Motor surge at cruise speed

[srwilson]

What will cause the motor to surge when cruising?

Car is a carbureted 79.

I notice a slight surging when trying to keep a constant speed between 2 & 3K RPM range. It's actually irritating. It's like riding with someone that's trying to keep a beat to the radio with their gas foot.

Seems to idle fine. Accelerates fair from a dead stop with mild hesitation at times. I don't really notice it when it's under a load either.

So far: New plugs, rotor cap, distributer cap, plug wires, coil, magnetic pick-up, fuel filter.

My timing is set a little before the 10° BTDC (seems to help with acceleration).

I'm thinking it carb or vacuum related.

The carb is rather new, 34 ADF, and it's sitting on top of a 78 single plane intake. I have only two vacuum lines connected to the intake; brake booster, and air breather. And only one line from the carb going to the charcoal canister. No reed valves, EGR, smog pumps, etc. All that's been removed.

I've tried to set the carbs idle mix using Brad's guide. But after the two turns out from the seated position I never really notice the rpms picking up when I turn it in or out. It stays pretty much steady until it starts to drop in both directions. In the out direction I can turn the mix screw several turns before I notice the drop in rpms. Is this normal?

Any thoughts? What next?

Sorry for the long post!

[AriK]

Here's a longshot. Such a longshot that it's probably not the problem, but you can't overlook everything. I'm more inclined to post it more for entertainment purposes.
I had this exact problem with another brand of car, and i, like you began changing components such as filters, cap, rotor, plugs etc. Nothing solved the problem, in fact this annoying surge would only appear at cruising speeds, not at idle, not under load. I endured it for about a year until one day i was told that my brake lights flickered while driving. The culprit was a badly adjusted brake light switch on the pedal. A relay flickered simultaneously as the lights flickered, thus causing the surging. Not evident at idle nor under load and evidently not visible or audible by me.
My experience, just sayin.

[76was124]

Are you sure it's surging vs.losing power which would feel like a head wind slowing you up?

How old is your guibo on the drive shaft?

[srwilson]

Are you sure it's surging vs.losing power which would feel like a head wind slowing you up?

How old is your guibo on the drive shaft?

It could be a loss of power.

Guibo, hmmm, I'm betting around 35 years old.

[azruss]

check for a very small air leak in the intake system. had this with a BMW 2800.

[vandor]

Yes, lean condition could cause this. Check for vacuum leaks.

[srwilson]

Found it!!! Vacuum leak, and it was a big one. What I had hooked up to the air breather, small line that's "T'd" into where the PVC hose connects, should not have been. Not sure how I came to the conclusion that it should have been connected there in the first place, but anyway, once I plugged the port on the intake, set the idle and mix, and put timing back to where it's suppose to be, all is well.

Now when I adjust the idle mix it makes a very quick difference in where the screw is set. I can see the RPM change very easily.

I had a great big smile on my face when I took it out for a test ride. Mucha Better!!!

Thanks for the help!!!