Nissan 5hp power problem

[Panamadave]

I have a Nissan 5 hp two stroke that has suddenly developed a power problem. Out of gear it will run up the rpms normally, but under load, it runs at about half speed. I have cleaned the fuel tank, the filter in the tank, the fuel line, the carburetor, jets, float bowl and checked the fuel pump. It still will not develop full RPM or power beyond about half speed under load. The motor dates back to the mid 1990s. Any suggestions on what to try next? Thanks.

 

[pvanv]

Re: Nissan 5hp power problem

What is the compression, and what does the plug look like?

 

[Panamadave]

Re: Nissan 5hp power problem

I replaced the spark plug just two days ago. I had the rings, bearings, etc., redone less than 18 months ago. The compression is unchanged since then.

Also, I should note that this is a duplicate of a posting by hopalong, which also happens to be me. I posted my original message but then could not find it so I reposted. Sorry about that. And thanks for coming in with some help.

 

[pvanv]

Re: Nissan 5hp power problem

Assuming you have an NS5B.

No way you had the con rod bearing replaced, unless you replaced the whole crank, as it is pressed, or someone foolish disassembled the crank press fit. Rings, sure, and I suppose the wrist bearing.

How many PSI is the actual compression? What color is the spark plug? If it's sooty, you are too rich. If white, too lean, which is likely the issue. What method did you use to clean the carb? Is the float height correct? Are you running TCW-3 oil at 50:1? Is the cooling system peeing correctly? Running too hot could limit RPM under load.

You should do a full diagnosis, including verifying the fuel delivery (by examining plug color), checking resistance of ignition components, and double-checking compression. Odds are that the carb still needs a proper and thorough cleaning and adjustment.

 

[Panamadave]

Re: Nissan 5hp power problem

Thanks for your help. It is an NS5B. The old sparkplug looked quite clean two days ago. I will have to check the new one. The float is correct if I understand how it should be done, which is that it should be parallel to the bowl edge when the carburetor is turned upside down. (Please correct me if I am wrong.) I run standard two-cycle oil 50-1, right now Mercury brand. Cooling system is peeing nicely. The lack of power occurs even immediately on start-up, when the engine would be its coolest. As for the carb cleaning, I disassembled it, including the jets, sprayed all channels several times with carb cleaner, using the little straw that comes with the can. Then I blasted it with compressed air. It certainly remains a strong possibility possibility that there may be a little piece of dirt in there somewhere that I can't see. I don't have the immediate ability to actually measure the compression right now or the ignition components, so I think I will tackle the carburetor again. I was wondering whether you think this might be a fuel pump issue.

Re the bottom end, it was some sort of bearing. I would have to check my records, which are buried right now, but that is probably not necessary to solve the immediate problem. The work was done in Central America so most of the conversation about it occurred in halting Spanish.

Again, thanks very much for your help.

 

[Sea Rider]

Re: Nissan 5hp power problem

Are you using both interal & external tanks ? Does your external tank has a grey or black hose ? Is problem same with both ?

Happy Boating

 

[Panamadave]

Re: Nissan 5hp power problem

It's an internal tank only.

 

[pvanv]

Re: Nissan 5hp power problem

That model would get enough fuel by gravity, as long as the tank is near full. Fuel pump restriction from a sticky check valve is a possibility, but unlikely. If starving for fuel due to inadequate delivery to the carb, the carb would "run out of gas" at higher RPM's, so that could possibly be your problem. Unfortunately, since the fuel pump is integrated onto the side of the carb, you can't easily check how well it's delivering fuel to the carb. Did you disassemble the fuel pump and carefully inspect the diaphragms and check valves? Probably a good idea to do that while you are doing a proper carb cleaning in real carb dip, instead of just a spray out with the aerosol. If in fact there is significant varnishing in a carb passage, the aerosol just isn't aggressive enough to fully dissolve it.