Nissan 9.8 cuts off under 50% throttle

I have a 1998 Nissan NS9.8B 2-stroke motor. The end of last year, it had sat in the marina on my sailboat for just over 3 months and unfortunately I forgot to run it dry. Then I started it up when I went to my boat, started and ran fine while at dock. I took it out and cranked throttle to full and it quit a couple min after running. It would not start back up so I kept it off while I sailed. After a few hours of sailing, I went to start and it started up fine, however it would not stay running unless it was a more then 50% throttle. As soon as I dropped it down to below 50% it would sputter and die. I took carb off and of course found dirt in it, I spray cleaned it all out and also the fuel filter. It started back up and it ran rough but seemed to be normal after about 5 min. It ran me back to the marina without a problem. A week after I came back to the boat with some more carb spray and took the carb off again and spayed it down all over and more thoroughly then what I had done while I was out in the lake in hopes to make sure I don't have anymore problems. Now here is what I done different this time... I also sprayed down into the Inlet Manifold throat where the carb attaches to the motor. Is this something that is not supposed to be done? Reason I ask is because when I done this, that is when it sounded like it was gargling and if I throttle too low it sputtered and quit. So I started it up again and while running sprayed more carb cleaner into the air silencer and when I did this it would seem to start running at its proper idle but seconds later it would start to run low again. I sprayed into silencer again, same results. I took the boat out then to run it wide open, I did this but it would still die when ran under 50% throttle and it also seemed to lack power. I also tried marine fuel tank treatment and refilled tank with fresh 91 octane, still same results. I replaced the fuel filter as well. Another thing, say if I am running it a about 60% throttle, I can tell it is running too low RPM for that amount of throttle and off and on it will run really high to its normal RPM and then slow back down to a struggling 60%. Someone I know suggested to get a carb kit and put that on. I just want some more opinions of what else I can do and I can only hope someone here maybe able to help. The shops over here are booked up and I hate to be without my motor for 1 to 2 months.

Change sparkplugs.----------Check and see if you have strong spark on both cylinders !

Oh I forgot to mention I did change the spark plugs

You did NOT clean the carb. You made it shiny. The low-speed passages are still varnished. the carb needs to be completely disassembled, including jets and emulsion tube nozzle, and submerged in real carb dip for 4 hours at room temp., then all bits blown clean with generic carb spray, then carefully reassembled (paying close attention to float height).

You must have been correct because I tried exactly what you said there above and it worked. Thanks for the help and pointing me in the right direction!

Unfortunately my problem keeps returning. At first I soaked my carb in carb dip for just over 6 hours. It ran fine at home, just after putting it back on my boat, same problem returned. I took carb off and spray cleaned with carb spray numerous times and problem was still there. So I brought the motor home again, only this time I soaked in the carb dip overnight for over 12 hours. Put everything back together and did notice that the screw which holds the float valve in was not completely tight so I tightened it up. Then I started the motor up and it ran fine. I turned it off, restarted up with one pull and a half pull at that and cranked right on up. I done this multiple times and it was running like a champ. I tried again the next day, started, put in gear, forward reverse, idle ran like a champ. Tried again the third day, still ran like a champ. Today which was a week after since i started it, I went to the marina to put the motor back on my boat, took a bit to start up maybe because I ran it dry on fuel last I tried it. Anyway it started and ran just great. I left the dock and about 200ft down, just as I went past half throttle, it killed as if someone pulled the switch. Then I had to struggle to restart. I got it restarted then it killed again and after this it would start but difficult and if it did start it would run for a few seconds and killed. I have taken this carb off at least 12 times, complete cleaning on it twice, I also replaced the spark plugs, fuel filter on motor, fuel line on gas tank too. People I am out of ideas other then getting a carb kit but I just don't know what that will do for me. Everything that comes with the kit, on my motor it appears to be ok. What has me confused is the whole floater valve assembly. I don't know much if anything about this. Could there be a problem with this? The valve moves up and down as the floater goes up and down but very little. I don't see any way to adjust this valve, is there a way to do it if that could be the problem? I am beginning to wonder if something is going on with this motor during transport to boat. I mean it runs fine at the house, the moment it goes back on the boat, here is the same problem again. I transport it laying in the bed on my truck but on the outboard dolly. I had it laying flat the time I brought it back from the outboard shop a year ago and never had a problems so I just don't know what is going on here. It seems like dirt is getting in it from somewhere I just don't know where and when I take it apart it usually appears clean so I don't even know about this. Hope someone can help, July 4th is around the corner and I have a two week sailing cruise planned and need this motor running correctly or this cruise won't happen.

pvanv said:

You did NOT clean the carb. You made it shiny. The low-speed passages are still varnished. the carb needs to be completely disassembled, including jets and emulsion tube nozzle, and submerged in real carb dip for 4 hours at room temp., then all bits blown clean with generic carb spray, then carefully reassembled (paying close attention to float height).

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If you "accidentally" noticed the lock screw for the float valve needle was loose, you did NOT do a complete disassembly (including jets and emulsion tube), and then a thorough reassembly, paying attention to float height. If you used real carb dip for a period of much over 4 hours with the inlet needle installed, it was damaged by the solvents in the dip. If you don't understand how the float operates, you could use a hand. My recommendation at this point is to seek the assistance of someone who understands carbs. It still sounds like the cleaning was not thorough. Spraying the "mechanic in a can" through the carb is not effective enough. It is a good flush for getting the dip off, and it does make the carb shiny, but that's about all.

Hi and thanks for taking the time to answer. I guess I should have clarified more, but I did disassemble everything that I was able to prior to putting it in the dip, don't think I missed anything but it is possible... main jet, slow jet, the main nozzle (is this the emulsion tube you refer to?) all plastic and rubber such as gaskets and everything on the fuel pump, the float, float valve needle. That screw you mention, I did not tighten all the way as I was unsure if it was a way to adjust the height of the float or not as I noticed it would make a difference how I tightened it. The pin was in all the way but the screw seemed loser then I remember putting it so I tightened it all the way. When you mention to pay attention to float height, is there an adjustment I should locate as I never seen any way to adjust it other then that screw? The dip is certainly doing something because everytime I dipped and reassembled everything it would start right up and run fine. There must be something within the carb that the dip is getting that I am not. I think I am going to re dip it all again and order a carb repair kit, new fuel filter (again) and two new jets. If none of this works, I will have to either shop it or just replace the entire carb.

The float pin screw isn't an adjustment. Confirm float height, and if wrong, either replace the float or remove it and re-shape the arm on it... that may not be within everyone's skill set.

Flush out the entire fuel system. You may be re-contaminating the carb.

I ordered a carb repair kit and I have re dipped the carb again, it's been in the dip for over a day now. Once I get the kit I will replace the float (and see if I can confirm the proper height) and everything else that is there and hope it works. Even if it does work, next week I will still replace the jets and fuel filter (once again) to make sure I don't have anymore problems. I will post on here if it does work out, maybe it will help others. Thank you very much, I appreciate your help.

If you are using real carb dip, you will etch the aluminum of the carb beyond repair if you soak it for much more than 4 hours. If the jets are clean, there is no reason whatsoever to replace them. If the fuel filter is not damaged, and is not plugged (or otherwise contaminated), there is no reason to replace it.

Oh no... hope that won't happen as I had it in the dip for close to 48 hrs. I took it out this morning and it looks normal. What I am using is Gunk Carburetor[FONT=arial, sans-serif][/FONT]Parts Cleaner it's a dip that I bought at the auto store which came in a large can with a basket to submerge the carb and parts in, actually on the can it says to soak for 20 min. First time I soaked it for 6 hours, next time overnight and now like I said it has been a couple days. I had no idea it could harm it if soaked too long. Well I hope it all works out for me in the end.

Got the carb kit in today, just finished putting it all on... a new float assembly, new gaskets for both carb and fuel pump. Was definitely a difference between the old float and new, that valve was hardly moving on the old float, the new one I noticed has a lot more movement. I never did any adjustment on the new one, hopefully it don't require it but if it does the only way to adjust it is by bending the holder that the float valve clip is attached to and I think I can do that if necessary. It's just a matter of finding what the proper height is, no line on bowl. Maybe I can find a diagram online somewhere. At last, I started it up and motor is running fine now but I only wonder if it will stay that way and time will tell.

The float height is correct when the float is parallel to the bowl when the carb body is inverted.

Thanks for clarifying the float height, all setup correct now and I really think the problems are solved this time.

This motor, I tell you this motor! I don't know what to do with it anymore, problem returns everytime after about 4 days of sitting. I have read all over the entire internet and everything that people suggested to do I have done plus more. Here is what I had done... Dipped carb at least 6 times, spray cleaned it, put it back on. Runs fine, problem returns after a few days. Rinse and repeat, same story. So I figured tank must be dirty since carb is getting recontaminated over and over. Has to be getting dirt from somewhere, well there was some particles in my tank. I flushed entire fuel system, fuel tank, all lines, everything and bought new fuel line and bulb. I even put ethanol free gas in this time and only that. I bought carb kit, fuel filter, low speed jet and spark plugs and installed then. After I had done all of this I had dipped the carb once again before I would try to run. Then when this was all done I finally attempted to run it, fully cleaned fuel system, all hoses and lines cleaned, carb dipped and cleaned, new parts all over. Starts and runs ok, a tad bit hard to start but it started. I killed it and started again, ran fine. Then after sitting for about 4 days, what do I know here comes the same problem, poor idle and cutting off under 50%! I figured nothing else I can do to this motor other then perhaps run it since it has been sitting. Well today I put it back on my boat, poor idle and cutting off at low speeds. I said the heck with it I try it anyway. Well about 1/4 mile away from my slip I rev up to past 75% and what do you know, motor cuts off as if I pulled the kill switch. Then next thing I know I was heading straight towards two big yachts without power or control and yes I was alone, this unreliable motor is going to get me in trouble one day. I do fear I am going to find myself in a very bad situation if I continue to attempt to use this motor.

I question if a certified mechanic can even fix this thing at this point. If anyone here thinks they can fix it and want it, I will gladly sell it for $700 which is cheap because I bought all these new parts. It has been a complete nightmare for me and it is not worth it for me to bring it to a service center to pay over $500 at least for them to fix. Back in early last year, I spent well over $900 on this motor at a mechanic shop (have all papers too) just to get corrosion cleaned out and a few other things such as it was smoking alot. Bad taste in my mouth from the mechanics, it took over a month to get it back and I felt robbed since that was half the price of a new outboard. They said about $150 and somehow that number worked its way to $400, then $750, then a whopping $900+. They kept calling me saying they needed to do this and do that. I am a certified PC Technician and it is just as easy for me to take advantage of people but I don't do that (although I know others who so just that) and wish auto or outboard mechanics would not do it either. We have a hard enough time dealing with ethanol tearing up our expensive outboards. If I knew an outboard shop around here that was trust worthy I would have gladly brought them my motor a long time ago and once or twice a year for service every year.

This said, I think I am going to sell this motor as is for little or nothing to someone who knows they can fix it. I spent over $1100 on this thing in just a year so I spent enough time (nearly 6 months) and money on this thing, time for a new outboard as I don't want to be paying for a tow in or end up on the rocks of a shore somewhere with my beautiful boat getting torn up. The nerve of me to argue with the guys regarding yanmar inboard diesels as being real engines. I respond by saying oh the outboard can easily be taken off and worked on or replaced. Good thing because I lost count how many times I had to take off this motor to work on it. That must be why they are so easy to take off, because it is like caring for a baby or a dog, needs constant attention. I guess the yanmar guys just sit back and laugh as my outboard motor dies and I crank and crank my arms off while on a collision course with other boats or a dock or shore. Wow and I am a sailboater, if I was a powerboater I would be ready to pull my hair out, those poor guys no wonder their boats sit all the time on trailers. That's because their motors are probably not running half the time. My friend has an 18ft Chaparral with a 140hp evinrude outboard. We went to take it out, bought all the drinks food and everything. Launched the boat and low and behold, it would not start, we cranked, cranked and cranked and finally had to force it back on the trailer and take it straight home. Was not fun nor do I remember any fun out of my boat in months, money and work is all it's been lately.

Sorry for all the ranting but this has been one of the most frustrating things I have ever experienced. Paul if you happen to have any ideas I would really appreciate it. I just don't know what else to do with this thing and I am now scared of it on my boat for reasons I mentioned above. Cuts off at anything lower then 50% throttle and now cuts out at higher throttle such as over 85%. If I want to kill it, no problem just turn it down to lowest speed no need to pull the switch. I done this twice to dock my boat today. I am just about ready to raise the white flag here at any day now and surrender and just sell this motor.

I understand your frustrations very well. I distinctly recall an afternoon boat parade at our club, back about 10 years ago... and threatening to jettison an old outboard (year, make and model withheld to protect the guilty) as it had become unreliable. Yes, that motor has since been replaced (upgraded... to a more modern and convenient setup, actually), but not before I rolled up my sleeves and got it to be reliable... all the while chanting "You have to be smarter than your equipment, ya know!"

The thing is... we often feel that we have "all our bases covered" with our mechanical skills, and that we are "all set to go" without any problems. That may or may not be true. Motors are mere machines, and they operate by well-defined physical principles. When they malfunction, we somehow feel chided... but the truth is... motors being mere machines, we have probably overlooked SOMETHING. There are 3 primary areas that must work in concert for your motor to run reliably: fuel system, electrical system, and mechanical system. Thus far, as I read through all this, it seems that you have spent a lot of time and effort on the fuel system. If you are 100% confident that you have done everything possible in that area, and want a reliable motor, it's time to look at the other two systems. OTOH, there are lots of people who covet those 9.8 2-strokes for their light weight and brisk acceleration, and I'm sure you wouldn't have any problem finding a buyer at a reasonable price.

You are now faced with a choice: repair or replace. Buying a replacement is easy; just write a check. Repair, if you want co continue in DIY mode, especially at this point, will demand that you go through the Factory service manual until you understand all 3 systems and how they need to work in concert... then diagnosing each system, ad nauseum, until you have found the little gremlin culprit. That will require the Factory service manual, decent tools, a comprehensive understanding of what is and isn't working properly (and under what conditions you are having problems), patience, and most importantly, a clear head so that you can proceed with a logical diagnosis. Only after you have diagnosed the problem can it actually be fixed.

Keep in mind that some motors will have more than one issue. Also... you should understand that intermittent problems are the most difficult to diagnose, because you can test a system, find it OK at that moment, and be fooled. Some issues can only be caught when the problem is actually happening... and some issues may really fool you, for example, I had one oldie that appeared to have a thermally-related ignition problem, when in fact the problem was fuel-oil-ratio related. Patience and diagnosis won the day.

Thanks for the advice Paul. I supposed it would not have been so bad if I did not dump nearly 1000 on the motor for service just a little over a year ago. Actually the top end problem I was having is gone, it will now run good at WOT. I think the motor has basically been flooding itself. I could not get it started back up when I left the marina Saturday. When I kill the motor, I have to start it as if it is flooded so I open throttle and pull, wait a few seconds and pull and eventually it starts but when I last left it did not start. I decided I am going to bring it in to the shop. If it is going to be too costly to fix I will probably just sell it (to someone who can fix) and get a new 9.9 Nissan or Tohatsu. Not sure what the difference is between the two though.

If you are flooding (too rich) it's probable that the float valve isn't closing completely, or something in the carb isn't assembled correctly.
You cannot buy a new 9.9 Tohatsu/Nissan. They were phased out years ago, in favor of the 9.8. The Nissan and Tohatsu are identical except for the decals.

Did you ever get any resolution on this?

sailguy40 said:

Thanks for the advice Paul. I supposed it would not have been so bad if I did not dump nearly 1000 on the motor for service just a little over a year ago. Actually the top end problem I was having is gone, it will now run good at WOT. I think the motor has basically been flooding itself. I could not get it started back up when I left the marina Saturday. When I kill the motor, I have to start it as if it is flooded so I open throttle and pull, wait a few seconds and pull and eventually it starts but when I last left it did not start. I decided I am going to bring it in to the shop. If it is going to be too costly to fix I will probably just sell it (to someone who can fix) and get a new 9.9 Nissan or Tohatsu. Not sure what the difference is between the two though.

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Hi, no I sure did not. I had recently sold this motor but at the time I sold it, would you know it seemed like it was running fine? I told the guy who had bought it that it was having problems and when he came by to buy it, it fired right on up and was running as normal. Maybe my loss but I did not want to take anymore chances, last time I used it I had to get towed in. So I guess this ends the case of the motor which would run fine at home but not on the boat. I am actually selling my boat now only because I am going into a bigger one. If I were to keep my boat, I would put a new Tohastsu or Nissan 6hp long shaft 4-stroke on it. These seem to be the perfect motor for my boat as far as size, weight, and power.

No doubt he got a good deal. Likewise the MFS6 will work well for you!