Surge Cures

When driving my ’66 GTO at really light throttle (just cruising along) or coasting, the motor surges and seems to miss, as if it has an ignition problem or it is running really lean (or maybe both). When I hit the throttle hard and get out of the idle circuit, it runs fine. When the car is parked in Neutral and I press the throttle, by the time I get to about 2,800 to 3,000 rpm, the same missing seems to occur—it is rough. If I am stopped at a light and the car has been idling for a few minutes, it seems to stumble off idle, though in normal driving the off-idle response is good. After a long drive, the car idles roughly to the point that it rocks—in a bad way!

My GTO has a Muncie M20 four-speed, manual steering and brakes, and is fairly original. Its factory-installed 389 engine was rebuilt in 2005 with a 0.060-over bore. The 093 heads are mildly ported and fitted with stainless valves. It has a Comp Cams 270H cam, 1.52:1-ratio rockers. The original Tri-Power has been rebuilt; the center carb has 0.063 jets and the end carbs have 0.068 jets.

The distributor has a Crane XR-i unit instead of points, the coil is a Pertronix Flame Thrower, and there is no ballast attached, though there is a resistor wire going to it. At the coil, about 12-13 volts are present.

The plug wires are 7mm reproduction units, though I have had other wires and it made no difference. I use ACDelco 43R plugs, though I’ve tried others and the same problem occurs. Initial timing is at about 12 degrees.

Vacuum advance is currently coming off of one of the center-carb studs (which goes directly into the manifold). Previously, the vacuum advance came off a port out of the center carb. I have not tried switching this around.

I set the engine idle to 750-800 rpm. When the car is “colder,” it idles smoothly, but when it’s really warm (coolant temp between 180-200 degrees) then it idles roughly. In most cases, the car has the “normal” stinky smell when idling. I have ’67 H.O./Ram Air exhaust manifolds going into a 2.5-inch dual system that has an H-type crossover.

Less timing makes the problem less apparent, though it still seems to have it at the 2,800-plus-rpm range. With 0.062 jets in the center carb, it had the same problem. I can move up to 0.064 jets.

On a vacuum gauge at idle/slight throttle, there are no bad fluctuations. I’ve checked for vacuum leaks but haven’t found any.

Should I treat this as a carburetor problem, an ignition problem, or both? I’ve been fighting this for too long. I want to be able to enjoy the car more.

Adrian Jenkins - Renton, WA


Jim Taylor responds:

From the information you provided, I think you’re close to determining your engine’s issues. You believe the “surging, missing, and running rough” are being caused by a “carburetor problem, an ignition problem, or both.”

Since the invention of the internal-combustion gasoline engine, men have been very diligent in carefully assembling the fixed parts of the engine using the best components available, but that’s not the whole story. These parts have to be tuned to work well together, as the engine can fail very quickly if allowed to run too lean or with too much ignition timing.

Let’s deal with some obvious stuff first. You say you had a vacuum gauge on the engine and found “no bad fluctuations.” This would indicate no valve-sealing problems. Your manifold vacuum would be about 4 Hg lower with the Comp Cams 270H cam than the original “068” cam. The Comp cam has a 110-degree LSA; the original Tri-Power cam had 115.5-degree LSA. The missing 5.5 degrees of lobe separation, which lowers manifold vacuum, is not compatible with what Pontiac, Delco, and Rochester had designed for proper operation of their vacuum-advance units, distributor, and carb, so some tuning will be necessary.

You state, “The vacuum advance is currently coming off one of the center carb studs.” This means the vacuum-advance unit is receiving direct manifold vacuum. If it’s the original unit, does it still work? You also state “previously the vacuum advance came off of a port on the center carb.” Switch the vacuum line back to the carb and see if there is a difference. The surge you’re experiencing could be a half-bad vacuum advance unit. That is, the unit holds vacuum against its spring, but leaks. When the diaphragm slowly leaks, the opposing spring takes over and releases the breaker plate to a momentary timing retard condition. The surge is the vacuum timing recovering to advanced.

My advice is to disconnect the vacuum advance so that the engine uses only the initial and the mechanical advance. Order the Mr. Gasket #928G curve kit and install it. The springs will let you super-tune a timing curve just for your engine. The bronze bushing will limit total timing so you don’t scatter your engine. Try to dial in approximately 34-degrees total timing (initial and mechanical combined) by about 3,000 rpm.

Moving to the fuel side, check to ensure you have a completely clean fuel tank vent. If it is even slightly clogged, the engine will starve for fuel. If atmosphere cannot enter the tank as fuel leaves, it will stop leaving. Make sure no rubber fuel lines are cracked. I’ve seen them suck air but not leak fuel. Install a new fuel filter.

If your carbs were maintained as original, each will have a tube screen filter at the base of the fuel bowl. Make sure they’re not clogged. Of utmost importance are the small screen filters on the needle seats, as they are often clogged with small pieces of black rubber from inside the gas hose.

All of the areas mentioned could cause a lean surge.

The two 0.063-inch jets may need to move up to 0.066 because of today’s gas. A few thousandths larger could cure surging on the road. Make sure the power valve and the spring-and-piston assembly that operate it are in good condition. Rough idling and the stumble off idle you indicate when the engine is hot “after a long drive” or “idling for a few minutes” could be a float set too high or the fuel itself percolating (vapor lock) from a too-hot carburetor. Today’s fuel goes to vapor easier than previously, possibly because of the boiling point of ethanol.

We have cured similar problems by adding hardwood spacers under carburetors. I got the idea from dozens of original sets of Tri-Powers I took apart, which had three phenolic spacers under the carbs. A carb does not work well when the pure liquid it requires is turning to vapor.

You also said, “The car has the ‘normal’ stinky smell.” I guess this is the raw-fuel smell. Again, the lower vacuum at idle with your cam should direct you to things operated by vacuum on the engine. If the vacuum is not sufficient to hold off the power valve piston-and-spring assembly, it will be engaged, causing a rich, eye-burning condition at idle.

Generally speaking, it’s usually not the cam you have that causes driveability problems. It’s about tuning the carb and ignition timing to enhance the benefits of the more powerful cam. One more thing: You listed the spark plugs as ACDelco 43R. It’s probably just a keystroke error in your letter but check to make sure—because that’s what my lawnmower uses. Install AC R45S or the colder R43S. Good luck.

Email questions to: tom.demauro@sorc.com