Engine won't reach max RPM under load

[Dendiaz]

Hello everyone, I'm hoping someone here will be able to help me out. I have a 1992(I believe) Nissan NS40C that just started giving me some troubles. At idle and anything under about 3200-3400 RPM it runs flawless. Starts right up on electric start, fires right up with half a pull of the manual pull start, runs smooth(for an old 2 cylinder 2 stroke), and will accelerate perfectly to about 3200 RPM or so. But it doesn't want to get over that.

It feels as if it's sputtering or shaking and just maintains that RPM. Now, on a few occasions, I have been able to work the throttle up and down and if I was able to get it to crest over 4000 before bogging it would then take off and run up to 4800 and run great (should be 5200 to 5700 but the prop isn't exactly the right pitch, boat is new to me and this will be fixed soon). I have tested compression, 120ish on both cylinders. Spark appears to be consistent and strong on my tester. I ohmed the exciter and pulsar coils, both within spec. Spark plugs are new, fuel pump and lines are new, carb was thoroughly cleaned. Brand new reed valves installed. I'm kind of at a loss here so I'm reaching out to the community.

On a side note, I also recently adjusted the timing and noticed that it appears to be impossible to get it exactly within the specifications listed in the manual which has me boggled as well. It can get CLOSE, but not perfect and it appears to be simply the design of the linkages and travel distances. Long story short, when the adjustable timing link is adjust long enough to make a 2 ATDC to 25 BTDC swing there is not enough travel distance, it's either smashed up against the idle stop post to the point I'm afraid of damaging the cable or cranking VERY hard on the throttle roller.

But alas, playing with timing, unless I make a MASSIVE change, appears to have no affect on this issue. So to recap, runs perfect below 3400, sputters and shakes at/near that RPM and won't go faster if throttle is simply held in place, and IF I can manage to work the throttle just right to shoot over 4000 it takes off and runs WOT perfectly. Any ideas?

 

[MattFL]

It sounds like a fuel issue. Have you cleaned out the carburetors? Does squeezing the ball while or just before going full throttle make any difference? I suspect a plugged up main jet or stuck float, but also make sure you're getting good fuel flow at the motor and that any under-the-hood fuel filters are clean.

 

[Sea Rider]

For the time advance carb synch need to check the following : remove the remote throttle cable linkage from the lower advancer arm, with finger raise fully the roller upwards till stops and hold it there. Throttle the lower portion of the lever back and forth, is there a light or hard banging of the cam going on against the roller ? Should hear a very slight banging,



if hard release the sttoper screw's nut on which the left side of the advancer arm stops against to lessen the pressure of the cam against the roller, adjust till bangs slightly, readjust the nut once done. Report your findings, next step is to check the time advance setting the timing plate stops at to achieve a perfect carb/time advance synch.

Happy Boating​

 

[ahicks]

Do as you like, but I like checking the easy/most likely stuff first. I'm with Matt above. It's likely starving for fuel beyond a certain point. When that point is reached, it starves, creating issues. Below this point, it runs fine.

If you have an extra fuel tank and line available, start there to eliminate the possibilities. Fuel filters are well know culprits for this kind of thing. Remove it and see if you can blow through it. It should allow you to do that easily. If it seems like there's any restriction, you have your culprit....

 

[Sea Rider]

Let the OP decide that, will need to go that route the sooner or later if wanting his horse to run top and agile. If the tank, the internal mesh filter and the carb's fuel filter are impeccable clean now with a new fuel hose and fuel pump do the following : remove the fuel filter's hose that connects to the carb, place a container, open the tank's air vent, with bulb's arrow pointing upwards prime the bulb firm several times and check if the fuel flow exists strong and unobstructed, will show how healthey the entire fuel system tank to carb is woking and avoid wild fuel troubleshooting prolonged guessings.

If the entire fuel system fuel tank to intake is working spotless, that oldie 40C motor could have already eletrical related problems, exciter coil, CDI, ignition coils heats up and motor misses specially at wot when max power output is needed. Those issues are the hardest to diagnose properly and fast.

Asside still having fuel or electrical related issues need to change the factory delivered prop for same 3 blade alum prop with at least 2 less pitch sizes from installed one for the motor to rev towards max 5700 rpm.

Happy Boating​

 

[racerone]

Timing on these motors generally does not change.-----Adjusting and hoping it will run better may well lead to expensive damage.----Best to do some trouble shooting to find out what is wrong.

 

[Dendiaz]

Ok, so I'll try to reply to a bunch on these at once here. First, I had a freshly cleaned carb on there BUT I did just replace the fuel pump, I suspected it may be possible for there to have been some something in it from the factory that could have gotten into the carb so today I pulled the carb and cleaned it again. I will be able to take it out tomorrow to see if that made any change, one step at a time I suppose.

The only filter I have is the spin on fuel/water separator kind and it's only a few hours old. I will replace it just for kicks if the carb cleaning yields no results but I don't suspect that it's having any issues with that simply because when I'm able to crest over that magic 4000 RPM mark it runs perfectly fine at full throttle indefinitely.

Sea rider, I think I understand your description of adjusting the linkage and I did a similar method that should have produced the same result. I pushed my remote to max and checked to see how hard the cam was pushing against the roller and adjusted the stopper until I could just BARLEY make the roller spin by hand with the cam against it. Then I adjusted the cable length until it pushed against the metal stop in neutral. So now my throttle linkage is pressed firm (but not too much) against the metal stop when in neutral and pressed firm (but still just barely able to spin the roller) against the roller at max. The problem is that at these settings the next step should be to adjust the timing link to achieve 2 ATDC at idle and 25 BTDC at WOT and the cable physically does not have enough travel from stop to stop to accomplish this. It's close, probably about 0 BTDC to 25 BTDC but I can't get it down to 2 ATDC. And then vice versa, if I adjust the idle down to 2 ATDC It only moves enough to achieve about 22-23 BTDC. I can probably take some pictures of it if this is confusing, it's kinda weird to explain in text.

I concur that fuel seems the most likely culprit. And yes, I did try to squeeze the primer ball while it was struggling and it made no change. And like I said, the few times I was able to work it over that small RPM range it took off from there and ran perfect.

So to recap, I cleaned the carb today and will be taking it out tomorrow and see if there is any change. If not I will throw a new fuel filter on it. It that doesn't help I have a spare carb that I cleaned out as well that I will swap out and try. And once this issue gets resolved I will then be looking at changing props to get the proper WOT rpm. And about the timing, I know it typically doesn't just change on its own. The problem is the previous owner liked to "fix" things that he had no idea how to work on and the engine definitely didn't run right when I got it and timing was clearly off so I'm trying to get it set properly. It's running much better with the way I have it now but I'm still curious as to why there isn't enough travel between stops to achieve the factory specified settings.

 

[Sea Rider]

Timing on these mechanical motors factory adjusted don't change whatsoever, unless being factory delivered badly adjusted which has happened several times, an example is the posted 40C motor pic, boaters fiddling with the carb/time advance synch and boaters buying second hand motors in which there's no data about what the previous idiotic owner did as the OP mentioned earlier.

Being the carb adjustment well made, need to synch the time advance : leave the carb open at max. (1) back the ball joint nut some CCW turns, disconnect (2) downwards, manually advance (3 to the right, align 25º mark (4) at middle of both mating surfaces (5) shorten the rod's gap accordingly, push up the ball joint onto pin for the time advance mark to remain set at 25º, tight back nut (1). Will suggest to order and change all the plastic bits if with excessive play.



Have you primed the fuel bulb as suggested and checked if the fuel flow flows constant into the container ? if so, all the fuel filters are doing their homework right. If you guys think the fuel system is the only system that makes a motor to run top the answer is Nope, it's just half of the equation, the other half is the electrical system which most times is Olympically bypassed. Being a 29 year oldie motor don't expect all the electrical components to work spotless, one of them is surely with intermittent issues, the bummer, which one is the culprit ?

Happy Boating​