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  1. #1

    Default LM 318 running rough over 2500 rpm

    Hello,

    I have an 81 Silverton 31c with 318's. It just went back in the water, and on the first real cruise, the starboard motor is having trouble keeping up with the port motor. It runs quite rough and requires more throttle. I will say that last year I had some fuel issues, so over the winter I changed both fuel pumps. It starts and idles great. I will also say that on the way home from where we anchored, it ran fine, and kept even and smooth for about 3 or 4 minutes at cruise before starting to stumble and require more throttle. I've read a lot of threads on the subject, and I plan to replace the plugs and wires, check the timing and advance, look at the carb and choke, linkages and vacuum. I was even thinking of disconnecting the flexible fuel line just before the fuel pump inlet and letting it run from a gas can that has a good dose of sea-foam in it to maybe clean the carb a little. Am I missing anything else obvious to try?

    Thanks, Dave

  2. #2
    Join Date
    Mar 2009
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    Portland, Oregon,
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    12,026

    Default Re: LM 318 running rough over 2500 rpm

    Quote Originally Posted by Drfeno View Post
    Hello,

    I have an 81 Silverton 31c with 318's. It just went back in the water, and on the first real cruise, the starboard motor is having trouble keeping up with the port motor. It runs quite rough and requires more throttle. I will say that last year I had some fuel issues, so over the winter I changed both fuel pumps. It starts and idles great. I will also say that on the way home from where we anchored, it ran fine, and kept even and smooth for about 3 or 4 minutes at cruise before starting to stumble and require more throttle. I've read a lot of threads on the subject, and I plan to replace the plugs and wires, check the timing and advance, look at the carb and choke, linkages and vacuum. I was even thinking of disconnecting the flexible fuel line just before the fuel pump inlet and letting it run from a gas can that has a good dose of sea-foam in it to maybe clean the carb a little. Am I missing anything else obvious to try?

    Thanks, Dave
    When you look at the ignition advance, be more concerned about the progressive advance and the total advance regarding what RPM is becomes "Full In".
    Any gasoline marine engine will suffer from lack of performance if the ignition advance is NOT doing what it should be doing!

    All that said, to date NO ONE here has been able to find a Chrysler Marine ignition advance curve for any of the Chrysler Marine engines.
    The specs give us BASE or Initial advance, but nothing regarding the curve and/or total advance.


    Suggestion:
    Put a timing light on the Port engine and look at what the progressive and total advance are showing.
    Compare that to the Stbd engine's progressive and total advance.

    I'm not saying that this is your issued, but you can certainly eliminate it if all is OK with the advance.
    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.

  3. #3

    Default Re: LM 318 running rough over 2500 rpm

    Thanks, before posting I tried to read a few threads on 318 timing. Like you said, my port engine seems to run fine, so if I can maybe graph that advance curve, and compare it to the other motor it may show me something. Another question, if the advance is off either in total advance, or the progression, what can be done about it? would the only cure be to get a new distributor, or could the centrifugal advance be repaired? This may not be the issue at all, but I'm just curious.

  4. #4
    Join Date
    Mar 2009
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    Default Re: LM 318 running rough over 2500 rpm

    Quote Originally Posted by Drfeno View Post
    Thanks, before posting I tried to read a few threads on 318 timing. Like you said, my port engine seems to run fine, so if I can maybe graph that advance curve, and compare it to the other motor it may show me something. Another question, if the advance is off either in total advance, or the progressive, what can be done about it? would the only cure be to get a new distributor, or could the centrifugal advance be repaired? This may not be the issue at all, but I'm just curious.
    Find a shop who owns and operates an old school Sun, Allen or King distributor machine.
    With a known curve, an experienced tech can make the corrections to your system.
    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.

  5. #5
    Join Date
    Nov 2005
    Location
    Chi West, IL, USA
    Posts
    863

    Default Re: LM 318 running rough over 2500 rpm

    You might want to lubricate the distributor advance mechanisms even if it's not the cause of your symptom. I'd guess they are overlooked in tune ups by most. Even the automotive mechanics omit it.

    FWIW, Ray in PA has been rebuilding Chrysler automotive distributors for restorers and HP applications for some time now. I'm sure he could match a pair of marine distributor curves if one is good to base off of.
    https://www.forabodiesonly.com/mopar...faxhops.33547/

  6. #6
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    Default Re: LM 318 running rough over 2500 rpm

    Quote Originally Posted by Daves69
    You might want to lubricate the distributor advance mechanisms even if it's not the cause of your symptom. I'd guess they are overlooked in tune ups by most. Even the automotive mechanics omit it.
    Great point.


    Dave, typically, but not always..... the automotive guys do not quite understand the differences between (and the importance of) the Marine advance curves -vs- the Automotive HP advance curves.
    The automotive HP engine can handle a greater TA, whereas the Marine engine may become damaged if the auto curve is used.

    So.... IMO, it would be imperative that this is understood before any changes would be made to the Marine ignition advance curve.


    Again, several of us (over the last few years) have searched and searched for an OEM Chrysler Marine advance curve. All we have found are specs for BASE/Initial advance..... and even at that, some of the data is conflicting.
    Here is one the older threads regarding this:
    http://www.marineengine.com/boat-forum/showthread.php?400801-Timing-a-318



    .
    Last edited by RicardoMarine; 06-01-2019 at 07:30 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
    Join Date
    Nov 2005
    Location
    Chi West, IL, USA
    Posts
    863

    Default Re: LM 318 running rough over 2500 rpm

    Quote Originally Posted by RicardoMarine View Post
    Great point.


    Dave, typically, but not always..... the automotive guys do not quite understand the differences between (and the importance of) the Marine advance curves -vs- the Automotive HP advance curves.
    The automotive HP engine can handle a greater TA, whereas the Marine engine may become damaged if the auto curve is used.




    .
    I do agree Rick. That is the reason I mentioned "
    I'm sure he could match a pair of marine distributor curves if one is good to base off of."
    I do know a pair of aftermarket distributor curves were posted awhile back. Forgot who. Wonder how those are working out.

    Lol, Now get on over to my thread in steering and give me your input! Been looking for your post!



  8. #8

    Default Re: LM 318 running rough over 2500 rpm

    Thanks to all for the input. Here's what I found. After changing the plugs and wires (plugs looked fine, maybe running a little lean). I looked at the timing. at 650 rpm the advance was 5 degrees BTDC. I found that when I went up in rpm, the timing did advance, but got no where near 22 degrees. This is the CCW motor, and the timing gauge is indeed on backward on this motor. the timing mark advances away from it. I would say you'd be lucky if it got to 13 or 14 degrees. I checked it against the other motor, which runs fine, and it advanced the same. The actual issue was with a leak in my heat exchanger cap. It had gotten loose, and leaked quite a bit when the motor was revved past 2500 or so. So my checking the timing at higher rpms was fortunate, as the leak wasn't happening at lower rpms. I revved it to 3000 to see the advance, and the heat exchanger was basically like a shower head spraying on the spark plugs on the starboard side of the engine. I fixed the exchanger cap, and everything ran great. We took it down the river and both motors ran at the same rpm at the same throttle input and very smooth. So the behavior I saw last week was perfectly consistent with what was happening with the exchanger. While I was at it, I did look at the choke, vacuum, and leaks around the carb. Thanks again,

    Dave

  9. #9
    Join Date
    Dec 2014
    Location
    Ontario Canada
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    94

    Default Re: LM 318 running rough over 2500 rpm

    According to my manual the base advance on post 1971 marine 318s is 5 deg BTC.
    The centrifugal advance is added to the base timing as follows.
    0 deg at 275 RPM
    7.5 deg at 475 RPM
    6-8 deg at 550 rpm or higher
    This is 5 plus 8 equals 13 degrees total advance if I'm reading this correctly and this corresponds as well with your observation of 13-14 degrees of advance on your engines.
    I'm thinking these RPM values will be added to the base timing RPM.
    Click image for larger version. 

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    Last edited by Toonl; 06-20-2019 at 06:28 PM.

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