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  1. #1

    Default Do carbureted engines have a steady idle?

    There may have been something wrong with my engine, but my old carbed gm350 volvo I/O always had a little bit of a surge +/- 50rpm and it drove me nuts, then of course when you put it in gear it would drop about 200rpm, so It always idled a little high in neutral to prevent stalling. After we rebuilt the carb I think it only dropped 100rpm, but anyways, I replaced the engine with a MPI engine and the rpm is as steady as can be. When I shift gears I think it drops just a tad for a couple seconds, then it's back to sitting at a steady 700rpm, just like a car and boy am I a happy camper. So was there something wrong with my old engine or is that just nature of the beast? I love the fact that I can maneuver at the dock and not worry about the engine stalling.

  2. #2

    Default Re: Do carbureted engines have a steady idle?

    I know the MPI engine is can change the amount of required fuel instantly for the load, so I guess that's the difference?

  3. #3

    Default Re: Do carbureted engines have a steady idle?

    I have had carbureted engines idle very smoothly. Likely carb or ignition items were an issue but if you get the need to replace the engine any high wear engine where compression is significantly different cylinder to cylinder or valance wear is different could also contribute to idle varying.

  4. #4
    Join Date
    Sep 2008
    Location
    Maryland - USA
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    8,025

    Default Re: Do carbureted engines have a steady idle?

    When they are 'right' they are 'steady'...

    On the MPI engines, the ECU recognizes the RPM drop when shifting into gear and opens the IAC valve a bit, to compensate....

  5. #5
    Join Date
    Dec 2006
    Location
    Chaumont Bay, N.y., USA
    Posts
    5,896

    Default Re: Do carbureted engines have a steady idle?

    Ayuh,..... My carbed motors have always idle smoothly,...... do alotta that freshwater trollin',......
    Any Grease is Better,... Than No grease at All....

  6. #6
    Join Date
    Mar 2009
    Location
    Portland, Oregon,
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    11,580

    Default Re: Do carbureted engines have a steady idle?

    You mentioned "my old carbed gm 350 Volvo I/O".
    May we assume that this engine is equipped with a mechanically advancing ignition system?

    If not a carburetor issue, please read on.

    (disclaimer.... this is a long shot!)

    .... at idle speed, there is no mechanical advance being added to BASE advance.
    .... BASE advance should be constant at either 8 or 10 degrees BTDC.
    .... your system uses a flyweight/cam affair (2 of these).
    .... the flyweights are controlled by two small springs that pull them both back into (and keep them in) BASE mode at idle speed.
    .... the spring value allows the flyweights to
    progressively advance the spark event as RPM increase above idle speed.
    .... when/if the flyweight return springs have become rust compromised, they loose their spring value.
    .... when/if they have lost their spring value, they will not consistently pull and hold the flyweights back into BASE ADVANCE mode.
    .... if this was to occur, the spark "lead" at idle may be somewhat erratic or pulsating, so to speak.
    .... even with a good fuel delivery system, a pulsating BASE advance spark event (of let's say 2 degrees or so) will present itself as engine RPM "surging".


    Connect up your standard mode timing light.
    With the engine at idle speed, see if the BASE advance is steady or if it is erratic.
    If the BASE advance is erratic, your flyweight system is not holding BASE advance at idle speed, of which may indicate weak springs.


    CAUTION:
    Do not haphazardly replace the flyweight return springs (no matter what the supplier says) without the aid of a shop who owns/operates a Sun, Allen or King distributor machine.

    These little springs not only hold BASE advance while at idle speed, their spring value also controls the very important progressive advance.


    Again, this would only apply if all else is OK.


    .
    Last edited by RicardoMarine; 01-25-2020 at 08:52 AM.
    Rick ... aka Ricardo..... AQ series Volvo Penta repair
    Portland, Oregon
    28' SDN F/B w/ twin Volvo Penta 5.7L DP drives

    "Intellectual growth should commence at birth and cease only at death." Albert Einstein

    Please... no PMs! Post your questions on the forum.

  7. #7

    Default Re: Do carbureted engines have a steady idle?

    The old one had a prestolite ignition which I believe was electronic.

  8. #8

    Default Re: Do carbureted engines have a steady idle?

    This is an 89 GM350 volvo aq271c 275hp

  9. #9
    Join Date
    Mar 2009
    Location
    Portland, Oregon,
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    11,580

    Default Re: Do carbureted engines have a steady idle?

    Quote Originally Posted by Fishmaster23 View Post
    The old one had a prestolite ignition which I believe was electronic.
    Quote Originally Posted by Fishmaster23 View Post
    This is an 89 GM350 volvo aq271c 275hp
    1989 pre-dates Volvo Penta's use of the EST system. (EST = electronic spark timing, of which does not use a mechanically advancing system)

    Even with a Prestolite electronic triggering system, it will still incorporate a mechanical advance.

    Try what I suggest in post #6.


    .
    Rick ... aka Ricardo..... AQ series Volvo Penta repair
    Portland, Oregon
    28' SDN F/B w/ twin Volvo Penta 5.7L DP drives

    "Intellectual growth should commence at birth and cease only at death." Albert Einstein

    Please... no PMs! Post your questions on the forum.

  10. #10

    Default Re: Do carbureted engines have a steady idle?

    In first post, OP stated he replaced this engine with an MPI engine, so doubt he can check the advance mechanism anymore

  11. #11
    Join Date
    Mar 2009
    Location
    Portland, Oregon,
    Posts
    11,580

    Default Re: Do carbureted engines have a steady idle?

    Quote Originally Posted by Dieter
    In first post, OP stated he replaced this engine with an MPI engine, so doubt he can check the advance mechanism anymore
    Yes, the MPI engine would use an EST ignition system that would not incorporate any mechanical advancing.


    Original thread title: "Do carbureted engines have a steady idle?"

    In post #1 Fishmaster asks about his old carbureted engine regarding a bit of an RPM surge.


    Quote Originally Posted by Fishmaster23 View Post
    There may have been something wrong with my engine, but my old carbed gm350 volvo I/O always had a little bit of a surge +/- 50rpm and it drove me nuts, then of course when you put it in gear it would drop about 200rpm, so It always idled a little high in neutral to prevent stalling. After we rebuilt the carb I think it only dropped 100rpm, .................................





    .
    Last edited by RicardoMarine; 01-29-2020 at 08:49 AM.
    Rick ... aka Ricardo..... AQ series Volvo Penta repair
    Portland, Oregon
    28' SDN F/B w/ twin Volvo Penta 5.7L DP drives

    "Intellectual growth should commence at birth and cease only at death." Albert Einstein

    Please... no PMs! Post your questions on the forum.

  12. #12

    Default Re: Do carbureted engines have a steady idle?

    I think the old engine had a bad power valve too. Found out the adjustment screws on the carb were only .50 and .75 turns out yet it still ran.

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