Diagnose this

Discussion in 'General Discussion' started by WestPointMAG, Apr 5, 2009.


  1. WestPointMAG

    WestPointMAG Monkey++

    I was working on my tractor tonight and thought this would make a good thread.

    Put the group’s logic and diagnostic skills to the test after you have figured out what the problem was post all the facts, symptoms and diagnostic test results. Do not post until you know what the problem is or else it will become a help me thread and since most of us are not Docs, Vets or gunsmiths we could end up stepping outside what we are allowed to do. It could be a problem with your car that you solved, you cow has purple flop, a wiring and pluming problems, gun not working or like me trying to get a tractor to run right. It does not matter what the problem is as long as you have figured it out before you post it. Hopefully this will help every one hone there diagnostic skills.

    I will start it out.

    Some of you know I have been working on a 73-75 Sears SS16 tractor with an Onan twin engine.

    When I got it the points were stuck so I unstuck and cleaned them so I could get the tractor running so I could work on it and use it some to test the drive train. I also dissembled and clean the carburetor, installed new spark plugs, change the oil, replaced the diaphragm in the fuel pump, new fuel lines, new fuel filter, new battery and cleaned the rust out of the gas tank.
    After working it awhile it started back firing through the exhaust and carburetor and suffered a lose of power.

    This is where the fun begins.
    Detined the engine.
    Washed it with the high pressure washer to removed 30+ years of crud.
    I replaced the points and condenser.
    Set the points at .020 at TDC.
    Set the timing at 23-24 BTDC.
    Check for spark,
    Spark will jump a .045 gap on the spark gap tester. Spark is bright blue.
    Engine cold needs to be choked to start.
    Started the engine.
    #1 cylinder {front} runs cold #2 runs hot {back}
    Pulled the plugs and they were dry.
    Restarted engine. Engine warm needs to be choked to start.
    Cleaned them and reinstalled them. Problem not fixed.
    Restarted engine. Engine warmer needs to be choked to start.
    Swapped spark plugs front to back. Problem not fixed.
    Restarted engine. Engine warmer needs to be choked to start.
    Swapped spark plugs wires front to back. Problem not fixed.
    Restarted engine. Engine warmer needs to be choked to start.
    Swapped spark plugs wires on the coil front to back. Problem not fixed.
    Restarted engine. Engine warmer needs to be choked to start.
    Check compression has lots.
    Pulled #1 spark plug wire and started engine. No change
    Restarted engine. Engine warm needs to be choked to start.
    Reinstalled #1 spark plug wire pulled #2. Engine will not start.
    Removed cylinder head everything look ok but was some oil in cylinder.
    Resurfaced cylinder head, cleaned jug and reinstalled with new gasket.
    Restarted engine. Engine warm needs to be choked to start.
    Problem not fixed. But I know what the problem is.
    All of the info you need {plus some} to form a diagnoses is above.
     
  2. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    Fair to ass-u-me that is a pushrod engine?
     
  3. WestPointMAG

    WestPointMAG Monkey++

    Yes an Onan twin cylinder.
     
  4. ColtCarbine

    ColtCarbine Monkey+++ Founding Member

    I'll take a stab at this even though I haven't done much work on those little two cylinder engines.

    It's been my experience on gas fired engines that if it is backfiring through the carburetor it usually means that the timing is off 180 degrees. Not that I have ever accidentally put a distributor back in 180 degrees off to have it backfire through the carburetor.:oops:
     
  5. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    That would backfire both cylinders, no? I'm leaning toward busted valve springs or more likely stuck open valves (oil gallery plugged) on No.2, it'll fart both ways as described if so.
     
  6. ColtCarbine

    ColtCarbine Monkey+++ Founding Member

    I missed that it was backfiring through the exhaust also, guess it pays to pay attention to detail. [peep]

    Improper valve clearances (too tight) could attribute to this also, if the intake valves were always open. As would stuck intake valve or broken intake valve spring.
     
  7. WestPointMAG

    WestPointMAG Monkey++

    Not it. But thanks for playing and please try again.
    The next caller on the line is.
     
  8. WestPointMAG

    WestPointMAG Monkey++


    Not it. But thanks for playing and please try again.
    The next caller on the line is.
     
  9. WestPointMAG

    WestPointMAG Monkey++


    Not it. But thanks for playing and please try again.
    The lines are still open and you can call in as many times as you want.

    Feel free to use out side sources.
     
  10. ColtCarbine

    ColtCarbine Monkey+++ Founding Member

    This is a little easier to do in person. :D

    Bent valve from kissing the piston

    Vacuum leak-fuel mixture too lean, lean misfire

    Exhaust leak

    Bad ignition coil-firing too early or late

    Broken ground

    spark plug wires arcing

    If it's none of the above then I guess I'll need to do some research on the net.
     
  11. Cephus

    Cephus Monkey+++ Founding Member

    Rings me thinks ya checked or replaced everything else .
     
  12. Tango3

    Tango3 Aimless wanderer

    firs tguess is timing you said "
    tdc"But TDC for which cylinder? if its a`180 crank like small honda twins
    the pistons may go up and down at the same time but the valve timing is180 degrees different"

    "choked to start"( warm) means its lean:vacuum leak? Lean mixtures take longer to burn.lean also contributes to discolored exhaust pipe chrome.(late burning)
    pulling#1 plug with no change #1cyl not running. fuel will be cycled through,
    compression ck on #1?
     
  13. WestPointMAG

    WestPointMAG Monkey++

    Give Tango a pat on the back or a bat on the pack.
    The points caused misfire and it to also to go out of time. It backfired through the carb and blew out part of the intake gasket on # 1 cylinder.
    Here is a photo.



    ss16.
     
  14. KCwelder

    KCwelder Monkey+++

    It needs bailing wire on the fender. That works every time.
     
  15. Clyde

    Clyde Jet Set Tourer Administrator Founding Member

    Gas + Ingintion + Pressure = motion
     
  16. Tango3

    Tango3 Aimless wanderer

    :) backfire was probably caused by the lean mixture also..
    Daddy always said: "Son:The gasoline engine is made of iron and steel and rubber; by people no smarter than you or I. If it has (suitable):compression, spark at the right time, and air and fuel in the right mixture it HAS TO RUN IT, HAS NO CHOICE!. There are no "demons" inside...
     
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