Just put a rebuilt 2 stroke and brand new Mikuni SBN carburetor in my jet boat. Got her running no problem the first couple times, now it seems I can't get any gas into the carburetor from the primer bulb. The primer gets very firm and I am completely unable to pump it. If I remove the gas line from the carburetor inlet the gas flows beautifully, so no problems with the lines ( which are also brand new ) or the primer.

If I spray some starting fluid into the carburetor, she will start AND run perfectly fine with the gas line after, the only problem is the initial startup. ( I know starting fluid is not good for 2 stroke so I really want to get the carburetor issue corrected )

This carburetor only has 5-10 hours on it and I really hope I don't need to rebuild the damn thing.

I read thru the carburetor manual ( here ) and found the section on adjustments, but I'm not sure which ( if any ) will correct the problem I am experiencing.

Any idea's?

All boat detail can be found here: 1992 Boston Whaler Rage


I took the boat out yesterday and heard a large pop once I opened the throttle for the first time ( sounded normal while idling directly after the start ). I believe maybe one of the carburetor pins was stuck as a result of me trying to squeeze the primer bulb really really hard.

  • Make and model of jet boat would be nice.
    – Moab
    Commented Jul 29, 2016 at 21:51
  • Do you mean the bulb that is on the line between the fuel tank and the engine (like on a blood pressure cuff) or is this a primer mechanism on the carburetor itself? Also, will the engine start without priming it (without using starting fluid)?
    – dlu
    Commented Jul 30, 2016 at 15:41
  • @dlu, yes the primer bulb is in line between the gas tank and the carburetor. The engine will not start without the starting fluid
    – Calvin
    Commented Jul 31, 2016 at 11:35
  • Is there s choke? It's been a very long time since I've used one of those but I didn't think that the bulb was for starting.
    – dlu
    Commented Jul 31, 2016 at 13:52
  • @dlu, yes there is a choke.
    – Calvin
    Commented Jul 31, 2016 at 16:15

2 Answers 2


I'm not a watercraft Mikuni guy, but I've dealt with a few two-stroke carbs that work in this manner.

There's a few possibilities:

  1. The internal filter screen is clogged, but I'm discounting this as it's just rebuilt, and I doubt that would cause the primer bulb effect anyway.

  2. The fuel return is dirty or blocked. Verify with it running (after an ether bunny start) that fuel is returning from the return port.

  3. There's something out of order with the pump plate, either the internal poppet valves, or the pulse metering system. Are you certain the primer bulb isn't overpressurizing the pulse area? I probably dont't have the right terms here... I'm referring to an issue somewhere between #20 and #21 on the following diagram (especially #21 with poppet "check" valves):

Mikuni SBN Exploded Diagram

Here's a cross-ection that I found, which gives a clear view of the operation. If the fuel return is working properly, it seems nearly impossible to pressurize this area or "overprime" it, as excess fuel would simply return. Unless, of course the pulse area above the inlet is causing a problem with the valves. It might even be possible that the rebuild installed the pulse check valves upsidedown, the wrong direction, etc. Even more likely there's too much restriction in the fuel return "restiction". Here's the diagram which to me at least, gives a good clarification of the operation (from the Mikuni SBN technical manual):

Mikuni SBN Cross-Section

I hope that helps. Best troubleshooting I can do from my laptop on my first gin and tonic. Check the return flow first, and verify that flows. Easy. Luck to you.

  • Thanks for the detailed response! I don't know if it would make a difference to your answer, but the carburetor isn't rebuilt, it's brand new with 5 - 10 hours of use. Because it's brand new, I assume it was assembled correctly and this would pretty much leave us a possibility 2, but hey, you never know. I will check to see if the return is working properly today, should fuel come from the return line when the boat is not running and I am pumping the primer? I assume it will not due to the fact that the primer gets hard as a rock and I am unable to squeeze it after 1-2 pumps.
    – Calvin
    Commented Aug 5, 2016 at 13:34
  • 1
    @Calvin Another thought (no insult here) ... Are you sure you didn't accidentally swap the fuel inlet and return lines? Anyway, from the second diagram, it looks like fuel should flow past the pulse pump assembly, and whatever doesn't get "used" down the filter screen bypasses to the return. This is the way it must work while running, so I don't see how "primer bulb" motivated fuel is any different than "pulse diaphragm" motivated fuel -- either way excess fuel bypasses through the same restriction (which is maybe blocked) and out the return port.
    – SteveRacer
    Commented Aug 5, 2016 at 20:09
  • 2
    Holy Crap, You were right!! The carburetor even had arrows on it show the inlet and the outlet!! You my friend are a life saver, thank you so much. If you were in the southern NH area, the Gin and tonics would be on me all night.
    – Calvin
    Commented Aug 5, 2016 at 22:09
  • 3
    @Calvin be sure to award Steve the bounty - he deserves it! Internet-armchair diagnosis is a black art :)
    – Zaid
    Commented Aug 6, 2016 at 18:45
  • 1
    @Calvin Thanks... sorry I was away, but yes, it would probably run okay with the lines reversed, because as you can see the from the second diagram fuel would siphon or gravity feed into the internal filter. But the primer bulb would fill everything, and seal the "exhaust" valve of the pulse pump, allowing no more fuel until the engine was started and began consuming it. The pump would never prime properly from the inlet side.
    – SteveRacer
    Commented Aug 8, 2016 at 10:02

There's also the slight possibility that there could've been some old bad gas or gunk in the tank that could've just gotten sucked into the lines or the carb jets. If that is possible in your case you can flush everything with started fluid or brake cleaner and make sure to remove the fuel tank and put about a half gallon of gas in it and shake it well and dump it. Repeat until clean. Good luck with it.


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