Carb. Rebuild or Adjustment?

Question

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

UPDATE:

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.

Answer 1

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):

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):

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.

Answer 2

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.