My car stalls and has a rough idle

Question:

I have a 1957 Bel Air with a 283 smallblock and a two-barrel carburetor. It starts and runs just fine, but after driving a while it runs like it is running out of gas at times. Also, it stalls when I come to a stop, even when warmed up, and its idle isn’t as smooth as it should be. I gave it an ignition tune-up recently so I know the ignition system is in top shape. What do you suggest?

Answer:

It sounds like you may be getting air into the fuel system. Let’s start by checking the obvious things first.

Inspect the air filter to see if it is dirty and clogged. If it is a dry mesh filter, clean it and shoot light oil onto the filtration element. And if your Chevy has an oil bath air filter, wash it out and put in new oil.

Check your fuel filter. It may be clogged, even though it looks fine. The original fuel filter on your car would have had a glass bowl containing a ceramic filter inside. With that design, you need to remove the glass bowl carefully and clean or replace the element. Check later, after market disposable fuel filters by removing them, pouring the fuel out of them, and then blowing into them to see if there is resistance. If there is, replace the filter.

Next, check the vacuum advance line and fittings to the vacuum advance on the distributor. Make sure that it is in good condition and connected securely so it doesn’t leak. The metal line itself is not likely to give you a problem but any rubber flex lines are perishable and may need replacing.

Inspect all of the fuel lines and fittings for leaks and corrosion. It is possible that they are loose and leaking or have corroded or been chafed enough to develop a pinhole that would let air into them and cause the symptoms you are talking about. This may sound like a remote possibility but it is not that uncommon on older cars.

Now let’s check for the most likely reason for your problem. It sounds like you have an intake manifold leak. For this test, work outdoors and have a fire extinguisher handy. I have done this procedure many times without problems, but it is risky.

Start your car and let it come to an idle. If it won’t idle, have a friend rev it just a bit to keep it running. Now grab a can of flammable brake cleaner or carburetor cleaner that has the little plastic straw inserted to direct your spray. As the car is running, shoot a little of the cleaner around the areas where the intake manifold bolts to the heads.

If you have a leak at the joint, the car will rev a bit, and the idle will smooth out temporarily. Just spray a little along the gasket line, but don’t get sloppy with the cleaner because you risk a fire if you do.

An intake manifold leak will cause the cylinder that the manifold runner serves to run lean, and over the long run could damage your engine. You can try tightening the manifold evenly from the center to the corners to the torque specs in your shop manual and see if that solves the problem, but if that doesn’t do it you will need to order new gaskets. Also, get a tube of high-temp gasket sealer intended for exhaust manifolds. Gaskets for Chevrolet small-blocks are commonly available at most auto parts stores.

It would be prudent to check the manifold ports for flatness before putting in new gaskets. The intake manifold may have warped just enough to allow a leak. Use a steel straightedge and lay it across the ports. And then use feeler gauges and a flashlight to check for gaps. If you find any, have the port runners resurfaced at a machine shop before putting in new gaskets and torqueing in the manifold. When tightening the bolts, start from the center and work out, and torque the bolts to the specs in the shop manual. Finally, start the engine, warm it up completely, and then shut it off and tighten the bolts to specs again as needed.