hinemadm

I have a 1972 Vette with base engine, rebuilt 10k miles ago. I am on my 3rd carb, 2nd dist. With adjustable advance. When I am cruising, 40-45 mph, not accelerating, not slowing down, the engine kind of stutters.
It is not a cylinder miss or a engine knock. I have read on Willcox page that the engine is dithering, searching for the right advance. No matter what the timing is set at (8 degrees initial) I cannot get rid of this "dithering". I have gone back and forth on the vacuum advance to no avail.
Any help is appreciated
Dave

billschroeder5842

If your engine vacuum is steady, vacuum advance on distributor working I'd say you have a fuel flow issue. Timing does not tend to jump around if your vacuum is steady.

I'd start with basics on fuel pressure, fuel filter or (please mention carb) and double check choke operation.

Hammerhead Fred

Too much advance will introduce a hunting/jerky/"dithering" result at cruise speeds. Might want to verify that balancer mark is correct - that your balancer ring hasn't rotated resulting in erroneous timing setting.
Another simple check is that your Dwell is steady and the vac advance plate is solid - try shooting your timing light at the plate through the cap window and see if it's steady across the rpm range.

7T1vette

Great tip!!!

billschroeder5842

Good point- a simple spin of the distributor will answer that question

plaidside

Did you disconnect the vacuum advance and drive the car to see if it still does it?
What is you vacuum at idle? You should have a vacuum advance that is all in 2" below your idle vacuum and should add 14-16" of timing max.
Joe

hinemadm

Thanks for all the tips...I disconnected my vacuum advance, checked my timing (spot on), watched my mechanical advance (ok) and went for a test drive....still dithering. Would the float level in the Quadrajet be set too low, causing a fuel sputter? This is my third carb in the last 24 years, and I recall having this dithering since way back. I am thinking of going with a aftermarket carb from Jegs or Summit. Any advice.
Dave

Hammerhead Fred

Checked timing is not the same as verifying that the damper hasn't slipped on the hub. You need to verify that the timing mark on the damper is accurate and shows zero/TDC when the #1 piston is mechanically at top dead center. Only then will your timing results be "spot on". Easy to verify before you throw more money and time at part swapping activities.

Alternatively, as stated by others, you could simply disconnect the vacuum advance and/or retard the timing to see if your "dithering" goes away.