vacuum advance

[captden29]

i searched old threads but could not find how to check if the vac advance unit is bad. i disconnected mine at the carb and sucked on the line and i could suck air thru it. i assume the diaphragm in the unit is bad, but is that a reliable test? i am having a stumbling problem at normal driving speed which does go away when i step on the gas pedal hard. however, if you try to hold a steady speed it will surge or stumble. i was told it could be the advance unit. any suggestions would be appreciated. dennis

[oldmopar]

If you have a vacuum pump you could use it and see if it holds the pressure. Not sure what is correct pressure but I usually use about 15 lbs. Sucking on it works just as good you are still creating a vacuum. You should see the arm move if the unit is good

[steveplym]

Sounds like a vacuum leak possibly. If you don't find your advance bad would probably be good to check for that. I had that problem a few months ago and after checking the intake for leaks all over I found that I had the connection for the wiper motor loose. Tightened it up and retightened the intake bolts. Now does not stumble or miss at a steady speed.

Another problem I had was I put a new gas cap on my car. Didn't know it was pressurized, car missed and stumbled on take off. Took it off and put the old cap back on. Fixed the problem.

[TodFitch]

i searched old threads but could not find how to check if the vac advance unit is bad. i disconnected mine at the carb and sucked on the line and i could suck air thru it. i assume the diaphragm in the unit is bad, but is that a reliable test? i am having a stumbling problem at normal driving speed which does go away when i step on the gas pedal hard. however, if you try to hold a steady speed it will surge or stumble. i was told it could be the advance unit. any suggestions would be appreciated. dennis

Attach a handheld vacuum pump with gauge to the advance unit.

First test: does it hold vacuum?

If yes, second test with engine running and a timing light, pull the vacuum shown in the manual and see if the advance changes by the specified amount.

You don't say what car/distributor you have, but the 46-54 service manual shows under the "Maximum advance (vacuum control)" part of the table 15 to 19 degrees advance at 16 inches vacuum for the P15 and the same advance but at 14 inches of vacuum for the P17 through P25 models.