Dithering
Thread Starter
Cruising
Joined: Feb 2008
Posts: 14
Likes: 0
Dithering
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
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
Zen Vet Master Level VII
Joined: Mar 2016
Posts: 5,213
Likes: 1,173
From: Southlake, TX
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.
I'd start with basics on fuel pressure, fuel filter or (please mention carb) and double check choke operation.
Melting Slicks
Joined: Mar 2005
Posts: 2,093
Likes: 297
From: Midlothian VA
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.
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.
Zen Vet Master Level VII
Joined: Mar 2016
Posts: 5,213
Likes: 1,173
From: Southlake, TX
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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
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
Thread Starter
Cruising
Joined: Feb 2008
Posts: 14
Likes: 0
Dithering part 2
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
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
Dave
Melting Slicks
Joined: Mar 2005
Posts: 2,093
Likes: 297
From: Midlothian VA
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
Dave
Alternatively, as stated by others, you could simply disconnect the vacuum advance and/or retard the timing to see if your "dithering" goes away.
Last edited by Hammerhead Fred; Aug 23, 2016 at 05:19 PM.
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