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Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600 RPM.

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  • Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600 RPM.

    What would cause a non vortec carburated Chevy small block to idle well at 16 degrees BTDC but idle rough and erratic at 8 degrees? The engine will run good at that high spark advance but will obviously ping at higher RPMs. When the engine is set to 6 - 8 degrees BTDC the engine idles rough at 600 - 700. It runs great between 1000 and 5000 though.
    Better run away!
    I gotta twin engine stringer powered cabin cruiser!

    sigpic

    1980 28' Imp Kansa w/2x OMC 260HP 350 Stringers.


  • #2
    Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

    Idle mixture on the carb needs adjusting or the carb needs a proper rebuilding.
    Don S.

    sigpic

    Please, no PM's (Private Messages) regarding boat/engine problems.
    That is what the forums are for.
    Only forum/moderator issues will be answered in PM's.

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    • #3
      Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

      timing chain could be very loose, you can check by removing the dist cap, rotate motor in normal direction untill timing mark is at "0", mark rotor, rotate crankshaft in opposite direction untill rotor moves. a tight chain moves the rotor instantly, a worn chain might get 10-20 degrees difference before the rotor moves.
      NO PERSONAL QUESTIONS, THIS IS WHAT THE FORUM IS FOR.

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      • #4
        Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

        I thought that too. But few marine engines live long enough to wear out a timing chain. Another symptom would be the timing mark jumping around a little bit and a general loss of power.
        But Don's on a roll today. If I see one more of his posts like this, I'm starting a game show "Name that Problem" with him as the star! (like name that tune - I can name that engine problem in one symptom!). I would yield to his expertise before ripping the front of the engine apart. Adjusting the carb would be a prerequisite anyway.
        Besides, we don't have clue what else might have or have not been done or how old it is or anything else.
        sigpic
        Nothing is so broken that Government can't make it worse.

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        • #5
          Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

          What year is this 305. Don s is, as always on the money. Carb must be ruled out before anything else.

          With what little info we were given the conclusion must be the carb first.

          Was this discovered cause you just bought the boat ?

          You were doing a tune up and was timing the motor ?

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          • #6
            Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

            I tried rotating the crank as suggested and found a 10 - 15 degree difference between the distributor and the crank. I took off the timing cover and was amazed that there was a very loose double roller chain in there. I replaced the chain today and look forward to seeing how it runs tomorrow. The new chain is as tight as a barbed wire fence. I also have a new Q-jet on the way.
            Better run away!
            I gotta twin engine stringer powered cabin cruiser!

            sigpic

            1980 28' Imp Kansa w/2x OMC 260HP 350 Stringers.

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            • #7
              Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

              They don't call him the Doctur for nothing.

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              • #8
                Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

                Took her out to the lake today and ran her for about 2 - 3 hours. The idle is much much better and the thing fires right up, but the idle is still a little bit erratic. When I say erratic I mean sometimes it will hum at 650 RPMs and after running it hard for a few minutes or after it has been shut off for about 20 minutes it doesn't want to idle at more than 450 RPMs. I think changing the carburetor will make a big difference.
                Better run away!
                I gotta twin engine stringer powered cabin cruiser!

                sigpic

                1980 28' Imp Kansa w/2x OMC 260HP 350 Stringers.

                Comment



                • #9
                  Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

                  You may have water in your carb and/or the water seperating fuel filter. Have you checked?

                  How about a little information on your engine. Need answers to #1 in the link below.
                  http://forums.iboats.com/showthread.php?t=288430
                  Don S.

                  sigpic

                  Please, no PM's (Private Messages) regarding boat/engine problems.
                  That is what the forums are for.
                  Only forum/moderator issues will be answered in PM's.

                  Comment



                  • #10
                    Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

                    I do not have water in the fuel. I have checked that.

                    I'm sorry I didn't put all the proper info on the engine.


                    1982ish Chevy 305 HO with 4MV Q-Jet and stock intake bolted up to an OMC Stringer 800 with a 14X14 / 14X16 prop
                    Better run away!
                    I gotta twin engine stringer powered cabin cruiser!

                    sigpic

                    1980 28' Imp Kansa w/2x OMC 260HP 350 Stringers.

                    Comment



                    • #11
                      Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

                      Have you tried adjusting the idle mixture on the carb? Not idle speed, idle mixture?
                      Don S.

                      sigpic

                      Please, no PM's (Private Messages) regarding boat/engine problems.
                      That is what the forums are for.
                      Only forum/moderator issues will be answered in PM's.

                      Comment



                      • #12
                        Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

                        Yes I have, I found the port idle screw to be a lot more sensitive than the starboard idle screw. The port side will kill the engine if closed where as the starboard side will make it just run a bit rougher when closed. I followed the instructions on adjusting the carb's idle mixture and speed and have the screws set to the "optimum setting". I have just ordered a remaned Q-Jet 4MV with an electric choke (My current carb has no choke at all). I have a feeling this one may be bad. I will post what happens after I install the new carb.

                        Engine compression is good and I put a new gasket on the intake when I installed new cylinder heads. That leaves either ignition or fuel delivery.
                        Better run away!
                        I gotta twin engine stringer powered cabin cruiser!

                        sigpic

                        1980 28' Imp Kansa w/2x OMC 260HP 350 Stringers.

                        Comment



                        • #13
                          Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

                          a non responsive idle mix screw is a good sign there is something wrong wiht the carb

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                          • #14
                            Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

                            I don't want speak out of turn here but if the carb is replaced and issue still persists would the next order of buisness be the ignition module that controls the spark advance?. If in fact this engine has electronic ignition?. And if so don't purchase after market by OEM.
                            1996 Searay 175 Five Series
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                            • #15
                              Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

                              I wish it had electronic ignition. It has a points system and an old prestolite mechanical marine distributor. Because I plan on doing a lot of trolling I have considered upgrading it to a high performance electronic ignition.

                              I find it hard to believe that I can run the engine at full throttle with a bad carb for minutes at a time without any issues other than a possible lack of power. I would think it would run lean and cause an overheat or detonation issue.
                              Better run away!
                              I gotta twin engine stringer powered cabin cruiser!

                              sigpic

                              1980 28' Imp Kansa w/2x OMC 260HP 350 Stringers.

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