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Nissan NS5B questions

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Gshell422

New member
I recently got a new-to-me Nissan NS5B. It looks to be in excellent shape, but I have a few questions. It has been in storage for several years.
  1. It has a flush port hose attachment. When operating in water, there is a steady stream of water coming out of the hose attachment. Water is also streaming from the pee port. Is this normal operation, or should it be closed off when not being used for flushing?
  2. The engine starts fine and easily. Runs great and has full throttle range when on a stand with the flush attachment. Water seem to come out several places when hose is attached. Pee port is strong, as well as water through the propeller, and two small holes forward and above the cavitation plate. No water is coming out of the 2 small exhaust ports above the propeller. Engine appears to reach full RPM when flushing on a stand.
  3. However, when installed on an inflatable dinghy, it seems to bog down slightly at full throttle and not reach full RPM. Propeller is clean. What might be the cause of this issue? I have read articles that the water impeller might cause this issue, but considering water seems to be coming out strongly from the pee hole as well as the flush port hose, it seems that the water pump is working fine. I've also checked the fuel filter at the internal tank outlet and it seems clear. Is there another filter anywhere? Any suggestions as to what might be the issue, or maybe it is just the way the motor works and I am not used to it, but I feel that my old 2.5Hp Tohatsu had more power. At least it was a lot noisier when you opened up the throttle.
Thanks
Gshell422
 
I was told by a service technician that the engine has an overheat protection system that limits the motor power if it is getting hot and that the water flow from the flush port attachment could be making the engine hot enough to limit it's power. I have added a plug to the flush port attachment and will attempt another sea trial and see if it makes a difference. However I don't understand how this overheat protection works. I can not find any heat sensor on any of the engine diagrams from Tohatsu. Is there one and where would it be located? I would think any type of "heat protection" would require a temperature sensor and some sort of logic system to function.
 
Thank you. Another dead end path.

The plot thickens. Today I cleaned the carburetor main jet. Not particularly dirty but I did it anyway. Another sea trial on the inflatable and no change. Runs great but does not seem to reach full RPM. In yesterday's sea trial, it did seem to momentarily rev up for about 2 seconds before bogging down again, so I thought the carburetor might be the issue. Today, after the first run, I decided to pop out the grommet on the side of the drive shaft housing. BOOM! suddenly we had RPM and power at full throttle. The bow of the dinghy came up out of the water and we were moving. Based on another report I had read on the internet, that could be a clogged exhaust system. The previous report had sea shells clogging the exhaust behind the propeller. I pulled the propeller and it looks clean as a whistle in there. When I put the garden hose into the hole where the grommet was, all of the water streamed easily out around the propeller shaft. So what could be causing an exhaust restriction when the motor is in the water? Also of note is that the motor starts on the first pull every time when out of the water on a stand. However, when in the water it generally takes 3 or 4 pulls if the engine is cold. That also makes me think there is more restriction to the exhaust when in the water making it a little harder to start. Any ideas as to what I should do next?

BTW, compression is about 110 psi.
 
Another observation. When runniing in neutral, the prop rotates moderately in the reverse direction. I don't know whether that is a normal operating thing or an indication that something is not right.
 
Since the motor has full power when the grommet on the side of the drive shaft housing is removed allowing the exhaust gas to exit above the water line, I dropped the lower unit to inspect the interior for possible blockage. All was as clean as a brand new motor. I did some more sea trial testing. With the grommet removed I get about 10% more boat speed than with the grommet in place blocking the above water exhaust exit. I can't imagine a 10% loss of power simply due to the force needed to exit the exhaust gas below the water line versus below the water line. As I removed the cowling and the baffle plate at the carburetor input, the motor seemed to run some better but not as good as good as it does with the grommet removed, so I am suspecting an improper fuel/air mixture, or possibly a bad float making it run lean or rich at full throttle. Does anyone know which of the screws on the carburetor controls the fuel/air mixture and which controls the idle speed? I see that I can purchase an entire new carburetor on Amazon for around $40 to $50. That may be a good option, but I may need to know how to adjust it if needed. Thanks for any advice.
 
I am starting to suspect that this is essentially how this engine operates. Part of my confusion is what appears to be a non-linear throttle. The throttle rotates through roughly 1/4 turn of the handle. From minimum throttle to what is marked as the starting position is about 1/8 of a turn. I got a digital tachometer and took some RPM readings in both a barrel and using the flush port. Using the flush port, idle RPM was around 1400 RPM and the "start" position was around 6400 RPM. Full throttle was over 7200 RPM.

In a barrel in neutral, RPMs were slightly lower but not much: 1100, 5500, and 6900

In a barrel in forward gear RPM dropped significantly: 840, 3800, and 4900. All seem good as max RPM is supposed to be 4500 to 5500. However, the first 1/8th of a turn increased RPM by 3,000 and the second 1/8th of a turn increased by only 1100 RPM. So the throttle rotation is non-linear which I was not accustomed to having previsouly used a Tohatsu 2.5 with a throttle lever on the front of the engine.

When I removed the shift linkage access grommet on the side of the drive shaft, the exhaust gas could exit above the water line and RPM values increased by about 10% due to less exhaust restriction: 1040, 4500, and 5300. That more or less confirms the 10% speed gain I measured when powering the dinghy without the access grommet.

I am very surprised there is a 10% degradation of RPM and power due solely to the under water exhaust through the propeller. Although the full throttle RPM is within specification, are there any things that I could do to increase the top end RPM? I'd like to have the max in-gear RPM readings with the grommet in place closer to the 5500 rather than the 4500. I suppose if I ever needed to enter a dinghy race, I could always pop the access grommet and go into "hot rod mode".
 
HELP!
I finally did a complete carburetor soak and rebuild. There was no change. I took the digital tach down for another sea trial. Max RPM while in neutral was around 6500 RPM, but when put in gear tied to a dock, max RPM never got above 3500. When operating in a barrel at my garage, max RMP in gear was around 4500-5000 RPM. When operating via the fresh water flush port, max RPM in gear was 6600 RPM. I have no idea what is wrong with this engine. It seems as if it starts and runs fine as long as there is no load on the prop (I assume that the barrel operation winds up creating a void around the prop reducing the load).

The "pilot screw" (Item 1-17 on the parts list) which I believe adjusts the fuel/air mixture seems to have no effect on full RPM. It was initially backed out 1-1/4 turns. While running on the dinghy tied to the dock, in gear at full throttle, I adjusted it through its entire range from fully closed to backed out over 2 turns and there was no noticable change in RPM.

Any suggestions as to what I could test or change next?
 
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