SB350 cutout at freeway speed, revs fine in neutral

Question

Was hoping someone could help me out with an issue I'm having since I'm running of ideas...

In case it helps, its a 71 Nova with a small block 350. Carbuerated and HEI, without any computer chips or sensors that could be adding extra complications to the issue.

Basically whats happening is that at freeway speeds, the engine seems to "cutout" if I go over a certain speed. Generally, it seems to happen if I go over about 2500 rpm. The weird part is that I can rev it up over 4000 rpm in my driveway and not have an issue at all. The cutout makes a "clicking" sound that almost seems to be coming from the distributor but since I can rev higher in my driveway I'm leaning towards it being a fuel issue rather than electrical? Here's a video that might help describe it better. The clicking sounds louder in person and you feel a slight jolt forward as though quickly taking your foot completely off the pedal. http://youtu.be/UdbuKAmVRHs

I started out trying to pinpoint the issue at the cheap end and I've been working my way up to more expensive. Already replaced all the spark plugs and checked the wires. Replaced the fuel filter (inline and the ones inside the carbuerator). Hadn't officially set the timing last time I pulled the distributor, so tried that. Was 5 degrees initial, 25 degrees total before setting it, used the timing light to set it at 15 initial and 34 final (with vaccum line sealed), still no luck. Adjusted the floats in the carb as per edlebrock's manual, also picked up a rebuild kit to replace a couple gaskets. Went ahead and changed out the "needle and seat assembly" on both sides that open adn close with the float to let fuel into the bowls.

As I said, at this point I'm almost out of ideas. I'm guessing the next step would be to replace either the fuel pump or the whole carbuerator all together? I've always been told that fuel pumps either work or don't, but maybe its not putting out enough volume? Seems to work fine while parked, but hard to check at freeway speeds. Any ideas?

Update: I went ahead and changed out the fuel pump this weekend and it didn't seem to fix it. Also noticed it seems to be a lot worse on hills. Anything I can try adjusting on the carb before buying a whole new one? Its an Edlebrock 1406.

Answer 1

This to me sounds like some form of vacuum actuator that's leaking somewhere under load. I would look for any dry or thinned out vacuum lines. I had something similar happen, though it was on a throttle body injected truck. Enough vacuum was pulled that a line either collapsed or flattened out enough for it to get pinched by something. Either way I would check the vacuum lines.

If it's naturally aspirated and only does this at speed and there's a clicking sound, I think I would be looking for a vacuum operated valve. Is there any kind of emissions or valve that lets some exhaust back into the intake to help warm the car up when cold? On my motorcycle, the carbs have a vacuum line that vents to atmosphere via a filter. If you blow air into it or it's in the airflow while riding it will cause the engine to stutter similar to what you are describing.

Answer 2

Sounds like a weak ignition to me. Falls down under load, but is fine with no load. Possibly the ignition coil (upstream of the distributor). Did you use a spark gap tester to check and make sure your spark at the plugs is at sufficient voltage? A too low voltage can match your symptoms. Coil may actually be finding a different path to ground under load (which might be your mystery sound).