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Author Topic:   Setting Engine Idle Speed
Salmon Tub posted 04-02-2004 06:14 PM ET (US)   Profile for Salmon Tub   Send Email to Salmon Tub  
The manual and the little sticker on the side of the [2000 Nissan 90-HP 2 stroke] call for the engine idle speed to be 750 rpm In-Gear, 900 Out-of-Gear. These settings are impossible to get since at 900 RPM Out-Of-Gear, she will idle at 650 RPM In-Gear, and at 750 RPM In-Gear, she rockets up to 1000+ RPM Out-Of-Gear. If I bring the Out-Of-Gear setting down to 800 RPM, she idles In-Gear at about 600 RPM. I like that, it is perfect for trolling, and there is little vibration, but since the manual calls for higher idle speeds, I worry about keeping her that low. Is it possible to do any damage with lower than specified idle RPM? Or are the posted speeds just there as a basic reference, or required to be posted due to EPA type stuff, as often is the case with automobiles? Thanks!
LHG posted 04-02-2004 06:49 PM ET (US)     Profile for LHG    
When there is larger than normal difference between idle and in-gear RPM, that is usually an indication of problems, possibly compression, out of round cylinder bores, etc. Under load, the engine is not putting out the power it should.
Salmon Tub posted 04-02-2004 08:39 PM ET (US)     Profile for Salmon Tub  Send Email to Salmon Tub     
LHG, I have often thought of that, but it has been like that from day one. Compression is on the mark for all three cylinders. As for out of round cylinders, a loss of compression would indicate this as well as even if they were oval, as long as the profile of the cylinder, wall and ring matched, they would hold compression and thus not cause a loss of power. I check the plugs for fouling and the compression regularly (every 3 or 4 months).
Moe posted 04-02-2004 09:04 PM ET (US)     Profile for Moe  Send Email to Moe     
Are you using the boat tach or a much more accurate (especially at idle) shop tach?

Split the difference until you can get it checked on a good tach. 700 RPM IN GEAR, 950 RPM Out-of-Gear.


jimh posted 04-02-2004 09:19 PM ET (US)     Profile for jimh  Send Email to jimh     
I think Moe is on the right path. The accuracy of the tachometers may be in question. By that I include the tachometers used to make the measurements in the recommendation, and the tachometer used to make the measurements in this specific case.
Salmon Tub posted 04-02-2004 09:19 PM ET (US)     Profile for Salmon Tub  Send Email to Salmon Tub     
It is the tach that came with the motor. 950 Rotations Per Minute Out-Of-Gear (:<>) seems a bit high and the engine gets a just a little jerky at 700 RPM In-Gear. I swear, ignorance is bliss. If it weren't for that stupid sticker, I would just leave it at 800/600 and be happy. I don't like discrepancies. Now, I have once or a few dozen times, seen Japanese literature (manuals) that have not been, how shall we say, reflective of the general control of quality found in Japanese products. None the less, can this lower setting cause any damage?
jimh posted 04-02-2004 09:34 PM ET (US)     Profile for jimh  Send Email to jimh     
[Thread was closed briefly while substantial editing was performed to help with the readability of this interesting discussion.]
Clark Roberts posted 04-03-2004 06:23 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Salmon, I believe that each carb has an "idle air" needle valve adjustment (should be near top of carb and may be vertical or horizontal) . This is screwdriver adjustable and can be recognized typically by its spring loaded feature. If you adjust all linkages (throttle cable, throttle arm roller/cam follower) etc. to factory recommended settings then you can adjust these idle needle valves. Then set idle rpm via timing or carb linkage adjustments, whichever drill your engine uses, and this may allow those rpms that you indicate. If one of the carbs is not adjusted properly for the idle fuel:air ratio then this may be the problem. To adjust these needle valves: turn each clockwise (cw) until it lightly seats and then turn counter clockwise (ccw) 1 1/2 turns, this allows starting and running the engine. Crank engine, warm up, in gear, tied to dock... turn valve on top carb cw until engine vibrates or misses and then turn ccw until same then try to adjust to midpoint. Do this on all carbs in succession and it's done. Hey, it's worth a try?... Happy Whalin'... Clark.. Spruce Creek Navy
Salmon Tub posted 04-05-2004 04:15 PM ET (US)     Profile for Salmon Tub  Send Email to Salmon Tub     
Clark, thanks, but I have already done that. Everything is in sync and pilot screws are where they should be. I was more interested as to how 'set in stone' those specs on the sticker are, or are they more of an arbitrary number so as to satisfy any requirements set by emissions authorities.

Has anyone ever used an inductive(?) timing light on their outboard? It would not assist in setting the timing, but I am more interested in measuring the spark. I would imagine that it would show if I had an inconsistent spark, but perhaps not a weak spark. What would you use to check for a weak spark, other than visual test in the dark?

One last culprit, as I have suspected for a while now, could be my wire harness. It is the original one that Came with the 40 hp. They were identical so I still left the original installed and have the new one at home stored away for future use. Even with the 40 hp as well as the 90 hp, I noticed that at low rpm's, the needle would drop every so often, as if there may be a small short. Never any problems at high speed. If this was an engine related problem, with the 40, I thought it was an engine related problem, but since it showed on the 90 as well from day one, that seems less likely.

Chuck Tribolet posted 04-05-2004 09:53 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
More likely a small open than a small short. Opens require
one point of failure, shorts require two.

Regarding what Clark told you: right on, but do it multiple
times, as adjusting one carb, affects the others. You aren't
done until a couple of passes result in no net change. This
is the voice of someone who owned a 240Z with only two carbs,
and tweaked a bunch of buddies cars to the "is it running?"
state. One pass would make it OK, two would make it better,
three was real close, and at about pass six it would be
really good. I suspect that with three carbs, you have the
same problem in spades. (Actually, there were three variables
on the Z: idle throttle posision, half throttle position,
and jet height (affected mixture over the whole range).


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