98 carburated Nissan 50 sudden loss of RPM / Power

[Normer]

Has anyone experienced a sudden drop of power in the Nissan / Honda 50 horse? I recently bought my x with a low hours nice condition Nissan that ran great for the first 20 hours or so, then one day suddently decided it only wanted to run at no more than 3500 RPM. Was running great one minute then suddenly dropped RPM & power. I changed out the plugs and cleaned the fuel filter (it had a significant amount of particles in the base of the bowl, but the interior screen was clean). It ran fine again for about 5 minutes at about 5200 RPM WOT and about 12 knots with full ballast. Then suddenly dropped again to about 3500 and seemed happy running at that speed. Same problem as before. Checked for pee stream from water pump and oil in the tank.

Wassup with that?!

Any thots much appreciated...

[craiglaforce]

Probably crap in the carbs. Can be intermittant. Might be time to pull the carbs and have them cleaned. (they are pretty easy to pull. )

Also check to make sure there are no air leaks in the fuel hose or fittings.

[Normer]

Thanks for the reply.

I'll clean the carbs.

Is there a "Chilton" type book for the Nissan on how to do that? Or would it be obvious enough when I get in there?

[cuisto]

Not likely but just maybe you need to just open the gas tant vent cap. on my first boat i had an aouto vent so never thought about it. when i took mt new one out for its first spin simmilar results...just needed to allow air flow to the tank..

[Frank C]

If the fuel tank was airlocked, the Nissan couldn't run "happy" at 3500 rpms. An unvented tank will cause "absolute" fuel starvation, stalling the motor. OTOH ... I suppose the vent could be open just enough to support a part-throttle operation?

Partial fuel starvation can happen from a weak fuel pump, the tank pickup tube leaking air, the fuel hose leaking air, a carb base leaking air (maybe even a partly closed tank vent). Partial starvation will cause the motor to run only to the rpm limit of that partial fuel supply ... and it might sound "happy" as long as you hold a partial throttle that matches the limit of fuel. But as soon as you open the throttle beyond that point, the motor will miss and stumble because of too much air.

Just went through it last year on my EFI Suzuki - air leaking into the fuel line. It's not clear that your problem seems simlar ... depends on a more detailed description of what happened when:
"Was running great one minute then suddenly dropped RPM & power."

[Miami]

Do you have an audible buzzer for the overheat alarm?

The Nissan/Tohatsu carb 50s have a problem with faulty overheat sensors. It's been mentioned here and on many other boards.

Mine (bought used) used to do that all the time when I pushed it to wide open throttle for more than 5 minutes...the overheat buzzer would sound, and it would drop to lower rpms until the buzzer stopped.

Some owners have removed the overheat temperature sensor totally.

I modified my overheat sensor so that it would not be as sensitive. Now I can run wide open and not have the darn buzzer come on and drop the engine rpms. No noticeable effects on the engine by reducing the sensitivity of the sensor. And it is still there to protect against real overheating.

I reduced the sensitivity of the sensor by moving it up about 1/8 inch from the bottom of the cavity where it is bolted into the cylinder head. (Putting the air gap in there slows down the heat transfer to the overheat sensor, which is basically a temperature sensor).

[Scott]

Probably crap in the carbs

Specific to the jets. Easiest way to determine is to run engine and check the exhaust individually. If its cooler than the rest that is your suspect cylinder.

If your exhaust is the box type you may not be able to tell that way, if so touch the side of the head. Cool one is still the problem.

Last test. Choke the cylinders individually by putting your hand over each intake 1 at a time. If you cover an intake and there is no change in running it is the problem. If you cover an intake and the motor runs worse it is a good cylinder.

[Night Sailor]

I once fixed a friend's boat that did the same thing. It was simply the clamps holding the throttle cable to the throttle assemlby weren't tight enough. Under full power and vibration it would creep down, closing the throttle a bit, although the cable and throttle didn't move on the operator end. A twist with a screw driver did the trick.

[craiglaforce]

If you do a search, you can find where I described how to pull the carbs on a Tohatsu 50, which I assume is the same or nearly as a Nissan.
Remove the top cowling of course, then there is a bolt or 2 holding the oil tank, remove these and move the oil tank out of the way.
Remove the black plastic intake silencer thingy, then each carb is held on via 2 long bolts, watch the linkages carefully while removing them, so you can put them back properly.

It would be good to have a set of carb gaskets in case any rip, but I got lucky and reused my old gaskets. I had ordered new gaskets, but they did not arrive in time.

Then I put the 3 carbs in a steel bucket and took them to the marina shop, where the helpful young lad unscrewed the bowls, and put them into a carb washing machine for a few hours for a reasonable fee. then just put it all back together.

I have the shop reapir manual, but it was worthless for this task.

[Normer]

Thanks for all the tips -

More details: I've got the NS50D2EPTO which does have the overheat buzzer. Haven't heard it go off at any point. I've run the motor WOT a couple times and each time it runs for about 5-10 minutes just fine then suddenly drops to the 3500 RPM level and down to about 7 knots. Vent on tank is open. I'll try the informal temp check on the individual cylinders and covering the air inlet check. Then I guess it's on to cleaning the carbs...

[Scott]

I was under the impression that it ran at 3500 all the time. If it is running fine then not, does it run fine again after cooling off??

When a motor drops out like that it sounds like yoiu are pulling a vacuum on your fuel supply and starving a carb. Could still be jetting but you should try taking your fuel cap off when it does it and see if it cleans up. Dont do it with a full tank or in rough conditions for fear of fire.

One other thing it could be is a fouled plug.

[LOUIS B HOLUB]

Normer...My 1999 Nissan 50 HP, carb. engine would hit 2500 to 3500 rpm, and it also had low hours last Oct 2005. I had the carbs cleaned, and it runs great now...plenty of rpm and gusto...also, the mechanic advised that storing the engine ought to be level, instead on a slant. He recommends this because of the floats. Im not sure if this is necessary, but Ive followed his advice. I naturally trailer the engine in a locked upward and slant positioned.
Happy Sailing...

[Scott]

Stored motors have less probs if you run them out of fuel before storing. Then it doesnt matter level or tilt

[waternwaves]

Bad check valve or fouled check valve in the squeeze bulb can also initiate these issues, try the other tank.

I have had also had this power loss caused by both fuel restriction and the overtemp control, as well as once not venting the tank cap sufficiently. Most disconcerting.

But on this motor, I have never had any RPM change due to carb fouling. The jets seem to be big enough to handle any sediment present on the fuels I run.

[Frank C]

... It ran fine again for about 5 minutes at about 5200 RPM WOT and about 12 knots with full ballast. Then suddenly dropped again to about 3500 and seemed happy running at that speed.

Thanks for all the tips -

More details: I've got the NS50D2EPTO which does have the overheat buzzer. Haven't heard it go off at any point. I've run the motor WOT a couple times and each time it runs for about 5-10 minutes just fine then suddenly drops to the 3500 RPM level and down to about 7 knots. Vent on tank is open. I'll try the informal temp check on the individual cylinders and covering the air inlet check. Then I guess it's on to cleaning the carbs...

Seems to me that these symptoms are completely unrelated to fuel starvation. There's no way a WOT motor, suddenly fuel-starved, will just drop 1500 rpms and still run happy. No way, no how. Just one opinion, but I think this covers all avenues of fuel starvation, fuel pump, tank, hose and carbs or jets.

A sudden fuel starvation at WOT will cause sudden, severe missing and stumbling. Once the throttle is closed to match the available fuel, it might even-out to "run happy at 3500" .... but that process seems lacking in Norm's amplified problem description (quoted). Anything so sudden as this seems electrical to me - and seems programmed. Does this motor have any advanced engine controls - i.e. electronic ignition, distributor, spark advance - some safe-mode controller?