2010 Nissan 8hp 4stk idle, throttle, power issues at break-in

I've been undergoing break-in of my new 8hp Nissan tiller S# 038884AX. First two outings went completely normal, kept the throttle low except for a few minutes here and there. No problems first two outings equaling 3 hours on the motor.

Third outing this weekend was not so good so I'm trying to figure out what might be the problem.

This outing my motor started up just great in choke but no matter how long I let it run in choke, when I released the choke the motor would not idle with the tiller in less than halfway past "restart" section and I would almost have to give it a little throttle boost to keep it running. It was hard to restart without just using the choke to get it going. Seemed to choke great, starting quickly. I ran the boat briefly (bout 400lbs. Worth of Jon boat) and it was hard to keep the rpm's low to shift into gear. As the boat moved in gear it seemed to fluctuate power or rpm's slightly in a pattern up and down slightly in whatever throttle range I tried the boat.

I adjusted the brass screw on the throttle arm to give it a slight amount of gas right at the restart section of the throttle (I did not touch the idle screw) and it seemed to work but rpm's seemed high and idling in gear seemed to fast. Plus power still was a little fluctuating. I think I made a mistake messing with that screw now, I realize that was probably not the way to fix it.

On a side note, I have not been running the gas out and it has been exactly 4 weeks. I did add gas and I only use 93 grade.

After I added oil for the first time, it measured almost barely under full, now it almost seems like there is more oil in there and maybe over full.

Another thing I notice is that from 3/4 throttle to full throttle there is no difference and actually seems to get more power back at 3/4. This has been briefly done while I'm still in break-in hours.

I never cleaned the gas tank before using it and disconnect the line from the motor between uses.

Any suggestions on what might have happened would be greatly appreciated. I bought this motor online and would like to learn to fix it myself if possible.

warranty--------------Find your nearest dealer.

I could obviously do that...

I was hoping to get a professional opinion on what may have caused this, or see if this is quite common from a forum of people who may know more than just "take it to a shop"

If there is too much oil, could that be the cause? It was fine first 2 outings, but like I said, it seems like there is more oil in there when I check the stick now.

High oil level ----------often caused by a failed fuel pump diaphragm.-------Also caused by improper float valve adjustment or leaking float valve.

It sounds like your motor is not getting enough fuel, especially at low speeds.

"On a side note, I have not been running the gas out and it has been exactly 4 weeks."
That's probably the cause of the problem. You need to run the carb out every day you use the motor. The reason is that today's fuels are not anything like what we used to get, and as they evaporate from in the carb over the course of several days, they leave varnish deposits behind. The MFS8 and 9.8 run extremely lean, especially at low speeds, to satisfy the EPA. That is accomplished by having very small, very high-velocity passages in the carb, especially the low-speed circuit. So varnish deposits that would not have affected the old pre-mix 2-strokes very much will radically hurt the new 4-strokes' low-speed operation. If that's the problem, and it is in over 90% of these cases, the carb needs to be cleaned out. The standard fix for this in our shop is to pull the carb, completely disassemble it, and give it a 4-hour bath in carb dip, such as Tyme. (Check with your local auto parts house for the dip, available in 1-gallon cans). This is not covered by warranty, since the problem was caused by operator error and poor fuel. OR... You might get lucky, and if you run a "super dose", say 10 times the recommended dosage, of injector cleaner, such as Chevron Techron in the fuel, it may dissolve some of the deposits, maybe enough so that it runs correctly again. I haven't had much luck with that technique, though. YMMV.

"I only use 93 grade"
That's unnecessary, but OK. These motors run fine on 87 octane. That said, some of the "premium" fuels have more stabilizing and/or detergent additives added at the refinery, so it shouldn't hurt.

"I adjusted the brass screw on the throttle arm to give it a slight amount of gas right at the restart section of the throttle (I did not touch the idle screw)"
Aside from the drain screw on the starboard lower part of the carb (uses an 8mm socket), there is only one user-adjustable screw on these carbs, and that is the idle stop screw on the throttle arm. Sounds like you have turned up the idle stop screw some, partially masking the problem. The "pilot" screw should be buried behind an EPA-mandated plug in the carb. If that plug was drilled out by the dealer, the dealer is in violation of Federal law, and in any event, the pilot adjustment is made with precision electronic instruments at the factory, should be correct for the life of the motor, and if tampered with, is extremely difficult to re-adjust correctly in the field -- requires a very good shop tach and a well-trained ear. After the carb has been properly cleaned, you should be able to set the idle stop screw back to the correct idle RPM, so no harm done yet. However... If you slam into gear with the motor running too fast, you risk damaging the driveshaft, LU, and prop hub.

"After I added oil for the first time, it measured almost barely under full, now it almost seems like there is more oil in there and maybe over full."
All 4-stroke outboards "make oil" due to combustion byproducts, and cool oil temperatures. I would be surprised if you have a defective fuel pump diaphragm, as that is very rare on these models. The "making oil" syndrome is more pronounced during break-in, the colder it is, and the slower the motor is run. When we set up and rig these motors, we fill the crankcase to half way on the dipstick, knowing that they will have an increase in oil level by the first oil change. After break-in, we fill these motors to almost barely full, but never above the full mark. If the oil is overfilled at all, even a drop, the crankcase pressure will be high, and can result in a lot of problems, sometimes even damage to the motor. You can drain some oil out to get the level down where it belongs, or you can change it. BTW, Be sure to change the oil at 10 hours, as it will have accumulated combustion byproducts, as well as possibly some bearing material from the break-in process. Since the MFS8/9.8 does not have an oil filter, the only way to get rid of wear particles in the oil is by changing it. Always confirm oil level by using the dipstick, as some oil always remains in the crankcase, even after draining it, and you don't want the oil overfilled ever.

"Another thing I notice is that from 3/4 throttle to full throttle there is no difference and actually seems to get more power back at 3/4."
You may be over-revving. The MFS8 has a rev-limiting ESG (electronic speed governor), that is intended to prevent you from exceeding the recommended max WOT RPMs. It engages at about 6250 RPMs. Your prop may need more pitch to slow the motor down a little on that light boat. The only way to know for certain is to put a tach on it and see what your WOT RPM reads. You never want to exceed about 5500-6000 RPM on this motor, or you risk severe motor damage.

"I never cleaned the gas tank before using it and disconnect the line from the motor between uses."
If at all possible, we always install a water-separating 10-micron fuel filter, Part # 9999937851, $30.18, close to the motor. That removes virtually all tank sediment, and catches the vast majority of any water that accumulates (which is worse due to the use of E-10 fuels). At the end of the day, disconnect the fuel line at the motor and run until the motor stalls. That way you don't get the carb varnishing, and if the motor gets into the hot sun, the fuel won't expand and flood into the crankcase, diluting the oil.

"I bought this motor online and would like to learn to fix it myself if possible."
While many online dealers often have good prices, most bricks-and-mortar dealers will match their prices (our dealership does). "Real" dealers also typically offer a lot of valuable free advice about how to do routine service yourself. About half of my customers get parts and supplies from me, and do their own service. Many, but not all, online dealers just grab your money, and then it's "bon voyage, Titanic!" as they laugh all the way to the bank. They know you aren't going to drop by their office for any advice, support or service. The best tool you can have in your arsenal, in addition to basic skills and tools, is the Factory service manual, Part # 003-21056-0, $26.52, available from any dealer (including me).

Thanks for the detailed analysis, that's why I tried to give so many details.

Paul, do you have some company info I can receive?

thanks!

You can call my office 716-877-8221, email [email protected], or website http://obersheimersails.com. Also check out http://www.nissanmarine.com.
Tohatsu America (the North American distributor for Nissan Marine) is near Dallas; you can always call them at 214-420-6440 as well.

I know this is an old thread, but I own a Nissan 8hp, 4 stroke that had similar symptoms when I first bought it, new.

In short, it is the idle jet that clogs too, too, too, way too, easily! Then the motor only runs on the high speed jet, and is annoying in no-wake zones trying to make it back to the dock.

I absolutely have to drain the carburetor, and gas lines after every use. Otherwise, the fuel gets more like tea, and then milky, as moisture gets into it. (There is an air vent in the top of the carburetor.) How many days that takes to happen will be unique to your area, but I personally do not go more than three days without running the motor or draining the carb.

I am now extremely practiced at dismantling the carburetor and clearing the middle of the idle jet with mono filament fishing line. (It is a tiny hole that I can barely see light through.) Then, it idles just like new.

Sometimes I dip the float bowl into carburetor dip to get rid of the crusty oxide that forms in the float bowl.

Only by emptying the gas out of the gas lines and carburetor, can I have dependable, and predictable, idling. And, I only use Premium Chevron gas, with no ethanol.

Reservoir Dog said:

I've been undergoing break-in of my new 8hp Nissan tiller S# 038884AX. First two outings went completely normal, kept the throttle low except for a few minutes here and there. No problems first two outings equaling 3 hours on the motor.

Third outing this weekend was not so good so I'm trying to figure out what might be the problem.

This outing my motor started up just great in choke but no matter how long I let it run in choke, when I released the choke the motor would not idle with the tiller in less than halfway past "restart" section and I would almost have to give it a little throttle boost to keep it running. It was hard to restart without just using the choke to get it going. Seemed to choke great, starting quickly. I ran the boat briefly (bout 400lbs. Worth of Jon boat) and it was hard to keep the rpm's low to shift into gear. As the boat moved in gear it seemed to fluctuate power or rpm's slightly in a pattern up and down slightly in whatever throttle range I tried the boat.

I adjusted the brass screw on the throttle arm to give it a slight amount of gas right at the restart section of the throttle (I did not touch the idle screw) and it seemed to work but rpm's seemed high and idling in gear seemed to fast. Plus power still was a little fluctuating. I think I made a mistake messing with that screw now, I realize that was probably not the way to fix it.

On a side note, I have not been running the gas out and it has been exactly 4 weeks. I did add gas and I only use 93 grade.

After I added oil for the first time, it measured almost barely under full, now it almost seems like there is more oil in there and maybe over full.

Another thing I notice is that from 3/4 throttle to full throttle there is no difference and actually seems to get more power back at 3/4. This has been briefly done while I'm still in break-in hours.

I never cleaned the gas tank before using it and disconnect the line from the motor between uses.

Any suggestions on what might have happened would be greatly appreciated. I bought this motor online and would like to learn to fix it myself if possible.

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