1986 Nissan NS15B will not run above 2500-2700 rpm

rzrbkrd

Cadet
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Aug 18, 2017
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21
Serial number 62244 15hp Nissan 1986 model. This is on my nephew's 15 ft aluminum Polarcraft flat bottom. It will just get on plane but not build full rpm. Here is a list of things I have checked and tried with no change in problem.
​1. Checked compression and it was 150 on each cylinder.
​2. Checked spark with tester and it showed a very solid blue line on both spark plugs.
3. Checked resistance on red wire from stator (should be 200-300 according to CDI) and it reads 280.
​4. Checked resistance on blue wire from trigger (should be 30-46 according to CDI) and it reads 42.
​5. Checked DVA on red wire (should be 110 v min) and it checks 260 cranking and the same running.
6. Checked DVA on blue wire (should be 4 v min) and it checked 30 cranking and running.
​7. Hooked different gas tank, fuel line and primer bulb to it and there was no change.
​8. Removed carburetor and cleaned it but there was no trash in fuel bowl and all jets were clean. Reinstalled and there was no change.
​9. Tried squirting fuel mixture directly into carburetor throat while it was running but it made no difference.
10. Talked to tech at CDI and told them all I had done and they seemed to think the ignition pack was breaking down.
​11. Ordered the pack 119-2400 which has the two spark plug wires coming out of the ignition pack, installed it and there was no change.
​I am not a boat mechanic so I am at a lose as to what the problem might be. Nothing I have done has changed anything. I also took the fuel pump apart and removed the two check valves and kept the fuel bowl full with the primer bulb but it made no difference. If any one has any advice or suggestions on what it might be I would be very grateful . Thanks, Jim
 

rzrbkrd

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Aug 18, 2017
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All passage ways and jets were clear but I sprayed carburetor cleaner in all and blew out with compressed air. Throttle valve is fully horizontal at full throttle on twist grip handle. Manual choke is fully open. Should I be seeing a small plume of atomized gas coming from the brass tube that comes from the fuel bowl into the venturi area of the carburetor? I do not see any when I accelerate the motor. I cannot find any svc manuals on this engine. The only ones I find online only go back to 92 and I think the engine schematics do the same. Any specific thing to check on the carb? Thanks, Jim
 

pvanv

Admiral
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Apr 20, 2008
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6,500
Did it never work properly, or is this a new thing? If this has been a problem all along, you may need a different prop.
Assuming this problem is recent, Elvin is on the mark. Spraying carb cleaner does not clean the carb passages. It makes the casting shiny. Proper cleaning requires complete disassembly and a 4-plus-hour soak in real carb dip, followed by a liberal spray-out... then a careful reassembly, verifying float height.
That motor is very obsolete; no carbs are available, and few other parts either.
I have no idea why CDI would suggest changing the CD (except to make a sale), when your readings were all fine.
What method are you using to verify the RPM are you getting? You need a good tach to verify.
 

Sea Rider

Supreme Mariner
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Sep 20, 2008
Messages
12,345
When OB will not go over 2.5-2.7 K revs is that at medium or full throttle ? OB runs even, misses ? Clean carb as recommended, include a prop hub slip test to rule out if with hub spun issues. If all comes good, could be a bad head gasket issue

Happy Boating
 

rzrbkrd

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Aug 18, 2017
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First of all thanks to everyone who has replied to this topic. It has been on hold while I worked on my own project. I need to clarify that on the old CDI it showed a good spark but when I marked tdc on each cylinder and hooked up a timing light I could see that it was occasionally missing. That was what lead the tech guys and myself to think it was breaking down under load. When I installed the new CDI it ran smoother at idle and I thought I had it but it still would not go any faster rpm wise. So asked my nephew if he had changed props recently and he had. Him and his buddy had ruined their prop and had replaced it with a four blade. When they tried to get on plane it would just blow out. So their fix was to get some J-B Weld, spread it on the prop hub, insert the prop into the lower unit and spin it to even it out and then they pulled it to let it set up. So now I am thinking they have it sealed off too good. I remove the J-B Weld and sure enough it is blowing out but at least it is turning more rpm. I come back home, remove the prop and machine .060 off the splined spacer that fits on the shaft. Go back to the water and now it has stopped blowing out and I have picked up a little rpm and speed. The prop they had is a 4 blade 9p that's not in the best of shape. He usually has at least one fishing buddy and sometimes two, so that sounds too high on the pitch to me. I told him to get a new 3 blade 7p and I will try that and report the results.
 

Sea Rider

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Sep 20, 2008
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12,345
Have someone messed with the timing advance & carb synch ? If so put them back to specs. 3 blade alum props performs excellent, a matter to go for less pitch props to pull wot revs towards middle to max wot rpm range as per factory specs. If wanting to get full 15 HP out of that under powered OB powering a large boat buy an induction tach and check max achieved wot revs with whatever pitch you go after. Ear tach is no bueno for any OB...

Happy Boating
 

rzrbkrd

Cadet
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Aug 18, 2017
Messages
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Installed the three blade 7p prop and its running fantastic. Nephew says it has never ran as good as its running now. So I guess the intermittent misfire on the CDI and the wrong bad prop were the culprits. He is tickled to death and I'm glad to be thru with it, lol. Thanks again to all who replied. First time I have used this site but it won't be the last.
 

Sea Rider

Supreme Mariner
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Sep 20, 2008
Messages
12,345
Be aware that although OB has made a dramatical hole shot improvement with a 7 pitch prop, OB could be over revving badly and nephiew not knowing all about it if likes running full throttle and very lightly loaded. That cond is not good for any OB, the only way to know would be installing an induction tach to rule that out..

Happy Boating
 
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