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Technical 21 stud flathead running rough

Discussion in 'Traditional Hot Rods' started by rat bastad, Oct 30, 2014.

  1. Hey guys I have what seems to be a common issue with the roadster atm [​IMG]

    I went for a run in the roadster about a week ago - ran Ok but them progressively began to run rougher, less smooth on the rev up thru the gears and the odd backfire. Barely made it home.

    Knowing about the crap quality/condenser issues, I swapped it out with a spare I had as its a quick test. Instantly back to her old self but lasted all but 1/2 an hour's worth of driving then back to the same crapola.


    The engine is a good condition 37 21 stud, mild cam, Edelbrock slingshot, shaved heads and headers/dual 2" exh. Ign is a nice rebuilt 36 Helmet running a 12V conversion plate with new everything that has now done perhaps 2K miles all up. Coil was new, run with a ballast resistor and has run flawlessly for 2+ yrs.

    So I undertook the following checks:

    i. Volts at battery: 12.78
    ii. Volts going INTO ballast resistor : 12.78
    iii. Volts OUT of ballast resistor: 8.0
    iv. Ign "ON" Volts at coil+ve post: 8.0
    v. check cranking spark at coil COLD: light blue/white spark - HOT: same
    vi. check spark at plugs cranking COLD: blue/white spark - HOT running:
    vii. check plugs: clean no carbon /fouling (set carbs originally with vacuum
    viii. checks dist caps - look fine
    ix. check for arcing in the dark : NONE
    x. Coil lead resistance: 4.75 Ohms
    xi. Coil Pri side: 1.8 Ohms Sec side: 10200 Ohms

    Upo these tests I proceeded to pull the distributor and disassemble. Was spotless inside one point set was slightly pitted but pretty much cleaned up with wet n dry. Prob pitted because of the toasted condenser on the first long drive. Rotor looked almost new and no cracks that I could see.

    I added some heat shrink (like Ive read on the Barn) to the shaft and reassembled the dist and regapped the points at .016". Fired right away once again when reinstalled but not running or reving up smooth - same as before.

    At this point Im thinking even though the 12V late style coil checks out OK, it might be playing up when hot and under load. Would this trash the condenser? The condenser used is the one that came in the 12V conversion plate/kit, was fine for 2+ yrs but not sure of its MFD rating.

    I will be swapping out the coil, ballast resistor and get some condensers and decent points from Bubba. As noted in the tests, the wires to the plugs put out light blue/white spark to the plug which I understand is HOT and the way it should be.

    Again the car ran great when swapping out the condenser the first time, but soon returned to running poorly with no power [​IMG]

    So I swapped out the 12V coil with a new one, added a new 1.6 Ohm ballast resistor and a new condenser from a Mopar 318 (which apparently is the correct MFD rating for the FH)

    Nope - STILL crap !!!

    As mentioned the plus are ALL getting blue/white hot spark via the wires. The rotor looks near new as do the caps and the points cleaned up OK.

    What am I missing....?

    Thanks in advance
    Last edited: Oct 30, 2014
  2. Christom
    Joined: Nov 3, 2011
    Posts: 217


    Had a similar issue when I first start to drive around in my coupe. Turned out to be small bits of rubbish sloshing around in the tank and getting sucked onto the outlet tube. Took a while to finally get it all out as new stuff kept coming loose and appearing in the tank. I thought I'd done a thorough tank clean during the build - got there in the end. Good to have a fuel pressure gauge at the carb to check for sure if that's the problem.
  3. Having had over 50 plus years messing with flatheads,I will add my 2 cents worth.the bulk of my engines(many) are big bore and stroke. having said that I "run" my flatmotors very hard on the street and track and no matter how hard I tried,I could not get satisfactory results from stock dizzy.I could replace everything in dist and go out and run it hard.They would rev to 5,000 plus only one time and then have to fiddle with them again; just could NOT make them right.Years ago there was a machinist out of Denver (Norm Frick)that made right angle drives for flatties.I bought one and stuffed it with a cut down small block dist with a special curve put in it by any competent dist shop.I figured if a stock z28 from 67 could rev to 7500 so could my flathead.I was right,it worked well for years,then I purchased a Petronics for small block Chevy and am still running peak rpm's to this day.Get rid of that stock igniter and get something new and modern.My motors now can pull 5,500 in any gear and not miss a lick.
  4. Petejoe
    Joined: Nov 27, 2002
    Posts: 10,849

    from Zoar, Ohio

    You have a tough one Rat...
    Try checking your fuel pressure under torque to see if you have 2-1/2 pounds consistent.
    I'd shy away from a major distributor change since your not beating on it and it ran fine before.

  5. I agree about fuel. Sometimes crap in the tank/filter/carb can also cause situations where it runs ok then gets worse to the point of seeming to misfire and conking out. Everything settles down, starts fine and does the same again.

    Not saying that's definitely what it is, but it could be, so worth checking.
  6. Fuel is the next check, thanks for the input fellas....
  7. alchemy
    Joined: Sep 27, 2002
    Posts: 16,734


    80% of all ignition problems are the carburetor. Or is that 80% of all fuel problems are the ignition? ;)
  8. I thought it was 90% imagination? ;)
  9. flatjack
    Joined: Feb 13, 2007
    Posts: 924


    It's been noted many times that many new condensers are often bad. Keep trying.
  10. BJR
    Joined: Mar 11, 2005
    Posts: 6,846


    90% of all figures are made up on the spot.

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