View attachment 235623 View attachment 235624 Ok I had some time today to work on my 66 300. With the vacuum Advance hooked up it stumbles & shakes badly when I begin to give it throttle. Diconect the Advance & it is smooth but accelerates very slow. I put a vacuum tester on it & removed the dizzy cap. It starts to move the Advance plate at 12 inches & stops moving at 17. Didn’t notice anything unusual about the movement. Everything looks surprisingly clean. Engine is a 383 low deck. Believe to be original. It still has points. The carb is a Edelbrock. The port at the carb pulls no vacuum at idle. I forgot to check at throttle. The other port has about 20 inches at idle. So I’m sure I’m using the correct port.
First thing I would make sure is that the gas is good. If you use ethanol laced gas, it can collect water just sitting in the tank. The crappy gas of today can deteriorate quickly and you can even get a slug of water from a fresh fill up. Bad gas will also cause an engine to run poorly - so this would be my first check.
You could have a carb problem, but I am going with an ignition problem first.
The 2 ports on the carb- The one with no vacuum is probably a ported fitting and will begin to pull vacuum as the carb is opened. The other port showing 20 inches is a direct vacuum port.
If you connect your vacuum can to the port having 20 inches, you are immediately pulling 20 inches on the vacuum can at the distributor giving you full vacuum advance. So you are getting Initial Timing (the timing set at the crank balancer) PLUS the built in vacuum advance at the distributor. So at idle, your Initial may be set at 5 degrees BTDC and your distributor's vacuum advance might be around 14 (?) giving you 19 degrees of Initial Timing at the balancer.
Have you checker your Initial Timing with the vacuum to the distributor unhooked?
If you reconnect the vacuum hose to the port having 20 inches of vacuum after setting your Initial Timing, initial timing should jump up. 19 degrees of Initial Timing would be too much and could cause the engine to run poorly because it is way too far advanced - then you disconnect the vacuum line and your engine is running solely on the Initial Timing of 5 degrees and it smoothes out again.
Check the other port on the carb to see if it pulls vacuum as you open up the throttle. If it does, try swapping your vacuum line to the distributor to it and see if this does the trick - remains smooth running and engine responds and is no longer sluggish.
I would check the operation of the Mechanical Advance of the distributor to make sure it is operating as it should or if something has broke or is sticking. When a car sits and is not run regularly, things happen. Check your Mechanical Advance with the vacuum disconnected.
You will need a "Dial Back" Timing Light. What you want to know is at what RPM your mechanical advance begins to kick in - typically around 1000-1200 RPM's. Next important number is to know what the Total Advance is - Initial PLUS the Mechanical Advance and at what RPM Total Advance is (your timing will stop advancing, let's say your engine no longer advances past 3200 RPM's, it stops moving any more). Then 32 degrees @ 3200 RPM's would be your engines Total Advance.
Once this is all dialed in, THEN you can work with the vacuum advance and dial it in.
I don't know what factory specs are or how much Total you need @ "X" RPM. But I do know that the timing curve is one of those often overlooked things and can cause a lot of issues in performance, detonation, & gas mileage. Set correctly for your engine, it can be like day & night when you are done.
I would also put a fresh tune-up on the engine and not screw around with trying to mess with points/condensor,rotor/cap/wires & plugs. I would change out the coil as well and inspect the wires going to it for breaks or frays. If it did not do anything, then you now have a spare to throw in the trunk - just in case. I also do this with the tune-up parts so I have a few replacement parts out on the road just in case of an emergency situation to get me home. Also new gas filter and make sure all rubber gas lines are replaced with ethanol compatible gas line. Parts, even new ones, can go bad on a car that is not driven regularly and sits for extended periods of time.
You can put a lot of time testing and chasing and scratching your head only to find out it was a $4 condensor that looked good, was replaced a year ago, and was indeed bad - one of our Pontiac members went through this. Just put a fresh tune-up on it, set your points/dwell, gap new plugs, and set timing so you know you have a good base to work with if you are still having problems. Get your plug wires straightened out. If you cannot buy a set that fits, get a kit that you can make up your own wires to the length you need. Not too difficult to do and then they can be neatly run using your wire looms so none of your wires cross/touch another.
Sometimes you have to know what it isn't in order to find out what it was.
Just my 2 cents worth.
