How To Set The Dwell On 308?

Discussion in 'Technical Q&A' started by donaldh2o, Dec 2, 2003.

  1. donaldh2o

    donaldh2o Karting

    Nov 10, 2003
    Irvine CA
    Full Name:
    Removed the distributors on my '76 308 to replace the O-rings. While I had them out I checked the point gap and discovered they were set at .004 to .003. The manual says .012.

    So I reset the gap and now the dwell measures only 10' rather than 39' as described in the manual.

    My 308, originally a U.S. version with dual points, was converted years ago to the Euro setup. Euro cams, single Euro points and Euro mufflfer. (The change made it a very fast car)

    How do you set the dwell? The shop manual describes a test bench setup but I don't have that. The points were last changed by the local Ferrari shop and since both distributors were the same when I took them out (very small point gap: .003), I suspect they know something above and beyond the manual.

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  3. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    I don't have your exact year/version OM, but you might be mis-interpreting the (somewhat poor IMO) translation slightly. When it says something like:

    "-Dwell: on advanced breaker point R1: 39 deg +/- 3 deg."

    that really means "when measured at the R1 point connection and the R2 point microswitch is closed (i.e., R1 and R2 are active) the measured dwell should be 39 deg +/- 3 deg."

    If you've only got a single R1 point set you'll want to set its dwell to 34 deg +/- 3 deg per the:

    "On advanced breaker point R1, with retarded point wire R2 disconnected from ground (i.e., only R1 is active) 34 deg +/- 3 deg."

    Another slight oddity is that you need to use the 4 cyl scale on your dwell meter due to the "1 dist per bank" architecture -- are you? (I sort of hope that you're not since that would make your mechanical set-up better match the electrical measurement results -- the units are deg not arcminutes). But bottom line is you'd change the mechanical spacing until you get the desired electrical dwell result -- i.e., set mechanical spacing, mount on car, measure dwell, if necessary remove and reset mechanical spacing (smaller for more dwell, larger for less), repeat as needed ;).
  4. Mike Florio

    Mike Florio Formula Junior

    Jun 19, 2003
    NW Rural Nevada
    Full Name:
    Mike Florio
    The dwell is the period, expressed in degrees of rotation, that the points are closed. It is almost impossible to get it right with just a feeler gauge. Over the years I have tried various ways to get around this. I built a distributor machine out of an old sewing machine motor and a couple of 5-1/4" disk drive platters, one with a slot cut in it and an led under the slot to illuminate through a clear protractor. I even built a digital tachometer for it. It worked pretty well, but didn't have the range of RPM to check advance.

    I recently broke down and bought an Allen Synchrograph distributor machine, and have rebuilt it. The electronics package is 1955 vintage vacuum tubes, and I'm working on that now. Several other guys on this board have done the same thing (RG are you out there?).

    Basically you need to spin the distributor at a known RPM then use an external Tach/Dwell to measure the dwell. Someone on this board previously suggested using a variable speed hand drill to spin the distributor. That will work. Clamp the distributor in a bench vise and slip a piece of 1/2" fuel line on the splined shaft. Force a piece of 1/2" metal dowel on the other end of the rubber fuel line and chuck that into the drill. Make sure you are rotating the distributor shaft clockwise (looking from the top of the distributor).

    Now get an external 12V power supply. The easiest thing to use is the car battery, since you have to supply 12v to the tach/dwell meter anyway. Wire the positive post of the battery to the distributor input post. Wire the distributor case to the negative post. You may want to splice in a 12V 5W bulb into one of the wires as a current limiting resistor (plus its fun to watch it blink).

    Connect your tach/dwell meter and set for Tach, 4cyl. Spin the drill until you get to 500 RPM on the Tach/Dwell meter. 500 RPM is what the distributor turns at 1000 RPM crankshaft speed. While holding that speed on the drill, switch the Tach/Dwell meter to Dwell. Viola! you can now read the dwell outside the engine. Adjust the points and try again (many times).

    Again, I stress: you cannot check the advance curve with this setup. And you really need to check that given the mechanical advance.
  5. donaldh2o

    donaldh2o Karting

    Nov 10, 2003
    Irvine CA
    Full Name:
    I am concocting this gizmo in my head as I type.

    I have a bench drill press. I have a 120V variac (to fine tune speed of drill press). Also, drill press has several belt/pully settings to adjust speed.

    I have wood (1x6 comes to mind) for mounting the distributor which can then be placed on the drill press. I have 1/2" rubber hose to fit onto the spline. Attach distributor to 1x6 piece of wood (firm but loose), place wood mounted distributor on bench press adjustable table. Raise table so rubber hose (not spline) fits into drill chuck, tighten chuck on hose (may need small wood dowl in hose), center everything, secure wood to bench press table (firmly, but above all don't stress the damn distributor shaft). Voila, mechanical is done.

    I have an old Panasonic AC adaptor that puts out 12V DC. I have 12V light bulbs. Radio Shack has DC receptacle and small buss strip to attach all the electrical.

    I have a cheapy dwell/tach tester. May have to upgrade since this is now getting serious. The one I have doesn't have a scale for 4 cylinders, it notes to multiply the 8-cylinder reading by 2. When I tested the dwell, the meter was reading 5', hence 10'.

    Question: In the event I can't get .012 gap with 34' dwell, which is more important gap or dwell?

    Question: How does the advance take place on the single point distributors.? Is it mechanical? Are there weights on the points?
  6. 4Webers

    4Webers Formula Junior

    Nov 12, 2003
    Full Name:
    I just finished refreshing my distributors and saw the same thing: 0.012" gap only gave me about 28 deg dwell. Since it's the electrical properties that matter more, I would say to make sure that the dwell is correct.
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  8. Ferrari Fanatic

    Ferrari Fanatic Formula 3

    Apr 2, 2003
    One of the things that you need to be REALLY carefule of is a worn out SHAFT or BREAKER PLATE.

    If you set the gap with a feeler gauge, the dwell will be radically different on the car. The breaker plat and shaft woobling back and forth will never allow you to get the right dwell.
  9. Mike Florio

    Mike Florio Formula Junior

    Jun 19, 2003
    NW Rural Nevada
    Full Name:
    Mike Florio
    I agree with the above two posts. The dwell is what is important, the gap is only a mechanical way to get you in the ballgame. I also agree that you must be careful if the bearings are worn.
  10. tuttebenne

    tuttebenne F1 Rookie

    Mar 26, 2003
    Bay Shore, NY
    Full Name:
    Distributor goes in the bench vice

    variable speed electric drill goes on floor

    flexible drill cable (like an oversized speedo cable) goes in drill chuck on one end. Install a short (2 inch) piece of threaded rod into chuck on the end of the cable

    connect threaded rod to distributor shaft with short rubber hose. Now drill is connected to the cable which is connected to the distributor shaft

    connect a 12volt battery to the points

    attach dwell meter to points (don't forget to use the 4 cylinder scale)

    power up the drill and lock it at half speed; set the dwell to the 39 degree spec.

    done with distributor number one.
  11. joeyy

    joeyy Karting

    Nov 11, 2003
    long island
    Full Name:
    the drill thing is simply genius.

    Thanks Mike
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