Nissan NS9.8 B Engine starts, runs, needs choke to keep going then stops.

[Reich965]

Hello, I would appreciate any advice. My NS9.8 B has 300 hours pushing a 5,000 sailboat; it is the remote control variety. The motor bogs and then quits. It acts like the tank is not vented ultimately resulting in the engine shutting itself down. I have dissasembled the carb and soaked it and reassembled including the fuel pump diaphrams. All passages were clear and there was no residue anywhere. The motor still acts like it is starving for gas. When it starts to die, I can keep it going a little longer by pulling the choke on and off, but it always dies. I tried a different hose and bulb combination. Fuel filter changed. The tank is clearly vented. The bulb does lose pressure while the engine is running. Gas is new and oil is mixed 50:1. Plugs look fine. I can't think of anything else to work on. So, I was thinking about buying a new carb ($170.00) thinking maybe something isn't right with the float. Also, I've never understood how those diaphrams are able to pump fuel--maybe I need to get a fuel pump kit although the diaphrams look fine and I have soak them in carb cleaner too. I will appreciate any feedback. Jerry

 

[pvanv]

Re: Nissan NS9.8 B Engine starts, runs, needs choke to keep going then stops.

Yes, it does sound like a fuel starvation issue. You may have a fuel pump problem. Did you did soak the diaphragms in carb cleaner dip? They probably wouldn't survive intact.

FWIW, the fuel pump operates on the principle of alternating pressure/vacuum from the crankcase. That operates a diaphragm which pushes/pulls against the chamber with the check valves. The fuel then is delivered to the carb bowl.

You could verify whether the fuel pump is weak by operating the primer bulb when the motor is getting the near-stalling symptom. Assuming the primer bulb is good, and the only other issue is a weak pump, that should deliver enough fuel to refill the carb.

 

[Reich965]

Re: Nissan NS9.8 B Engine starts, runs, needs choke to keep going then stops.

Thank you Paul, You know I wondered about whether I should have put those diaphrams in the carb cleaner or not. They curled up some and became a little lumpy making them hard to get in place again. I will order the diaphram kit from Internetoutboards.com right now.

Thank you for explaining how the pump works. It makes sense now. I thought the diaphrams might have been some type of semi-permeable membranes allowing fuel to pass through them and that is why I put them in the carb cleaner.

I really like this motor; I added the 5 inch extension kit about five years ago making it a 25 inch shaft with remote control. Wish I could find another 9.8 B as a spare.

Thanks again Paul.

 

[pvanv]

Re: Nissan NS9.8 B Engine starts, runs, needs choke to keep going then stops.

Hope it works out for you. Remember that you can verify the fuel delivery by "helping" the gas to the carb. If reasonable, you can even elevate the gas tank, so gravity assists the gas delivery. It doesn't require much fuel pressure. Remember that the primer bulb is on the "suction" side of the pump, so it will not necessarily stay firm while running, especially if the tank is a bit below the motor.

FWIW, a leaky diaphragm can also dump gas into the pump's suction/pressure line, causing a really rich mixture under some conditions. Leaky check valves in the pump are just as common as a bad diaphragm, and will cause poor fuel delivery.

BTW, as a side note... check out the 4-blade High Thrust props available for the 8 and 9.8. They fit either the 2-stroke or the 4-stroke versions. I have a 5" pitch version on my personal 9.8 4-stroke, pushing my 7,000+ pound sailboat. Nice slow speed control in forward, and it BACKS UP. Something that the regular props don't do well. Your lighter boat would probably work well with the 7" pitch version.