Engine Surging, Tach Bounce

So, I identified the source of the problem I've been having for the last week or so. A little background ... Rebuilt 351C 4V, Holley 750 on Edelbrock dual plane, FMX trans. Starts fine. Idles pretty smooth. On the road, as the speed gets up to 35 - 40 mph the engine starts surging, the tach bouncing up and down. The problem seems to start when you let off the gas slightly. If you don't gradually bring the engine back down under 35 mph, and instead try to hold the speed, the rough engine eventually stalls.

Last weekend I checked the carburetor float levels and adjusted the idle mixture. Didn't solve the problem. Changed the coil and points. Didn't solve the problem. This weekend I changed the carburetor base gasket, and checked other connections, thinking there might be a vacuum leak. Didn't solve the problem. At my wits end, I decided to disconnect the vacuum advance and plug the port on the carb. Car drives MUCH better! Got it all the way up to 60 mph, varied the speed up and down, with no surging or tach bounce.

Mechanically, the vacuum advance is working. If you suck on the hose, the assembly moves. You let go, and it snaps back. But, there's obviously something wrong with it, since the engine does not run right with it hooked up. This is something I've never had to mess with. Are these units adjustable? Am I looking at replacing the advance unit or the distributor?

Thanks!

What's your base timing? Timing with full mechanical advance? Timing with full vacuum advance?

What it your vacuum reading when trying to hold that speed ? What RPM are you seeing at that speed ?

Could be mechanical springs too light and then vacuum low at that speed.

Also what vacuum source are you feeding the vacuum advance from? Ported vacuum or manifold vacuum ?

It could be a vacuum leak that is only apparent when you let off of the gas. I believe under those conditions, vacuum drops significantly.

The advance vacuum line is connected to the carburetor port behind the primary metering block. The problem seems to start above 2000 rpm or so, and doesn't go away until the engine is slowly brought back below. But, even gradual acceleration is much rougher with the advance connected. Tach wobbles more. Very smooth with the advance disconnected. The base timing is 6 degrees BTDC. If I'm reading the marks correctly, the advance with vacuum connected is somewhere around 45 degrees at 2000 rpm. The manifold vacuum is about 13 inches at idle, and 20 inches at 2000 rpm. I suppose it could be a vacuum leak in the advance system? The engine improvement is so dramatic when the vacuum advance is disconnected, it almost has to be related.

Cheezesnake,

I seem to remember and issue many years ago that a base plate was grounding out and arcing somewhere in the advance process. It was easily repaired / solved once it was found. I wish I could remember more, maybe after I sleep on it !!! lol

Thanks, Jay

Cheezesnake,

I seem to remember and issue many years ago that a base plate was grounding out and arcing somewhere in the advance process. It was easily repaired / solved once it was found. I wish I could remember more, maybe after I sleep on it !!! lol

Thanks, Jay

Click to expand...

+1

Also, the vacuum pulls the mounting plate the points/pick-up are mounted to and it could be the wiring has become fatigued, pinched or have worn insulation (resulting in a short to ground).

Some good points to look at, Guys. Thanks! I did find this interesting discussion on a forum that argues using manifold vacuum instead of ported ...

http://www.bangshift.com/forum/forum/bangshift/tech-section/32851-ported-vs-manifold-vacuum-advance

Some good points to look at, Guys. Thanks! I did find this interesting discussion on a forum that argues using manifold vacuum instead of ported ...

http://www.bandwidth.com/forum/forum/bangshift/tech-section/32851-ported-vs-manifold-vacuum-advance

Click to expand...

We've had similar discussions here, just type "ported vacuum" in the forum search box (upper right).

Here's what I've discovered in road testing this problem the last couple days ...

Ported vacuum = poor performance

No vacuum = better performance

Manifold vacuum = best performance

I need to go back and double check timing with each of those, and it's been awhile since I've listened for engine knock, so need to check that also with the manifold vacuum. I haven't yet checked inside the distributor for possible shorts.