Troubleshooting your fuel system is crazy easy and simple and cheap to do
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  1. #1
    PWCToday.com Is My Home Away From Home fox river pwc's Avatar
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    Troubleshooting your fuel system is crazy easy and simple and cheap to do

    More fun stuff.
    I see so much bad/incomplete/misguided info on this subject...figured I'd put something together to help make sense of it.

    First off, the moment you have a fueling issue, no, you don't need to immediately "rebuild your carb with genuine mikuni parts...", no matter how many times this gets posted by "helpful" folks, and even though you might need to do a rebuild...doing that as a first step is the exact opposite of actually using your brain to troubleshoot in a logical manner. If you like to fix things accidentally, then go ahead and just start rebuilding and/or replacing parts until your issue magically disappears. If you want to actually understand your issue and only rebuild/replace what needs doing to actually resolve your issue, then keep reading.

    Step 1:
    Actually understand how your fuel system works.
    (For this example I am using a very basic Yamaha single carb setup. Your details may vary a bit. Relax. I understand the differences and variations. You don't need to tell me about them, lol.)

    See the picture.

    You put fuel in your fuel tank.

    The carb has a fuel pump, diaphragm style. It uses pulses from the crankcase to move the diaphragm and create a suction. It sucks the fuel through the fuel system, uses the fuel to pressurize inside the carb, then returns all excess fuel to the fuel tank via the fuel return line. It's a closed loop system. Very common.

    The pathway from the tank to the carb is simple. Fuel travels upward, under suction, through the fuel pickup tube (ON or RES), over to the fuel selector (item 8), then over to the fuel filter (item 1, which also functions as a water separator), then over to the inlet fitting on the carb.

    It is important to note that as fuel exits the tank, air must be allowed to enter to compensate for the volume of fuel lost. There is a check valve (item 17) that is designed to allow air into the tank (via the fuel vent line) while preventing fuel from exiting through this line (unless excess pressure build up occurs in the tank).

    1650461282915_image001.png


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  2. #2
    PWCToday.com Is My Home Away From Home fox river pwc's Avatar
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    Re: Troubleshooting your fuel system is crazy easy and simple and cheap to do

    Step 2:
    Get the supplies you need to troubleshoot.
    You need 2 things. A 1/4" barbed coupling and a few feet of clear vinyl tubing, also 1/4".

    1650461282915_image002.png1650461282907_image003.png
    Last edited by fox river pwc; 04-20-2022 at 11:14 AM.


    WHEN IN DOUBT, GAS IT!

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  3. #3
    PWCToday.com Is My Home Away From Home fox river pwc's Avatar
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    Re: Troubleshooting your fuel system is crazy easy and simple and cheap to do

    Step 3:
    OK, so now you're suspicious that your ski is starving for fuel, so it's time to determine why.

    What are the really basic possibilities/variables?
    A) Carb isn't doing its job.
    B) Fuel system isn't doing its job.
    C) Both.

    So ask yourself...How well can a carb do its job if the fuel system isn't able to perform as it should? It can't. The mostest bestest workingest carb on the planet can't make your engine run correctly if the fuel system is sh1tted up, right? Even better, if you believe (via guessing) that your carb is sh1tted up inside, what do you think your fuel system looks like inside? So, with all this basic common sense floating around, why would it make any sense to even touch the carb before you've tested and troubleshot the fuel system...especially when this can be done easily and in minutes? I know why, cuz random strangers on the interwebs posted, "Start by rebuilding your carb with genuine mikuni parts....", Bwaaaaahahahaha!!

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    Last edited by fox river pwc; 04-20-2022 at 10:41 AM.


    WHEN IN DOUBT, GAS IT!

    Yeah, I'm an @sshole, but I'm not a complete @sshole.

    http://badgerlandjetpilots.com/

    I don't come here to make enemies - only to identify them...

  4. #4
    PWCToday.com Is My Home Away From Home fox river pwc's Avatar
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    Re: Troubleshooting your fuel system is crazy easy and simple and cheap to do

    Step 4:
    Now we are getting somewhere.

    Time to test the entire fuel system in one fell swoop.

    Crazy easy to do.

    Disconnect the fuel line at the INLET on the carb. That's the line running from the external fuel filter TO the carb.
    Stick your barbed coupler in the now open end of this line.
    Stick a decent length of clear tubing on the other end of the barbed coupler.
    Make sure you have fuel in the tank above the height of the ON pickup tube.
    Put the fuel selector to ON.
    Grab an empty fuel jug.
    Suck on the tube and get a siphon going. Don't swallow the gas.
    Watch the flow.
    Is it steady?
    Is it strong?
    Air bubbles?
    Weak?
    Piddly?
    No flow at all?
    Can't get a siphon started?

    If your fuel system is healthy, getting a smooth and steady and strong siphon going is easy to do. If your fuel system is not healthy, it won't be easy. It may be impossible.

    This siphon is essentially mimicking what your fuel system does under operation. If it can't pass this siphon test, guess what? Then it can't supply the carb correctly either.

    Assuming you get a good siphon going with the ON setting on the fuel selector, then switch to RES.

    If the RES portion of your fuel pickup tubes and fuel selector is also healthy, then the siphon should continue without any noticeable difference in flow. If the siphon stops, or gets weak.....well that's a problem.
    Last edited by fox river pwc; 08-14-2022 at 10:42 PM.


    WHEN IN DOUBT, GAS IT!

    Yeah, I'm an @sshole, but I'm not a complete @sshole.

    http://badgerlandjetpilots.com/

    I don't come here to make enemies - only to identify them...

  5. #5
    PWCToday.com Is My Home Away From Home fox river pwc's Avatar
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    Re: Troubleshooting your fuel system is crazy easy and simple and cheap to do

    Step 5:
    Let's say that the siphon test shows that your fuel system flows beautifully and steady, under both the ON and RES settings, NOW it is time to remove your carb, open it up, have a look, and decide what to do next - with the carb.

    Let's say your fuel system fails the siphon test, now what?
    Well, it depends on how it fails.

    If it siphons fine on ON, but not RES, then your issue is most likely within the fuel selector and/or the RES pickup tube. One or both is clogged. I can go into how to clean these if you want, but I don't feel like it now. Post below any questions or if you want more details.

    If it won't siphon well on either ON or RES, well then your issue could be both pickup tubes and/or the fuel selector and/or the external fuel filter.

    To narrow this down you can simply modify the test and keep applying the very same logic you've already been using during this process.

    The siphoning process is your friend. Use it elsewhere.

    You can connect the siphon hose directly to either fuel pickup, one at a time, and siphon directly from both. Same deal. Watch and determine if both are healthy or not.

    You can connect the siphon hose AFTER the fuel selector but ahead of the inlet to the external fuel filter and see what kind of a siphon you get. A good siphon here would tell you the external fuel filter is clogged.

    Side note: A stuck check valve on the vent line can prevent a good siphon as well, since this could keep the fuel tank from drawing air IN as the siphon draws fuel OUT. A simple test here would be to loosen the fuel cap. If the siphon improves greatly with the fuel cap loosened up, then you probably have a sticky check valve.


    Important to keep in mind: You could have MULTIPLE fuel system issues that all add up to enough of an issue to then starve your ski of fuel. Remember, that same crappy gas that sh1tted up your fuel selector and clogged it ALSO spent time in your pickup tubes AND your external fuel filter, so you might have issues there as well.

    Troubleshooting is all about identifying and isolating variables.....and then carefully examining the variables, if you want to actually know and correct your root issue(s).

    All of this is totally simple sh1t.
    Last edited by fox river pwc; 04-22-2022 at 11:36 PM.


    WHEN IN DOUBT, GAS IT!

    Yeah, I'm an @sshole, but I'm not a complete @sshole.

    http://badgerlandjetpilots.com/

    I don't come here to make enemies - only to identify them...

  6. #6
    PWCToday.com Is My Home Away From Home fox river pwc's Avatar
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    Re: Troubleshooting your fuel system is crazy easy and simple and cheap to do

    Almost forgot. So let's say your fuel system initially fails the siphon test. Then you narrow down the issue, resolve it, and now it passes the siphon test...but your ski is still showing signs of starving for fuel. Now what? Now you remove carb and proceed with any needed carb repairs and/or rebuild....and don't be surprised. After all, the same crappy gas that sh1tted up your fuel system simply managed to sh1t up your carb too. LOL. Again, simple reality in action here. Nothing surprising whatsoever.


    WHEN IN DOUBT, GAS IT!

    Yeah, I'm an @sshole, but I'm not a complete @sshole.

    http://badgerlandjetpilots.com/

    I don't come here to make enemies - only to identify them...

  7. #7
    PWCToday.com Is My Home Away From Home WFO Speedracer's Avatar
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    Re: Troubleshooting your fuel system is crazy easy and simple and cheap to do

    Truefully most of the time I replace very little when I clean-rebuild a set of carbs and I never use Mikuni carb kits. I do use OEM Mikuni parts that I buy separately from Western Power Sports, most of the time its the main diaphragms that need to be replaced and maybe a fuel pump diaphragm or two, very rarely do check valves need replacing, if the needle and seats pop off to spec and hold pressure they are good, period.

    Needle and seats get rep[laced as needed as do screws inside the carb due to rust and corrosion or the occasional screw that just won't come out without damaging it.
    Last edited by WFO Speedracer; 04-20-2022 at 01:57 PM.
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  8. #8
    PWCToday.com Is My Home Away From Home Myself's Avatar
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    Re: Troubleshooting your fuel system is crazy easy and simple and cheap to do

    WAIT A MINUTE!!!! You mean all these hundreds of carbs I've actually rebuilt through the years with AFTERMARKET kits because they had been sitting for 7 years with ethanol fuel in them.....didn't actually need it???












    Just kidding. I agree though, let's figure out where the issue is before just throwing parts at it. I don't know HOW many times I 've seen people suggest on a Seadoo to replace the fuel switch 'they always leak'. I bet I've only replaced 3 or 4 in the last 15 years!
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  9. #9
    PWCToday.com Is My Home Away From Home fox river pwc's Avatar
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    Re: Troubleshooting your fuel system is crazy easy and simple and cheap to do

    Quote Originally Posted by WFO Speedracer View Post
    Truefully most of the time I replace very little when I clean-rebuild a set of carbs and I never use Mikuni carb kits. I do use OEM Mikuni parts that I buy separately from Western Power Sports, most of the time its the main diaphragms that need to be replaced and maybe a fuel pump diaphragm or two, very rarely do check valves need replacing, if the needle and seats pop off to spec and hold pressure they are good, period.

    Needle and seats get rep[laced as needed as do screws inside the carb due to rust and corrosion or the occasional screw that just won't come out without damaging it.
    Yup, mikunis don't frequently need 100% complete rebuilds in order to return them to functional condition......but the person working on the carb does need to understand how it works, lol.


    WHEN IN DOUBT, GAS IT!

    Yeah, I'm an @sshole, but I'm not a complete @sshole.

    http://badgerlandjetpilots.com/

    I don't come here to make enemies - only to identify them...

  10. #10
    PWCToday.com Is My Home Away From Home fox river pwc's Avatar
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    Re: Troubleshooting your fuel system is crazy easy and simple and cheap to do

    Quote Originally Posted by Myself View Post
    WAIT A MINUTE!!!! You mean all these hundreds of carbs I've actually rebuilt through the years with AFTERMARKET kits because they had been sitting for 7 years with ethanol fuel in them.....didn't actually need it???












    Just kidding. I agree though, let's figure out where the issue is before just throwing parts at it. I don't know HOW many times I 've seen people suggest on a Seadoo to replace the fuel switch 'they always leak'. I bet I've only replaced 3 or 4 in the last 15 years!
    Yup, having been a troubleshooter and a trainer as a major part of my real job for the past 3 decades.....yeah, I kinda sorta prefer to find and correct root issues versus chasing down and only "fixing" the symptoms.

    Side note: Aftermarket carb kits are too often treated (and blamed) just like SBT motors. People throw an SBT motor in a ski without fixing the issue that blew up the first motor, so the SBT blows up, and all the random strangers on fukbook have a pile on SBT party, lol. Install an aftermarket carb kit in a mikuni without actually knowing how a carb works or how a fuel system works or without even troubleshooting the fuel system...and when sh1t goes wrong, yup, just blame that aftermarket carb kit.

    The masses today are much too simple to be paying attention to critical details, so the demand for over simplified blame is strong and easy to hand out...


    WHEN IN DOUBT, GAS IT!

    Yeah, I'm an @sshole, but I'm not a complete @sshole.

    http://badgerlandjetpilots.com/

    I don't come here to make enemies - only to identify them...

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