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15hp bogging down and dieing under load

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Matt04

Member
I am afraid I might have been badly ripped off on a 98 Nissan Ns15d2 motor that I just bought used. I just took the boat out to the lake yesterday and the engine starts up fine, first pull usually, idles OK, maybe a little rough, but when I try to go if I give it gas to fast or just trying to go full throttle it would bog down and die. The problem seemed to get worse through the day. Both Spark plugs looked good at the beginning of the day, by the end the bottom one was wet and pretty dark the top was brown and dry. I've done a compression test on it and it seems good to me 110 and 112. Gone through the fuel lines up to the full pump and they are all clear the filter is clean. Took the carb off and it was very clean inside so I think the previous owner had rebuilt it to try to fix the problem, fuel pump diaphragm looked good as well. Tried starting it up and disconnecting the plug wires one at a time and it wouldn't run with either plug disconnected. I also tried replacing the plugs, and that didn't help. I'm a little stumped about what the problem could be at this point. Does anyone have any ideas about what else I can check?
 
Is the wetness fuel or water? If the spark plug is getting black and wet it might be running very rich. Is the choke operating properly?

Stuart
 
The wetness is definitely fuel and 2 stroke oil, Attached is a picture of the plugs, the right one is the wet one, it is a lot less wet then it was when I took it out, because I wiped it to smell if it was fuel or water. The choke is working correctly as far as I can tell... Going to both stops and staying at its setting. I should also mention that the fuel bulb was staying hard, after a while running I could get half a pump at most and trying to pump while it was running didn't help at all.
 
Typically, your symptoms -- can't take a fast tip-in of throttle -- are due to a too-lean condition.
Likewise, running out/stalling at wot is usually due to insufficient fuel. Is the float height correct? Is the fuel pickup clear?
Even though the carb looks clean, since you do not know that the PO had a Proper cleaning done, that's a good place to start. Occasionally, there is a bit o' crud loose in a passage, that gets drawn under high flow, that then restricts the passage. A good cleaning should help that.
 
I'll pull the carb agian and check it out better. I'm also leaning towards it being a problem with not getting enough fuel, I'm new to outboards but I have worked on a lot of engines. I just got worried when I saw that he had cleaned the carb, because it seems like he already checked the fuel system and found out that the problem is more serious. I pulled out the main jet and that was totally clear, I assumed that the problem would have been with the main jet. I will pull it back apart and see if I can get it cleaner. Do the compression numbers sound good? Seems low to be based on other engines but from googleing it, I guess it's normal for a outboard? I assumed if it starts easily that the compression is good enough. Also could it be a timing issue causing this?
 
The compression readings are a little low, but not catastrophic. They are also even which is good as the gauge may just be out of calibration. If the fuel line connected correctly? Look for a closed vent plug, a poorly connected fitting or a spit in the fuel line. As well as the float adjustment that pvanv mentioned these can cause the symptoms you are experiencing.

Stuart
 
the carbs are defo worth cleaning i recently got a boat with a 50 hp mariner died when throttling open. serviced everything plugs new fuel tank fuel lines and connectors still was not any better took carbs off got a outboard mechanic to sonicly clean them had boat out monday and problems gone seems it doesnt take much to block the carbs
 
Ultrasonic cleaned the carb, got it spotless. Replaced all the fuel lines under the cowl, and ran a concentrated sea foam mixture through it, and it still has the same problem. Any other ideas? Could it be a partially clogged exhaust? I can try to check the timing, but I'm not sure how to do it on this engine.
 
Matt04 said:
Ultrasonic cleaned the carb, got it spotless. Replaced all the fuel lines under the cowl, and ran a concentrated sea foam mixture through it, and it still has the same problem. Any other ideas? Could it be a partially clogged exhaust? I can try to check the timing, but I'm not sure how to do it on this engine.
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sounds like its getting enough fuel if primer bulb staying hard. Might sound silly but is vent in fuel tank open and un restricted? My outboard cut out a few times the other day and it was as silly as something had fell on top of the vent closing it without me noticing took 5 mins to realise what had happened. Have you pulled a spark plug 1 at a time grounded to engine to check the spark condition probably need someone to help turn engine while you check for spark. My own next step would be checking coils or possibly the stator pick up you can check these with a multi meter but you would need to do internet search on your engine and find what it says regardind what readings should be. I dont imagine exhaust being the issue.outboards need 3 things to run fuel. Compression. Spark.nothing worse when you get a issue like this.hard to find. 1 other thing i know it was mentioned in a earler post choke. If its staying on this would cause these symptoms mine is a pre heat type and if not wired correctly at solenoind would be on all the time flooding carbs good luck
 
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Get the Factory service manual from a Tohatsu dealer, and go through all systems: fuel, mechanical, and at this point, also electrical. Be methodical and diligent, and you will find the malfunction.
 
Finally got the boat on the lake agian today, kinda hard troubleshooting it because I can't tell if it's fixed without taking it to a lake and the nearest one is 1.5 hrs away. I thought I had fixed it by running a bunch of sea foam through it, I figured maybe I had a couple of stuck rings. And although it did seem to run a little better it is still not fixed. On the lake I ran some more sea foam through it and it would get a little better after running that through it, temporarily. The other things I did notice at the lake today is it seems to idle fine and if I can get it to high revs it seems to run fine, but I don't think it's putting out full power ever, it's no more powerful than my 86 evinrude 9.9 hp. Also in order to get it to go from idle to high revs without it dieing I had to choke it for half a second and then give it gas, or it would die when I gave it gas. It really seems like it isn't getting enough fuel, but I don't know what else to do, as far as I can tell there is no float height adjustment. It does seem to be acting like too low of a float adjustment though.
 
The manual will tell you the height the float should rest at. If it is out you often can't adjust them, only replace them.

Stuart
 
It was totally disassembled, except for the throttle and choke valves. I have taken it back apart and I am going to run it through the ultrasonic for like 10 or more cycles this time just to make sure it is totally clean.
 
Ran the ultrasonic for about 20 cycles and left the carb in the heated solution overnight. Then blew all of the passages out with compressed air, the carb is spotless and all of the passages are clear. Started up the motor today, same problem. I ordered new fuel line for all of the under cowl in case the bulk stuff I got at the auto parts store is kinking, but of that were the case than shouldn't the problem get better when I squeeze the fuel bulb? Right now it doesn't make any difference if I squeeze the fuel bulb.
 
Do they make different size main jets for outboards like they do for dirt bikes? Maybe this motor was jetted for the higher altitude lakes, could that be a possibility? Pulled the plugs and it is definitely running too lean, especially the top cylinder, the bottom almost looked normal but a bit lean as well.
 
I can't find any information about high altitude jets, and the factory service manual doesn't give and information about jet size. I have been testing the motor at 2600-3800 ft elevation though, so even if it did have a high altitude jet, it should still work at these altitudes, right? Any other ideas to look at? Could it be a partially clogged exhaust?
 
Checked for water in the fuel, no water. Checked to see if I could burn the plug, it would not light, I think it's oil on the plug and not fuel, it's not very wet. I think it is running lean, the plugs are a light tan. I also think it's running lean because if you pull the choke for half a second it clears up and starts running better for a few seconds.
 
Right now I tighten it all the way and then backed it out two full revolutions, that's what the factory service manual was saying for a starting point, I think, it was a little confusing how it was worded. I have moved it in and out to see if it improves it, and haven't noticed a improvement. What is the proper way to adjust it?
 
In the parts catalog they call the jets MAIN JET #110 and SLOW JET #80 do the 110 and 80 relate to a measurement in the jet that I could measure to figure out if I have high altitude jets installed?
 
Matt04 said:
Turning it in made it worse, it got a little better when I turned it all the way out, but still not running right.
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That indicates that the carb is still messed up.

The jet numbers are etched into the jets, and you may need a magnifying glass to read the numbers. You did pull the jets and emulsion tube when cleaning, right?
 
Pulled the carb agian, the main jet is 110 but the pilot jet might be 60, it's either 60 or 80, need to find a magnifying glass to be sure. I pulled both jets when I cleaned it, I did not pull the emulsifying tube, I'm not sure how to remove it, the factory service manual, just says remove and doesn't tell you how to.
 
If you did not pull the emulsifier tube nozzle, the carb may still need a proper cleaning. It slides out and is retained by the jet. It may well be stuck or corroded in place. We use shop-made tools to extract recalcitrant tubes. The float does not normally need adjustment, and such adjustment is difficult, requiring controlled heat and much meticulous patience and skill. If it does not sit parallel to the casting, replace the float as well as the needle valve (in the carb kit).
 
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