I have a 400 that does not run smooth at 3000+ rpm. but if you unplug the vacuum advance hose from the distributor and plug the hose, it will run smooth. the 400 has been upraged. .030 over, edelbrock dual plane intake, holley truck avenger 670, hedman headers, comp cam xe262, comp valve train to match cam, msd digital 6a ignition with super conductor wires and blaster hvc2, distributor is the stock one except lighter springs have been installed. the lighter springs should bring the vacuum advance in sooner, could this be the problem?

 

Just to elaborate. The timing is set to 12 degrees Btdc and the vacuum advance hose is hooked to the timed port on the holley carb.

 

Took the distributor apart tonight and found that the mechanical was on a 18L slot so the total advance would have been 46 degrees. I changed it out for a 13L and dropped the initial timing to 10 degrees for a total of 36 degrees. full advance comes in at 2500 rpm with the lighter springs. The engine does not studder at all up to 5000rpm with no vacuum advance hooked up, but as soon as I hook it up, the stumble comes back around 2800-2900rpm. Has no one ever had a problem like this???

 

Sounds to me like you must be using manifold vacuum.

 

Is it doing this when your on the throttle or crusing ?

 

It is on timed/port vacuum. and it does it when I am on the throttle. I think the problem is fixed. The adjustment screw on the vacuum advance diaphram was at 10 ccw turns and making the timing jump up 16-18 degrees so I turned it ccw untill the diaphram tightened up so the timing would only advance to 4-6 degrees at 3000rpm. Tomorrow I will test drive.

 

Did anyone ever find a resolution for this? Same problem on a 351. Distributor advance hooked up to my MOUTH runs fine. Hooked up to port or manifold vacuum, runs like ***, timing is way off and misfires.

 

Probably, it's a 12 year old thread
There is usually an Allen screw inside the vacuum nipple in the vacuum advance
You can turn that screw and adjust the degrees of vacuum advance

 

Need a little more detail. But if your 351 has a carburetor and is running a Duraspark distributer then I suspect you have a bad vacuum can. The best I have found for a street driven engine is the BWD V391 it is a 10 degree can. The Ford cans are adjustable only in the amount of vacuum it takes to start them moving the amount of advance they pull in is fixed by a slot in the arm they run from 4 cam degrees to 12 cam degrees, double that for crank degrees. They are stamped with a number on the arm.
BWD V391 Distributor Vacuum Advance Control | eBay

 

Vacuum canister is brand new. Stamped 12. Vacuum is 15" and it causes far too much timing to run smoothly. It advances so far it can't run. LOL. Need to limit it or somehow dial back how far it can go.

 

Did you read post 8 and did you check?
That's how it's done

 

This is how these Ford cans work. The amount of advance they will pull in is determined by the slot in the arm. When it starts to move is determined by the amount of tension on the spring inside, that tension is adjusted with an Allen wrench thru the nipple. A 12-degree can is going to pull in 24 crank degrees no matter what if it has enough vacuum on it. Here are the basic instructions on adjusting one. Instructions for Replacing Vaccum Control

 

Well, those arms are not replaceable, and the vacuum advances are calibrated to each engine calibration
Once you have the correct one for your altitude, you can play with the screw

 

used a 6. Doesn’t backfire. Called it good and closed it up.