Lean surge at cruise speed

[heus]

My Newport with a 383 runs great under acceleration. When cruising, however, it surges or misses. I assume this is a lean condition, much like I had with my Corvette. I cannot find any vacuum leaks. I used WD40 and also an unlit propane torch. Car stalls when carb is completely covered with a cloth. Therefore, I dont think its a vacuum leak.
It is a Stromberg WWC 2 barrel carb. Any tips for richening the fuel mixture?
One new interesting thing that I discovered was my fuel was gurgling near the gas tank when I got back from a ride. I assume this means my vents are clogged? I then took a ride without the gas cap on, and i swear the surge was almost all eliminated. Could clogged vent lines cause a lean surge at cruise but allow the engine to run fine under light to heavy throttle?

 

[Davea Lux]

My Newport with a 383 runs great under acceleration. When cruising, however, it surges or misses. I assume this is a lean condition, much like I had with my Corvette. I cannot find any vacuum leaks. I used WD40 and also an unlit propane torch. Car stalls when carb is completely covered with a cloth. Therefore, I dont think its a vacuum leak.
It is a Stromberg WWC 2 barrel carb. Any tips for richening the fuel mixture?
One new interesting thing that I discovered was my fuel was gurgling near the gas tank when I got back from a ride. I assume this means my vents are clogged? I then took a ride without the gas cap on, and i swear the surge was almost all eliminated. Could clogged vent lines cause a lean surge at cruise but allow the engine to run fine under light to heavy throttle?

If the vent line is clogged, you will be running the tank in a vacuum. You should get a decided hiss when you remove the fuel cap. You might want to check to see if someone has installed a non vented fuel cap on a system that was designed for a vented one.

If you build enough vacuum in a fuel tank it can start to boil the fuel and cause the gurgling you describe. The tank could also be boiling if you have a tailpipe in close proximity to the tank. If you have a very high level of vacuum in the tank, that will make your fuel pump less efficient and starve the engine for fuel. As the car runs normally under acceleration, I do not think that is your problem.

The lean surge is most likely caused by a partially clogged jet in the carb or an idle circuit that is set much too lean. If the car idles normally, then the jet is a likely culprit. Today's blended fuels also do not flow the same as the fuels that these cars were designed to run on. You might have to locate a jet kit for your Stromberg and go one step richer. If the carb has been recently rebuilt, it is also possible that the float level is set too low and this is causing the lean surge.

Dave

 

[CBODY67]

When I had the OEM WWC-3263 on my '66 Newport 383, when it was "just a used car", it ran well and didn't have any performance issues. I'd tweaked the accel pump linkage for a longer pump shot that helped a little. Only later issue was the "air cleaner nut over-torque issue".

The "stoich" for E10 is 14.1, I believe, with E0 being 14.7. A little learner but not nearly enough to need bigger jets just because of the fuel. The emulsion tubes in the carb are generously-sized, so not a big chance they have any significant restrictions in them.

Over the years, there might have been some accumulation of deposits in the fuel passages below the jets and before the throttle bores. IF they're soft deposits, a good carb soak will probably get them. If they are hard deposits, not much way to mechanically remove them. Just mentioning that as a remote possibility.

The GAS CAP is probably the problem. Reading the C-300 articles of Curtis Redgap at www.allpar.com, When they took one of the first Plymouth Fury 2x4bbl cars to Daytona to run on the sand, the first run was close to 150mph, faster than the hallowed C-300 was, which was something of a "no-no", even if they ran it in the Factory Experimental class. On the return run, the car started starving for fuel and ran much slower. Enough slower that the two-way average was less than the C-300. When it limped (or coasted) back into the pits, a new gas cap cured the problem.

The correct gas cap, as I recall, is a vented cap but also one that is "Anti-Surge" so that it doesn't leak during acceleration or when the car is parked "uphill"? The fuel tank could well be a "no-tube" version, rather than a "two tube" version?

IF the fuel pump has enough vacuum, it can collapse the tank, from what I recall. Giving the bottom a concave look rather than generally flat on the bottom.

If the rear exhaust is in the OEM location, it should be far enough away from the tank to not really affect the heat of the fuel in the tank. Heat reflection from the roadway will have more of an affect, I believe. UNLESS there are a set of dual exhausts run under the tank instead of beside it!

Try the gas cap first. Easiest and cheapest, first.

Then go from there.​

CBODY67

 

[67Monaco]

Exactly what my Monaco did when the tank vent was plugged.

Granted I plugged it, just forgot to remove it.

 

[Clover]

You didn’t say what year but if it’s got points, make sure they’re in good shape with a good steady dwell reading.

 

[heus]

Ok, things I have done so far, but still have the lean surge:
New plug wires
Checked for vacuum leaks
Drove without gas cap (in case of plugged gas tank vent)
New fuel filter
Replaced points with a Pertronix Ignitor II electronic conversion with Ignitor II coil.
My next move is to replace the Stromberg WWC with an identical carb off my relative's 67 Newport.

 

[Clover]

Sounds like a good idea. It sounds like it has to be the carb!

 

[heus]

Report: It was the carb.

 

[LeBaron1973]

Well done identifying the problem, a new carb and happy motoring once again.