1978 Scout II 392 V8 Engine Problems...

Discussion in 'General IH Tech' started by wavesailr, Aug 6, 2009.

  1. wavesailr

    wavesailr Farmall Cub

    I've got a 1978 Scout II with a 392 V8 engine that's giving me problems only after it gets to operating temperature.

    The engine backfires, misfires, stumbles badly, and has nearly no power after the engine gets to operating temperature. Before it's warmed up it runs perfectly and has lots of power.

    I've replaced the mechanical fuel pump and that didn't help. I then bypassed the mechanical fuel pump and used an electric fuel pump. That didn't help either. The fuel pressure is about 3 PSI when the engine is running no matter if the engine is cold or hot at full throttle, or idle.

    The timing is good at 3 degrees btoc using the #8.

    The carb (Edelbrock Performer 600 CFM, electric choke) was replaced before I owned the Scout, but it looks fine.

    The distributor and coil were replaced and didn't help.

    All of the fuel lines are routed away from hot-spots.

    Please help!!
  2. Doc Stewart

    Doc Stewart Content Team Staff Member Moderator

    Vacuum leak. Use your propane torch [don't light it!]. With the engine running, pass it over the carb opening: the engine will either speed up or lug down from the extra fuel intake. Use that as your indicator. Now pass the torch around the base of the carb and the various fittings on the carb including the shaft pivots. Watch for the engine response. When it responds, there's your leak.

    Also check any of the vacuum fittings on the intake manifold especailly thee brake booster. Do the whole test in 5 minutes.
  3. wavesailr

    wavesailr Farmall Cub

    Thanks for the quick response. I"ll try it tonight.

    The bogging and backfiring is only under load/acceleration. The engine idles perfectly both when it's cold and at operating temp...
  4. scoutboy74

    scoutboy74 Y-Block King

    What Doc suggested has plenty of merit and should be followed. From your description, this could also be an air/fuel ratio problem. When cold, your engine craves more fuel than air, but this changes as the engine approaches operating temperature. Verify that your choke plate operates smoothly and that it opens fully as the engine warms up.
  5. Russ McLean

    Russ McLean High Wheeler

    >> The carb (Edelbrock Performer 600 CFM, electric choke)...

    >> Verify that your choke plate operates smoothly and that it opens fully as the engine warms up.

    Verify that the electric choke has 12 Volts and is properly grounded. Measure this on a warm, running engine.
  6. wavesailr

    wavesailr Farmall Cub

    Electric choke works fine. Choke plate opens smooth and is 100% open after running for a few minutes and stays open while hot and running.
  7. 1975IH200

    1975IH200 High Wheeler

    Be sure the electric choke is NOT powered from the ignition system.
    The has caused several people here problems, caused by the PO Monster.
  8. 70binderPA

    70binderPA Binder Driver

    i had a blown head gasket do the same thing. cept it was blown between 6 of the cyl's to each other
  9. wavesailr

    wavesailr Farmall Cub

    I just sorted it out.

    There was a missing ballast resistor. 12v was going directly into the coil and causing the coil to get REALLY hot.

    So, I bought a resistor and installed it. There's now about 6v going to the coil and distributor.

    That fixed the problem perfectly. Now there's tonnes of power when the engine is cold AND hot!!

    I tried the propane torch and it didn't change the rpm at all, even when I shot the propane directly into the carb... It sounded like a good idea though.

    Thanks for all your help guys!!

  10. 70binderPA

    70binderPA Binder Driver

    if you're only getting 6v something is wrong.
  11. Jeff Jamison

    Jeff Jamison Dreams of Cub Cadets

    Not if he is running points,6 volts fine.
  12. Russ McLean

    Russ McLean High Wheeler

    The coil used with an IH points distributor is 6 Volts. 70binderPA made a correct repair. The resistor limits the current (thus dropping the voltage) to the coil. This keeps the coil from overheating and increases the life of the point contacts.
  13. wavesailr

    wavesailr Farmall Cub

    I don't understand exactly why, but after the coil heated up the Scout started running terrible.

    The coil got too hot to touch. Not sure what was happening to the points, but they were getting 12v as well.

    Backfiring, misfiring, shooting blue flames out both tailpipes, and no power... It was kinda scary driving, I thought something was going to blow up. Actually, I thought it was vapor lock and replaced the entire fuel system from the pickup and sender through the fuel pump. It has a nice fuel pressure gauge mounted next to the carb now.

    If anyone can explain why the coil and distributor getting 12v instead of 6v caused the engine to run crap, I'd love to listen.

  14. 70binderPA

    70binderPA Binder Driver

    i remember reading on here that you wanted 8-10 volts

    guess it was a bit too cryptic?
  15. scoutboy74

    scoutboy74 Y-Block King

    Don't know about the 6 volts. That seems a bit low. Should be closer to 9. The more important number is the total resistance value. For an SV-8 with breaker ignition the total primary resistance should be roughly 3.4 ohm. This total is most often accomplished via a coil with resistance factor of about 1.8 ohm and ballast or resistor wire feed of about 1.6 ohm, with all readings taken at an ambient temperature of approximately 70 degrees. If you have a dvom, could you please find out what the resistance of your coil is across the terminals with no wires connected and do the same with your ballast resistor. Please report back with your findings.
  16. Eric VanBuren

    Eric VanBuren Lives in an IH Dealership

    The coil wasn't designed to handle the current that was flowing through it and that is what caused it to overheat. When the coil was overheated it's output potential was greatly reduced, and it was possibly shorting internally. I would replace the coil and points now to prevent possible problems in the future because they were overheated.

    The proper way to measure the voltage is with the key on and the points closed. W/o a ballast resistor in the circuit you should have seen battery voltage at coil (+) terminal and less than 1 volt at the coil (-) terminal. With the points open you should see battery voltage at both terminals.

    With the proper 1.8 ohm ballast resistor in place you should see a percentage of the battery voltage at the coil (+) terminal in proportion of the resistance of the ballast resistor and the coil. With the IH V-8 system that should be slightly less than 1/2 the battery voltage. So yes the ~6volts is correct, with key on engine off and points closed.

    Running with ~14 v at the battery you should see ~9 v at the coil (+) terminal.

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