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  1. #1

    Default OMC Cobra 5L won't stay running

    Starts right up, idles fine, goes into gear and idles fine, accelerate to 3500 RPM boat planes out and inside of two minute stalls dead. Starts right up…but this time will only goes 3K RPM (anymore and it cuts out), runs for few minutes and stalls dead. Starts right up…only 2K RPM…a few minutes and stalls dead. Starts and will only do 1500 RPM and stalls dead. Then hard to start, wait a few minutes and it starts but will only idle, any acceleration will stall the motor. Then on Idle, it only lasted about 20 seconds before shutting off for good.

    The next day on the trailer, it fires right up. I let it Idle for 5 minutes and I start to hear it "dogging". As in it's begging for juice (if that makes sense). So I put it in gear and rev her up to 2500RPM. After 10 seconds I hear the same "dogging" and throw it back in idle. One minute after that it dies. I have to wait 30 seconds or so for it to fire back up. Rinse and repeat till finally it won't start any more.

    THE NEXT DAY on the trailer, it fires right up and the same thing as the day before. .

    Temp is normal, voltage good, oil pressure good, and good gas

  2. #2

    Default Re: OMC Cobra 5L won't stay running

    This took forever getting back on here. I forgot my login and the emails i used is not longer.

    You replied to a post that is at least 4 years old about my OMC Cobra 5.7 problem.

    Dude!
    It was H E double hockey sticks for a few years.
    I'll try to keep it brief. Well, as I can

    Took it out, won't go more than a few thousand feet and then nothing, "dead in the water."
    I'm going to guess I thought I "fixed it" and took it to the lake 20 times trying to figure it out.
    During my time of exploration, I ended up overheating it three times, and rebuilt the out-drive all before getting it to work.
    In order - how I came to my conclusion:
    change the water separator
    cleaned the pick-up screen
    cleaned the carb filter
    cleaned the fuel tank
    rebuilt the carb
    new plugs and points
    replaced the fuel line
    cleaned the fuel tank again
    installed an electric fuel pump
    new carb
    new interrupter switch
    new mechanical fuel pump
    new fuel cell
    Here's where is started getting interesting...
    put the electric fuel pump back on
    boat runs all day
    Go home and think I have this licked.
    Remove the fuel cell
    still using the electric fuel pump
    take it out and it dies!
    Connect the mechanical fuel pump
    fuel cell again
    Boat runs great!
    Connect to the tank again
    Dies!
    Put the old carb back on and remove all the new and put back as was.
    take a rubber fuel hose and connect to external tank
    Boat runs.
    connect to fuel tank
    dies.
    Connect to fuel cell
    RUNS!
    put it back as it was and take it out...
    Wife drives while I play with the fuel pick up
    boat dies.
    I removed the bottom screen and it would go, just not very long.
    And I'd have to let it rest in between starts as it would stall quicker if I rushed it...
    (It was very much like vapor lock)
    Installed a inline fuel pressure gauge.
    tested the pick up again
    You could watch the pressure drop to zero and a few seconds later dead at any speed.
    I took it home and cut off the screen end to the pick up.
    Took it back to the lake and she ran all afternoon!
    The pick up looked as though someone had lengthened it with a splice in the middle.
    It was 3/8" copper to 1/2" copper to 3/8" and the screen end.
    I cut the whole pick up off from the 3/8" at the top down.
    Soldered a new 1/2" copper pick-up onto it.
    Took it out and it died.
    Come to find out the new extension was too long and sucked the bottom of the tank
    Notched both side at the bottom and still was stalling
    Cut 1/2" off the bottom of the pick-up and been boating carefree ever since.
    It would appear the screen end was only 1/4" and too small to deliver the fuel fast enough.
    Once I removed it, life was good!
    So, there was never actually anything wrong with the boat, motor, or out-drive, and I'm pretty sure the guy I bought it from knew there was some kind of problem as I found boat repair receipts stashed in the cabin storage with a manual. should have been my first clue.
    I'm not suggesting this is everyone OMC's problem. This is an old boat [1989] and it would appear it's been worked on several time.
    The part that was soooo frustrating was that it ran perfect in the driveway every time, I could throttle it up for an hour.
    Only under a load in the water would it starve for fuel through the under sized pick-up screen.
    I learned a lot about this boat working on it those years, but even the break downs were fun. The wife would bring food and we'd play cards, and swim and make a day of it every time. I only asked for help once to be towed on a really windy day.
    Every time it would stop running I would lash a rope and swim and pull the boat in myself, no mater how far out. Why? To tire the frustration.
    We have a great time now, the 5.7L OMC Cobra plans quickly, and moves the 26 foot cuddy to 38 mph with a 15.5x15 prop.
    I doubt you would have the same problem, but who knows! Just keep at it and eventually you will narrow it down and fix it.
    The worst part was the whole thing probably cost me about $500. I have a spare parts now, fuel stuff, a fuel cell, a new carb (spare), and more hours than I can count.
    No forum was helpful, all the know-it-alls all gave me the same simple and obvious rhetoric that was never helpful, but I was hoping.
    I also never gave into taking it to the boat mechanic, I saw what they were charging the other guy and it was never fixed. One of the receipts was for $600.
    One of the first things was how funked-up the fuel tank was, it took two times to get the 60 gallon fuel tank all the way clean.
    Drain the fuel from it, use detergent, power washer, and rinse it with water and a shop vac with a 1/2" copper pipe taped to the hose to suck it dry.
    I tried to keep it short. LOL
    Thanks for reading my story, and I hope in someway it helps.
    Good luck with your boat!
    See ya on the water,
    _Seasqwirl

  3. #3

    Default Re: OMC Cobra 5L won't stay running

    I can't believe you replied so quick LOL. I just googled my problem and found your thread 4 years ago. The thing is I've had my boat for 8 years now and this year has been the worst. Never experienced this problem and have been in and out of the mechanic 4 times now. Luckily I know the owner and if he can't solve something the first time, he doesn't charge the following visits. I'm not a boat mechanic by any means but I kind of understood your story. I'm gonna show this to my mechanic lol. Thank you so much for helping man!

    I'm in Michigan, boating the lakes. Wbu?

  4. #4

    Default Re: OMC Cobra 5L won't stay running

    Put a fuel pressure gauge before the carb and after any filtering and put it in the water, if you have constant fuel pressure, it’s not your fuel delivery.
    Has the mechanic tested and adjusted the interrupter switch?
    Those are tricky to play with.
    if it starts and runs in your driveway, you have a similar problem with it failing under a load. If it’s not fuel delivery, I would guess it’s thermal. Something in the electronics is overheating under load?
    Thats my two cents.

    Arizona

    Good luck

  5. #5

    Default Re: OMC Cobra 5L won't stay running

    Turns out the Carb was dumping fuel and harmonic balancer and timer were completely off. I have yet to test it out though.

  6. #6

    Default Re: OMC Cobra 5L won't stay running

    Carburetors can gum up and foul if left for long periods of time without use by evaporation. Introduce moisture to the fuel and the gumming gets worse.
    Timing is easy, the motor runs smooth, rough, or not at all if the timing is too far advanced or retarded. It will, would NOT cause your symptoms.
    Harmonic balancers are fixed and balance your cranks rotation with no adjustment. It also has the timing mark to set your motors timing from. The crank and the cam are connected with a timing chain on one end and you distributor gears are on the other end internally. Again, would not cause your symptoms. You set the timing by slightly rotating the distributor while pointing a timing light at the rotating harmonic balancer and fixed degree indicator.


    You need to start by cleaning and rebuild the carb. The needle valves and/or float could be gummed up and sticking closed not allowing the bowl to fill completely while underway starving the carb of fuel and then stalling.
    Set the timing to spec, about 8 degrees BTDC and any motor will run. Just saying.






  7. #7

    Default Re: OMC Cobra 5L won't stay running

    Do you have a fuel water separator filter?
    Have you checked the fuel filter in the carb?
    they are typically behind the fuel inlet at the carb.
    This is a micron filter and plugs very easily.


  8. #8
    Join Date
    Mar 2009
    Location
    Portland, Oregon,
    Posts
    12,020

    Default Re: OMC Cobra 5L won't stay running

    I'm posting to this thread as though the problem still exists. I'm pleased to see that you finally got a resolve!

    Quote Originally Posted by TheSeasqwirl View Post

    You replied to a post that is at least 4 years old about my OMC Cobra 5.7 problem.
    Searching the archives is great, but it's always best to start a new/fresh thread for a given issue.
    If need be, post a link to the thread that you found while searching!




    Took it out, won't go more than a few thousand feet and then nothing, "dead in the water."
    I'm going to guess I thought I "fixed it" and took it to the lake 20 times trying to figure it out.
    Try using the P of E.... process of elimination.... check/replace one item, and one item ONLY at a time.
    If you toss a bunch of parts at an issue, and if you are lucky enough to have solved it, you'll never know which item fixed it!


    During my time of exploration, I ended up overheating it three times, and rebuilt the out-drive all before getting it to work.
    In order - how I came to my conclusion:
    change the water separator
    FYI..... the only true water separating fuel filter will be the Parker/RACOR.

    cleaned the pick-up screen
    cleaned the carb filter
    cleaned the fuel tank
    Good on all of those.

    rebuilt the carb
    new plugs and points
    Are you adjusting the contact points by setting the dwell angle????
    Setting point gap is an estimate ONLY of correct dwell angle!

    replaced the fuel line
    cleaned the fuel tank again
    installed an electric fuel pump
    FYI.... in order to be legal, the electric fuel pump circuit must include a N/O Low Oil Pressure switch.

    new carb
    new interrupter switch
    There is also an "over-ride" switch. Both the Interrupt and over-ride switches must be adjusted properly.
    The ESA adjustment must be made while the boat is in the water.


    new mechanical fuel pump

    new fuel cell
    Please explain! External fuel tank????

    Here's where is started getting interesting...
    put the electric fuel pump back on
    boat runs all day
    What did you deduce from that?

    Go home and think I have this licked.
    Remove the fuel cell
    still using the electric fuel pump
    take it out and it dies!
    What did you deduce from that?

    Connect the mechanical fuel pump
    fuel cell again
    Boat runs great!
    Connect to the tank again
    Dies!
    Again, what have you deduced from that?



    Put the old carb back on and remove all the new and put back as was.
    take a rubber fuel hose and connect to external tank
    Boat runs.
    connect to fuel tank
    dies.
    At the risk of redundancy..... what did you deduce from that?
    (keep in mind that I'm posting as though the problem still exists.)

    Connect to fuel cell
    RUNS!
    put it back as it was and take it out...
    Wife drives while I play with the fuel pick up
    boat dies.
    Same as above????

    I removed the bottom screen and it would go, just not very long.
    What are you calling a bottom screen? The pick up tube screen?

    And I'd have to let it rest in between starts as it would stall quicker if I rushed it...
    (It was very much like vapor lock)
    Have you considered the possibility of mild ignition induced detonation?
    The Marine version SBC is almost always fitted with the GM Full Dished pistons, of which makes this engine very prone to detonation if the progressive and total ignition advance are excessive.

    Carefully watch your ignition advance on any gasoline marine engine.
    It is critical that it be correct throughout the RPM range.... especially at the TA RPM!

    Installed a inline fuel pressure gauge.
    tested the pick up again
    You could watch the pressure drop to zero and a few seconds later dead at any speed.
    As you know, the fuel pressure should not drop excessively. The pump should be able to keep up with the demand!

    I took it home and cut off the screen end to the pick up.
    Took it back to the lake and she ran all afternoon!
    The pick up looked as though someone had lengthened it with a splice in the middle.
    It was 3/8" copper to 1/2" copper to 3/8" and the screen end.
    I cut the whole pick up off from the 3/8" at the top down.
    Soldered a new 1/2" copper pick-up onto it.
    Took it out and it died.
    Dang... not good!

    Come to find out the new extension was too long and sucked the bottom of the tank
    Notched both side at the bottom and still was stalling
    Cut 1/2" off the bottom of the pick-up and been boating carefree ever since.
    Excellent!


    It would appear the screen end was only 1/4" and too small to deliver the fuel fast enough.
    Once I removed it, life was good!
    So, there was never actually anything wrong with the boat, motor, or out-drive, and I'm pretty sure the guy I bought it from knew there was some kind of problem as I found boat repair receipts stashed in the cabin storage with a manual. should have been my first clue.

    I'm not suggesting this is everyone OMC's problem. This is an old boat [1989] and it would appear it's been worked on several time.
    The part that was soooo frustrating was that it ran perfect in the driveway every time, I could throttle it up for an hour.
    Testing an engine with no load on it offers very little (if any), useful information.

    Only under a load in the water would it starve for fuel through the under sized pick-up screen.
    I learned a lot about this boat working on it those years, but even the break downs were fun. The wife would bring food and we'd play cards, and swim and make a day of it every time. I only asked for help once to be towed on a really windy day.
    Every time it would stop running I would lash a rope and swim and pull the boat in myself, no mater how far out. Why? To tire the frustration.
    We have a great time now, the 5.7L OMC Cobra planes quickly, and moves the 26 foot cuddy to 38 mph with a 15.5x15 prop.
    I doubt you would have the same problem, but who knows! Just keep at it and eventually you will narrow it down and fix it.
    Again, try using the P of E..... when used systematically and methodically, it will not let you down!


    The worst part was the whole thing probably cost me about $500. I have a spare parts now, fuel stuff, a fuel cell, a new carb (spare), and more hours than I can count.

    No forum was helpful, all the know-it-alls all gave me the same simple and obvious rhetoric that was never helpful, but I was hoping.
    Please don't be disrespectful to the forum members.
    And please understand that it is very difficult to diagnose a problem from a key-board.

    We have quite a few members here who are very savvy and helpful, but without actually being there with "hands-on" it is very difficult.
    Add to it that most people who join the forum for help/answers, are not mechanics, and often do not speak the language, let alone give the best and most accurate info from the get-go!



    I also never gave into taking it to the boat mechanic, I saw what they were charging the other guy and it was never fixed. One of the receipts was for $600.
    Tip:
    Always be willing to pay for proper diagnostics up front.
    Authorize the repair ONLY after proper diagnostics have been accessed in written form, and ONLY upon a written work scope "Quote" (not an estimate) that references the diagnostics in the work scope "quote".
    Otherwise, you may as well hand over a signed blank check!


    One of the first things was how funked-up the fuel tank was, it took two times to get the 60 gallon fuel tank all the way clean.
    Drain the fuel from it, use detergent, power washer, and rinse it with water and a shop vac with a 1/2" copper pipe taped to the hose to suck it dry.
    While rather expensive and perhaps not easy to find, the non-ethylene gasoline will not give us the same problems that we see occurring with the E-10 or E-15 gasoline.
    My family and I are into snowmobiling. Our machines are stored during the warmer weather. Warm weather accelerates
    E-10 or E-15 gasoline problems! If we were to use E-10 or E-15 gasoline, we could find ourselves having big problems.
    I run non-E in all of my toys, lawn mover, garden equipment, etc.

    I tried to keep it short. LOL
    Hey.... it was a good story, and I fully understand the length of it.
    I'm glad that all is good now!

    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.

  9. #9

    Default Re: OMC Cobra 5L won't stay running

    Have you looked into the ecu? Mine ended up being bad. Check out Gt40marine.com

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