1976 Johnson

Discussion in 'Bubba's Outboards' started by row, May 5, 2006.

  1. row

    row New Member

    Messages:
    3
    State:
    Australia
    i am looking for some advice. I have a 1976 Johnson 70 which is running rough at idle. I have read another member had a similar problem last month and he was told to clean carbs. I have done this. This is what I have done. Motor is running fine but mounts needed replacing $700 for set of mounts so I put a second hand trunk on with good mounts only $250. Since this has been done I have had the idle problem. Any other ideas
     
  2. Bobpaul

    Bobpaul New Member

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    3,039
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    Rowan, does adjusting the carbs in gear at idle speed have any effect on it's running.

    I can't see where changing the mounts would have caused this rough idle.
     

  3. Bobpaul

    Bobpaul New Member

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    BTW Rowan, it's nice to have a visitor and member from Australia,. Welcome.
     
  4. row

    row New Member

    Messages:
    3
    State:
    Australia
    My wife posted the question for me this morning. I will give you the full story so you have a clear picture of what has been done. I hope someone out there can help. Motor was running sweet, pulled off power head to replace top engine mounts 2nd hand trunk cheapa. Fitted up to transom, did new impellor and wear plate. Refitted leg- went boating. Noticed idle very poor and keep stalling- would hold just above idle ok when in gear.
    Performs normal at all other speeds. stripped old trunk out of curiosity to study water flow etc and noted resonator was completly burnt out so alot of exhaust gas was relieving to atmosphere at idle through bypass ports.
    Do you think increased back pressure is to much for the old engine? It is now going through the prop. Cleaned and syncronised carbs. Noted since lower cowling is hard on bottom carb bowl poss distorting flange and causing vac leak??? Also have to set up base idle timing book says 3-5 degrees ATDC in another section it says 0 degrees which is right?:confused2:
     
  5. Bobpaul

    Bobpaul New Member

    Messages:
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    A compression check should be done. From what your saying, I think the compression will be fine. I doubt that the new exhaust will cause the rough idle.

    I've used spray carb cleaner to detect air/vacume leaks around the carbs, hoses, and intake areas in the past. Some don't like it, but it works for me.

    When sprayed in a certain area and the engine speeds up or bogs down, you've found your leak.

    As for the timing, it'll have to waite until Monday for me to get back to the shop.
     
  6. row

    row New Member

    Messages:
    3
    State:
    Australia
    GREAT NEWS!!!!!!!!!! Found ignition timing at idle was 10 degrees ATDC. Had been held close before to correct just by cable adjustment etc....
    but after power head being off and good trunk and engine mounts on, everything AWOL
    Adjusted base timing up to 2 degrees ATDC and engine sounds as good as ever, max advance 17 degrees. In Australia I have to run unleaded premium which is 96-98 octane, or ethenol blend which is 95 octane when I can get it.
    Thank you so much for your advice its great to know where to look if I get into trouble!!:big_smile:
     
  7. Bobpaul

    Bobpaul New Member

    Messages:
    3,039
    State:
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    What help? You did all the work.

    It's good to here you found the timing info and set it right. With that high octane you have to use, to much advance could seriously hurt that engine.

    Now lets here a bit about the fishing.