and I cannot scan for check engine codes because this engine was dropped into a 1982 El Camino and I do not think any of that was set up. Any suggestions to make run in gear would be greatly appreciated. She is what gets me to work and back daily-(was).
Thanks in advance, Tom
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Tuesday, November 29th, 2016 AT 1:22 AM

1 Reply

There are not many carburetor experts around any more, so I will pretend to be one.

If the first carburetor caused problems when trying to accelerate, even a little, that is a common symptom of a worn accelerator pump. We used to be able to buy those separately. Today you might need to order a complete rebuild kit to get it. One potential clue is to watch down the center of the carburetor, with the engine off, to see if two nice streams of gas spray in when you work the throttle by hand. If that is missing, suspect the accelerator pump or check ball. If the sprays are weak, check the adjustment of the pump's linkage.

For the second carburetor, there are two metering rods that lift up to expose more open area of the jets. Those rods are held down, (closed), against spring pressure, by engine vacuum. On some models, the rods are lifted mechanically by a cam that rotates with the throttle blade shaft. Since carburetors do not have a means to self-adjust idle speed like Engine Computers do, a slowing engine has less vacuum, and that can be overcome by the spring that pushes the metering rods up. That allows more fuel in and is supposed to bring idle speed back up a little. You may need to remove the metering rods and unscrew the jets, then wash out the passages. You can use a spray can of carburetor cleaner, but that is designed to evaporate very quickly, often before it is done doing its job. I prefer to use brake parts cleaner. It works just as well, but takes longer to evaporate, so you have more working time with it. Wear safety glasses. It is a good bet your eyes do not need to be washed out.

Also check for vacuum leaks. If you have a non-standard camshaft, you will already be starting out with seriously-lower than normal vacuum, and that will go down even more when engine speed decreases when you shift into gear. You need vacuum causing air flow through the venturies, (I do not know how to spell that!), To draw fuel in. Some vacuum hoses need to be connected to "ported" vacuum, meaning the ports originate above the throttle blade as it sits at idle. Some hoses need to be "non-ported", meaning they are under the throttle blade all the time, or are connected right to the intake manifold. If you have an EGR valve, be sure that has all the required vacuum hoses and thermostatic switches in the circuit. The EGR valve must never open at low speeds. That will cause rough running and stalling.

If you have the round black automatic choke assembly on the passenger side of the carburetor is that working? I can't remember exactly how the system gets its heat to cause the thermostatic spring to open the choke blade, but if it is not working, there can be another related cause. Some of those assemblies added an electric heater element inside to speed up its opening, and that can mask this problem. There is an exhaust passage that runs from the right cylinder head, through the intake manifold, then out the left head. When the engine is cold, the "heat riser" valve on the right exhaust manifold closes and forces the exhaust from that side to go through that passage. On Chrysler engines the choke's thermostatic spring sits in that passage. I cannot remember how GM did it, but instead of that passage being wide open to exhaust gas flow, GM used a very tiny hole in the intake manifold gasket to regulate the amount of flow. That hole often became plugged with carbon. Where the problem comes in is that passage going through the intake manifold runs right under the base of the carburetor to warm the fuel. Liquid gasoline does not burn. It goes out the tail pipe, wasted, and not producing any power to keep the engine running. That can cause stalling even when you're getting what looks like plenty of fuel.

Installing headers eliminates the heat riser valve in the right exhaust manifold and renders that system ineffective. You may also be missing the "heat stove" on the left exhaust manifold. Incoming air is drawn past that to warm it before it goes through the air filter. That reduces the chance of icing in the carburetor when the air is cold and humid. All of those methods of heating the air and gas were found to be needed to solve the problems you're describing, so now that they are occurring, you might need to rethink the headers.

I seem to recall there is a small pipe that directs heated air to the choke spring housing, and that pipe will not get hot if the hole is blocked in the intake manifold gasket. You will see the choke not opening, or opening slowly, but you wont see the base of the carburetor not warming up. That hole may also have been purposely eliminated for the aftermarket intake manifold. Aftermarket parts, including headers, do very little to help the performance of a street-driven engine. They are meant for applications where we want the most power we can get under certain conditions, like when racing and constantly under high acceleration and / or speed, and we all know how poorly race engines run at idle and low speeds compared to passenger car engines.

That brings me to camshafts. The profile of the lobes is modified to achieve the desired characteristics of idle smoothness, throttle response, fuel economy, power, and emissions. We can never get all of them with a carburetor. Each one is a trade-off for the others. A GM 350 with a four barrel of this era was already a powerhouse as it came from the factory. What you gain with the aftermarket parts has very little chance of being used when driving around town. If you like the novelty of these modifications, as I do, realize there are going to be some driveability issues you will never solve. If none of my suggestions pan out, you will likely have to look at rebuilding the carburetor. Mostly that means taking it apart to clean the passages, and replacing the few gaskets. Be sure to watch out for little metal check balls. You should only do this once you have a copy of the manufacturer's service manual to use for reference.
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Tuesday, November 29th, 2016 AT 3:33 AM

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