The Cleveland Stall
Michael Roach; Fairfield, CT: First, Car Craft rocks. Here’s my problem. I bought a ’71 Mustang Mach I with a 351 Cleveland, a 650 Holley carburetor, and an automatic trans. The car’s in decent shape but it stalls out, especially when slowing down at a light, backing up, or anytime the engine is below 1,500 rpm. The guy who solid it to me said the cam is not stock. I’ve changed the plugs and wires, but so far no change. I’m not a mechanic and I’m a little short on cash, so if you can offer some advice I’d be eternally grateful.
Jeff Smith: Driveability problems are some of the most frustrating to diagnose, but thankfully we can offer a whole slew of suggestions to ensure decent throttle response. First we’re going to assume the engine is in good shape mechanically and that there’s probably only a few minor tweaks to perform. Most driveability issues are related to vacuum leaks, or more accurately, unmetered air entering the intake manifold. We’ll start with some simple diagnostic checks first to eliminate mechanical problems with the engine and then home in on the repairs.
First, let’s make sure the engine is in good shape. Borrow a compression tester and have a friend help you by warming the engine first, then yank all the spark plugs and disable the ignition by removing the positive lead to the coil so the spark plug wires won’t arc to ground. Using a compression tester, measure the compression in each cylinder and record your numbers. Make sure the throttle is open and crank the engine for each cylinder a minimum of three pumps against the gauge. Now compare the numbers from each cylinder; they should be within 10 to 15 percent. Assuming this is true, we now know the engine is in pretty good shape. Next, replace the spark plugs, enable the ignition, and fire the engine to check initial ignition timing. Be sure to remove and plug the vacuum-advance line from the carb to the distributor. At idle, you should have around 6 to 8 degrees of initial timing. If the number is less than 6 degrees, loosen the distributor hold-down bolt and twist the distributor to advance the timing. You might try 10 to 12 degrees initial timing. Now that we have the initial timing set, reconnect the vacuum advance line to the distributor.
Before we start this next test, borrow a vacuum gauge from a friend. Start the engine and read the vacuum in inches. Be sure to connect the gauge to manifold vacuum. The gauge should read between 12 and 18 inches of vacuum, but it can be more or less depending upon the camshaft. More than likely, the gauge needle will not be steady, but instead fluctuate between high and low readings. The more the needle moves, or hunts, the worse the idle quality. Now let’s try adjusting the idle mixture screws. On a 650-cfm Holley, these will generally be found in the primary metering block. With the engine shut off, first turn each idle mixture screw in until it lightly seats and count the turns to that point. Each idle mixture screw should be turned out roughly 1 1⁄2 turns. If not, you should set both idle mixture screws at that adjustment and see if the vacuum reading improves. If it does, turn both idle mixture screws in roughly 1⁄8 turn and reference the idle vacuum. If it improves, continue adjusting the idle mixture screws in this direction with 1⁄8 turns until the idle vacuum stops improving. If the vacuum drops on your first move inward (lean), then turn both screws outward 1⁄8 turn and run through a similar tuning operation.
If the idle does not stabilize or improve or it seems that the engine is not responding consistently to these adjustments, then the engine probably is suffering from a vacuum leak somewhere in the induction path. Start with a can of WD-40 or carb cleaner equipped with one of the red spray nozzles attached. With the engine running, squirt around the base of the intake manifold and watch for an instant increase in idle speed. If you discover an area around the intake manifold base, first try tightening the intake manifold bolts. If that doesn’t help, then it appears there is an intake manifold gasket vacuum leak, which is fairly common. This will require draining the coolant from the block, removing all the intake bolts, and prying off the intake. Luckily with a Ford, you don’t have to remove the distributor.
When installing a new intake gasket, be sure to use a dedicated gasket cement like Gasgacinch or that nasty dark brown gasket glue you can find in a bottle at the auto parts store. Don’t use RTV silicone around the intake ports because gasoline over time will dissolve the RTV and cause another leak. Make sure both the gasket surface on the heads and the intake are as clean as you can make them and carefully mate the manifold to the head and start all the bolts before beginning the torquing procedure. That should solve your driveability problems. If you continue to have problems, consider having the carburetor rebuilt, as it could be suffering from partially blocked idle passages or other common fuel delivery maladies.