Packard Forum Index » V-8 (1955-56)

Carburetor float level can be critical.

Does the surge problem occur at lower altitude the same as at higher altitude? Does engine temperature or ambient temperature seem to affect it?

Carburetors depend on air pressure differential to function, and so do conventional fuel pumps. At higher altitudes, less differential is available to move gasoline through the carburetor circuits in any particular operating mode. Perhaps a float level setting at the high end of spec, or even a bit higher, might help if you have a lean surge. A rich surge would require smaller main jets or a revised power valve setting.
Gasoline will vaporize more easily at high altitude, which would favor vapor lock, and conventional fuel pumps will lose efficiency at higher altitude. The combination of altitude, modern gasoline blends, and normal operating heat might induce mild vapor lock under load. An auxiliary pump located near the gas tank would prevent any fuel delivery problems related to reduced stock fuel pump efficiency.

Carburetor "icing" could be an issue, though that usually occurs where humidity is readily available.

A hairline crack or internal carbon track in a distributor cap or rotor can cause hard to trace issues.

Supression-type plug wires can look like new and still fail to function properly, usually evident when the engine is loaded.

Spark plug boots can carbonize and allow the current to jump to the plug base, especially if the boots are pushed too far down over the plug insulator. Any accumulation of carbon or other matter on the spark plug insulator can cause "flashover", which usually shows up when the engine is loaded. A very slight exhaust leak at the manifold can soot up a spark plug or damage a boot. A weak ignition coil or one with compromised insulation can cause surging under load.

Problems with an ignition resistor, if present, or a lack of one, originally equipped, can cause surge or stumble problems.

Some automatic transmissions are VERY sensitive to fluid level. Too much or too little can cause issue.Some transmissions require a specific procedure to check fluid level accurately.
A transmission filter can cause problems, as can trouble with a vacuum modulator, if present, and any plumbing related to it.

I drove it today and I think the problem is carburetion.

It only surges when the secondary butterflies are open. I can slowly roll into the throttle and it pulls smoothly until I reach the secondaries at around half throttle, then it starts surging after the secondaries open. It pulls strong with good power. I can feel the "rush" as the secondaries open...it surges while doing it.

I'll pull the carburetor apart and check it out. Maybe the secondary jets are the wrong size or something is clogged. A plugged air bleed could be all that it takes. If the fuel flow surges or the air flow surges, it will make the engine surge.

Inadequate "float drop" might be an issue. Float drop relates to how far the float can allow the float valve to open.