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  1. #1
    Chief Petty Officer rs2k's Avatar
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    Default 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.

  2. #2
    Honorary Moderator Emeritus Don S's Avatar
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    Default 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.


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  3. #3
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    Default 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.

  4. #4
    Captain mthieme's Avatar
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    Default 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.

    Nothing is so broken that Government can't make it worse.

  5. #5
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    Default 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 ?

  6. #6
    Chief Petty Officer rs2k's Avatar
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    Default 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.

  7. #7
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    Default 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.

  8. #8
    Chief Petty Officer rs2k's Avatar
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    Default 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.

  9. #9
    Honorary Moderator Emeritus Don S's Avatar
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    Default 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/mercruiser-i-o-inboard-engines-outdrives/adults-only-no-exceptions-288430.html
    Don S.


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  10. #10
    Chief Petty Officer rs2k's Avatar
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    Default 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

  11. #11
    Honorary Moderator Emeritus Don S's Avatar
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    Default 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.


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  12. #12
    Chief Petty Officer rs2k's Avatar
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    Default 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.

  13. #13
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    Default 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. #14
    Lieutenant Commander fat fanny's Avatar
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    Default 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. #15
    Chief Petty Officer rs2k's Avatar
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    Default 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.

  16. #16
    Lieutenant Commander fat fanny's Avatar
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    Default Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

    Well my input is ruled out. But I would have to believe the concensus that the issue is in fact the carb due to the inconsistant performance and the adj, on both side screws don't give similar results.
    1996 Searay 175 Five Series
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  17. #17
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    Default Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

    Quote Originally Posted by rs2k View Post
    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.
    Your idle problem wont effect the high speed operation of the carb. When you are using one, the other isn't in use

  18. #18
    Honorary Moderator Emeritus Don S's Avatar
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    Default Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

    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.
    Two completely different systems of the carb being used. You are having problems with the idle circuit, not the hi speed circuit.
    Don S.


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  19. #19
    Cadet BBK's Avatar
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    Default Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

    I had the same problem with my 1990 Bayliner 5.0l . After checking carb, replacing plugs setting timing I went to the distributor "prestolite mechanical". I noticed the wieghts seemed a little sticky. Used some electronic cleaner, then oil lightly with 3 in 1. Wiped it clean.....It worked no more rolling idle at low rpms or rough idles.

    BBK

  20. #20
    Chief Petty Officer rs2k's Avatar
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    Default Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

    I cleaned and oiled the distributor internals as replaced the carb. I currently have the timing set like so:

    700 RPM 8 degrees
    3000 RPM 36.5 degrees


    After clean cleaning the distributor the timing advanced at idle by 8 degrees even though the distributor had not moved. That obviously will make some difference. I also notice the new carb is much smoother when I advance the throttle. I look forward to trying it out on the lake after the wind finally dies down. It looks to be 25 - 40 MPH all week. It's not fun getting wet when the water temp is still 45 degrees.

  21. #21
    Honorary Moderator Emeritus Don S's Avatar
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    Default Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

    After clean cleaning the distributor the timing advanced at idle by 8 degrees even though the distributor had not moved.
    Not sure I follow what you are trying to say here. Did you have it set at 8 then cleaned it and the timing went to 16????
    Don S.


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  22. #22
    Chief Petty Officer rs2k's Avatar
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    Default Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

    Yes, that is correct. The weights were a bit sticky. I would have though that if would have changed going the other direction though.

  23. #23
    Honorary Moderator Emeritus Don S's Avatar
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    Default Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

    Get everything freed up and working, THEN set the timing and see what happens.
    Don S.


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  24. #24
    Chief Petty Officer rs2k's Avatar
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    Default Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

    I freed everything up and everything seems to be working just fine. It will idle fine at 700 RPM and 8 degrees BTDC. At 600 RPM and 8 degrees BTDC the engine will lope a little and can even die when the prop and exhaust have a load. It idles the smoothest at 14 BTDC (600 RPM). The problem is that idle timing would put it at 42.5 degrees at 3000 RPM which just won't work. Looking at the distributor weights it looks like I can adjust the stops so that it remains at 36.5 degrees BTDC but still idles at 14 degrees BTDC. Does this seem like a reasonable approach?

  25. #25
    Honorary Moderator Emeritus Don S's Avatar
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    Default Re: Chevy 305 has rough idle at 8 degrees timing but gets smooth at 16 degrees - 600

    The weights in the distributor are NOT adjustable, the timeing needs to be a t 8 BTDC at idle. Adjust the idle mixture on the carb at 700 and it should be fine. Every other SBC engine will run like that, what makes you think you should need something else.
    Don S.


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