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Thread: Vacuum Advance Causing Misfire?

  1. #1
    EngineGuyRich Guest

    Default Vacuum Advance Causing Misfire?

    I am troubleshooting a 1970 Chevy truck with a 350 small block. The truck had been running fine and then suddenly lost power and picked up a bad misfire as soon as you cracked the throttle but would idle fine. It has a 8361 Street-Pro vacuum advance distributor hooked to ported vacuum on a 1406 Edelbrock carb with a MSD 6A box. The distributor is out-of-the-box stock with the heavy silver springs and blue bushing.

    We cleaned the carb, finding nothing wrong, and had no change in the misfire. While checking all the basic tune-up stuff we found that the engine runs fine if we didn't hook up the vacuum advance. We pulled the distributor and disasembled it. I can't see a thing wrong. We stuck it back in and nothing changed -- still can't use the vacuum advance. What is going on?

    We checked:1) Distributor pickup reads about 600 ohms, removed the strain relief and wiggled the wires - no change in reading. 2) Verified timing mark on damper is correct. 3) Manually advanced timing w/o vacuum hooked up and the engine runs perfect.

    This one has me stumped! Any ideas would be great.

    Thanks, Richard.

  2. #2
    msdtech22 Guest


    Unfortunately, from what you describe, it does sound as if the vacumm advance canister is faulty.

    However, you are running the most sluggish advance that can be run with that distributor. You might consider changing that curve to a more open/easilyadvanced curve. Doing this will allow the engine to operate more efficiently.

    When timing the vehicle, use a 'plain Jane', non digital, non-dial back timing light. The electronic timing lights do not work as well with the MSD CD system as the plain Chrome timing lights.

    Remember, anytime you set the timing with a vacuum advance distributor, the vacuum canister should be disconnected when setting the initial timing and the hose plugged up using a golf tee or the sharpened end of a pencil.

    Make sure that the wires running from the MSD ignition to the coil and the pickup wires from the distributor to the MSD are seperated by 6-8" and not wired in a parallel run. In other words the orange/black pair should not be run in a parallel run of a foot or more with the violet/green wires of the distributor. If these two pair of wires go through the firewall in the same place, drill a new hole 6-8" from the other and route one of these pairs of wires through accordingly.

  3. #3
    EngineGuyRich Guest

    Default Tried that -still no luck!

    Hey Tech22 thanks for the promt reply:

    I should have mentioned that we did replace the v/a can and got no improvement. We have tried the base timing from two beforeTDC up to 14 with the vacuum disconected.

    I agree with what you say about wiring etc. but what do those things have to do with the vacuum advance? Just for a test I cranked the timing up to about 25 before and the engine will respond just fine. It is only when the vacuum advance plate rotates that the problem occures. The timing just jumps all over the place, or so it seems. This is why I though it might be a bad wire on the mag. pickup so I ohmed it out and can't see any problem (about 600 ohms), wiggled and pulled on the wires but the reading never changed. I even left the strain relief off just to see if the problem went away thinking that maybe some strain was being put on the wires when the plate moved causing an intermitant open -- nothing changed.

    I also understand that the stock curve is really lazy, (I have recurved many distributors on an old Sun machine), but the engine was running great with this setup for several years -- what suddenly changed to cause this problem?

    Thanks again, Richard.

  4. #4
    wev Guest


    I have same problem, shame you never got a responce or posted how to rectify :angry:

  5. #5
    msdtech1955 Guest


    sir, we are all on the phone right now, we answer 700 to 1000 calls a day .

    as soon as we get time, we'll be back .

  6. #6


    8 years later and still no time to answer?!?!?!?

  7. #7


    8 years ha good one. but anyways back to the main issue. where is the ground from the box going to? and how do you have the mag. pick-up wires routed?

  8. #8


    if you need a quicker response maybe you should try calling in to talk with a tech as it is very hard to get to these e mails on top of everything else we have to take care of. thank you

  9. #9
    Join Date
    Nov 2014
    Ont. Canada


    This is an old thread, but I will give my two cents. Your mechanical advance does not change the relationship between the rotor & terminals on the cap (rotor phasing), but vacuum advance does change the relationship between the cap & rotor. So when you connect the vacuum advance, your rotor phasing changes. You need to check to see if the spark is jumping to the wrong post in the cap. Get an old cap and drill a hole in it and have a look.

  10. #10


    yes please start a new thread

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