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Thread: Carby flooding

  1. #1
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    Default Carby flooding

    I have a 2006 4.3L Mercruiser with a TKS Carby. The engine was new when I bought the boat and had been fitted by the dealer. The engine has just over 500 hours on the clock and has run well throughout. I service the engine annually in September which includes a new fuel filter change. I now have have an issue that is driving me mad.

    I started the boat normally and idled out the channel before opening up the throttle and cruising for 20 minutes, when I brought the throttle back to idle the engine died and would not restart. I pulled off the flame arrestor and notice fuel dribbling down the Carby throat, without the engine running. I suspected a stuck needle and seat and an over full float bowl. I opened the throttle wide and cranked to clear, the engine starting after a while. I was able to keep the engine running at about 2000 rpm. I motored back to the dock without slowing down, the engine again died as soon as I went back to idle at the dock. I let it cool and removed the Carby top, cleaned out a little crud and cleaned out the needle and seat. After reassembly I restarted the engine and it ran until it warmed up after which it died again with fuel dribbling down the throat. So I suspected a bad needle and seat and I ordered a Carby kit.

    when the Carby kit arrived I fitted a new fuel filter (replacing the 3 month old previous filter), cleaned out the Carby, cleaned the Venturi block, replaced the needle, seat and float then reset the float level. I restarted the engine normally and my problem appeared to be solved. We ran the boat for the week and put about 10 hours on the engine before it started having issues again. I pulled the top of the Carby again and checked for issues, all looked fine. While there I reset the float level so that the fuel would be about 1 mm below normal fuel level. I restarted the boat and we ran it for an hour without issue before again having fuel dribbling down the throat. So now Iím starting to think that the problem must be elsewhere with the only thing coming to mind being the fuel pump.

    questions

    could the electric fuel pump be putting out too much pressure thereby forcing the float down to overfill the fuel bowl?
    is anybody aware of issues with these fuel pumps?
    does anybody have any other ideas as I donít just want to throw money into a new pump uselessly.

    oh before anybody responds, note that it has a fresh tank of fuel and we donít need to worry about wintering our boats here. Winter here gets down to a chilly 6 or 7 degrees C. It is summer now of course.

  2. #2
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    Default Re: Carby flooding

    why not measure the fuel pressure at the carb?

  3. #3
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    Default Re: Carby flooding

    My TKS on a 5.7L on a regular basis ( every 3 to 4 seasons) suddenly develops a total refusal to run under 1500 RPM. In every case the problem was an obstruction in the jet in the venturi. No amount of cleaner will clear this obstruction which always turns out to be a hard sandy colored deposit. A very fine wire is required to dislodge the crap.

    See item #21 on

    ww.marineengine.com/parts/mercruiser-sterndrive-parts/57l-carb-mie/0w090900-thru-0w689999/carburetor


    If that does not work, see Mercruiser Service bulletin #97
    Last edited by sandkicker; 01-26-2020 at 02:17 PM.
    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280

  4. #4
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    Default Re: Carby flooding

    Thanks guys.

    I’d measure the pressure but I don’t have a gauge nor the appropriate fittings.

    I’ve cleaned out the Venturi block and am certain that it is clear. Thanks for your thoughts though.

    Ive ran the engine a couple of times in the last couple of days with different results. I ran it yesterday morning and once warm it wouldn’t idle again. I checked the Carby and sure enough fuel was dribbling in causing a rich cut. Last night I thought more about it and thought that if the pump had too high a pressure then the bowl would get overfull and I’d have fuel dribbling even with the engine shut down until the bowl drained back to the set level. Anyway so without doing anything to the boat I thought that I would prove the thought. I started it this morning and ran it for over half an hour at various RPMs and it was all fine. What did I do between times? Nothing, not a thing at all, but I had different results. I therefore think that I’ll just spring for a new fuel pump as that is the only thing that I currently think could be intermittent.

    I hate replacing things without knowing that it is the actual cause. Otherwise all you really are is uncertain until it get a lot of use to prove that it has been fixed.

  5. #5
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    Default Re: Carby flooding

    I'm with you on the fuel pump being the culprit. The pressure regulation inside them is not a given--it can and does vary (as you have seen.)

    I use a Holley fuel pressure regulator on mine to eliminate the problem once and for all. Easy enough to install. Costs about $30 US.

    Jeff

  6. #6
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    Default Re: Carby flooding

    The blockages I've seen are not in the block per se, but rather in the two thin brass tubes staked into the venturi that suck gas up into it. You may not want to do this, but the test I use is to gently suck air thru the tube and check for free and equal flow.
    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280

  7. #7
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    Default Re: Carby flooding

    Thanks guys, yeah I have cleaned out the tubes and blown through them. They are clear but thanks for the heads up, any help is appreciated. If you have any other ideas I’d take them gladly.

    I have ordered a new pump and will try that, though I hate the idea of finding the fault by replacing items. I like to find the fault then replace/repair once I know what the cause it. Too many years working on aircraft where parts are horrendously expensive. The partially intermittent nature of the fault lends me to think that the pressure regulator may not be right. Letting through too much pressure sometimes but not others.

    I will report back once I know whether that fixes it. Hopefully to help others.

  8. #8
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    Default Re: Carby flooding

    Oh by the way I will look at pressure regulator costs over here. The exchange rate is killing us at the moment.

  9. #9
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    Default Re: Carby flooding

    Sorry, mate.

    Jeff

  10. #10
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    Default Re: Carby flooding

    short of a faulty carb or choke, the fuel pressure and a ruptured fuel pump diaphragm could cause the flooding issue

  11. #11
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    Default Re: Carby flooding

    Well the new fuel pump turned up, eventually, but still no joy. So Iíve pulled the Carby off and have taken it home, some 2 1/2 hours away, where I have my workshop with air. I stripped the Carby again and Blew compressed air through all the galleries. All looked clean with no build up being evident. The Carby body gasket was stuffed by now so Iíve made a new gasket and reset the float level. All assembled again and Iíll have to wait for a few weeks until I can get back to the boat to refit and see how it goes.

    It has to be right, there can be nothing else. If this doesnít work the only possibility is that the new needle and seat that I fitted was dodgy to start with. Give me a couple of weeks and I will let you know how I go.

  12. #12
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    Default Re: Carby flooding

    Try this before you driving 2 1/2 hours possibly for nothing:

    Find yourself an outboard gas tank, hook it to your carb, and pump the bulb stiff as possible. If the thing leaks, drop the float level a 1/16 of an inch at a time until it stops leaking.

    I do this all the time with OB carbs and many of them leak at the specified float level.

    Jeff

  13. #13
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    Default Re: Carby flooding

    Where did you get the rebuild kit from? Aftermarket Kits contain substandard needle/seat/float valves that leak.
    Chris
    Dockside Marine Services
    Jersey Shore.
    Mercruiser/Mercury Certified
    Let the insanity begin.

  14. #14
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    Default Re: Carby flooding

    2X !!!

  15. #15
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    Default Re: Carby flooding

    Thanks for the suggestions

    firstly, I now have the float level set to 20mm not the 14mm specified, so the fuel level should be 6mm lower in the bowl. And it still floods.

    Secondly, it is a Mercury Carby kit so not a cheap knock off. And yep still flooding Carby.

    Iíll get a pressure gauge and check pump pressure. That is all I can do.

  16. #16
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    Default Re: Carby flooding

    Agreed. So why the world went to EFI!

    Jeff

  17. #17
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    Default Re: Carby flooding

    the OEM specified float setting should be fine, but an adjustment of 4 millimeters could cause the engine to run lean when its pulling hard... I think a couple millimeters would be fine, but a lean running condition can erode the pistons at a fairly high rate.

    absolutely YES... check fuel pressure. 4-6 psi is plenty... getting much over 7psi can cause flooding as the fuel pressure can push past the closed needle seat.
    what are the exact symptoms you are seeing from the engine as it runs?

    what is the composition of the float?... check and see if they are able to float high in the fuel, or if they may be "sunk" and not allowing them to put the pressure to the needle seat that it needs to stop the fuel... both brass and plastic floats can become fuel logged, but its more common with brass floats...

    make sure the float moves up and down freely without any hanging up...

    also, look in the hole where the needle seat is and see if there is in fact a "seat" in there.... I dont know that carb, but ive seen carbs with removable seats that came loose letting fuel leak around the threads, and fixed seats that were damaged and couldnt let the needle to stop the flow of fuel... some carbs use a very small oring as a seat, and if this gets blown out during a "rebuild" the needle isnt going to be long enough to reach the bottom/top of its hole to stop the fuel... most production carbs have a seat that is machined, which is nothing more than a clean hole for the tip of the needle to seat in, and any ding or debris can let fuel thru when the needle is closed...
    also make sure the needle is attached to the float properly and not binding in its hole when its trying to close (I made this mistake myself once)...
    make sure the idle jet is still in place and snug...

    make sure the idle mixture screws are not screwed in too far.... they meter the amount of fuel being sucked up by the venturi by allowing air into the circuit... when they are tightened, there is no air bypass and the venturi will suck the maximum amount of fuel allowed by the idle jet.... when the mixture screw is backed out, it allows air into the circuit so the venturi cant pull as much fuel into the throat of the carb...

    I have seen cracked carb bodies and carb bodies that had manufactureing defects that allowed fuel to leak internally.... check it over real good.
    Last edited by centerline; 02-27-2020 at 12:03 AM.

  18. #18
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    Default Re: Carby flooding

    Quote Originally Posted by centerline View Post

    make sure the idle mixture screws are not screwed in too far.... they meter the amount of fuel being sucked up by the venturi by allowing air into the circuit... when they are tightened, there is no air bypass and the venturi will suck the maximum amount of fuel allowed by the idle jet.... when the mixture screw is backed out, it allows air into the circuit so the venturi cant pull as much fuel into the throat of the carb...
    The idle mixture screws meter the fuel that is delivered to the transfer ports; the main venturi has nothing to do with the carb's operation at idle, assuming the carb is setup correctly, the throttle plates in particular. The idle mixture screws are 'downstream' of the feed for the main nozzle in the venturi and again, have nothing to do with the fuel that flows from the main nozzle. The mixtures screws don't control the air flow into the engine, that's what the throttle plate is for (unless the carb has an idle air bypass circuit which is a rare occurrence nowadays)...

  19. #19
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    Default Re: Carby flooding

    Quote Originally Posted by makomark View Post
    The idle mixture screws meter the fuel that is delivered to the transfer ports; the main venturi has nothing to do with the carb's operation at idle, assuming the carb is setup correctly, the throttle plates in particular. The idle mixture screws are 'downstream' of the feed for the main nozzle in the venturi and again, have nothing to do with the fuel that flows from the main nozzle. The mixtures screws don't control the air flow into the engine, that's what the throttle plate is for (unless the carb has an idle air bypass circuit which is a rare occurrence nowadays)...
    you may have misunderstood what I was trying to convey.... I was attempting to explain how the idle circuit works (not the carb in general) so the OP could use it to help diagnose his issue with flooding...

    you are correct that it is not the main venturi that affects the idle circuit, but it is a venturi effect that brings the fuel for the idle in to the throat of the carb, and you are also correct that the the idle mixture screws dont control the airflow into the engine, but they DO control (meter) the amount of venturi effect (suction created or reduced by allowing or restricting air bypass) there is on the fuel in the idle circuit as its being pulled into the throat of the carb (which equates to the volume of fuel that gets pulled in)...

    a carburetor works on a very simple principal, but is only made complex by its several circuits working/transitioning together in such a small package, with each circuit designed to be as safe as possible for all conditions that a carb may be subjected to..
    Last edited by centerline; 02-29-2020 at 12:23 PM.

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