Question & Need Suggestion - Engine running erratic

[E-Orchestrator]

I have a 1986 Aquasport Osprey 200 with an 1986 Johnson 140 VRO with about 550 hours on it.

The motor runs like a top. Every year I always replace the fuel filter, primer bulb, water pump..    

At WOT I can usually hit 5500 RPM @ 41-43MPH.. but sometimes my motor only wants to top out about 5100-5200 RPM which makes a slight difference in speed (38-39MPH)...  When at 51-52 rpms it makes a sound like it's winding out (kinda like a rev limiter)..  When this happens I can stop the boat, put it in neutral, and give it a couple revs, and it will run correctly after that..   Anyone know what would cause this?   The motor runs awesome besides this little quirk that probably the average person wouldn't even notice.

Also, what type/brand plugs do you guys suggest?  Also, what oil do you suggest?

Any information would be greatly appreciated.

[John Jones]

Sorry you have not gotten a reply yet.  I don't know if we have any sure 'nuff OB mechanics on here or not.

You might try posting your question on the Florida Sportsman Forum Boating section.  Some sharp guys on there.

http://outdoorsbest.zeroforum.com/zeromain?id=1

[ddd222]

off the top of my head and this is guessing, and assuming motor is not hesitating, i would guess that you are comparing seemingly identical scenarios, but the TRIM of the motor was likely different. Assuming flat calm conditions your boat should trim out in the ideal attitude and get possibly 2-500 more rpm's them depending on the application. Trimming correctly takes a little getting used to but will give the boat enough lift to lessen drag, and also allow the prob to spin at optimum performance. Kinda a "rocky dennis" explanation but logical to the question. If i'm wrong i would check how true the prop is or the gear case

[John Jones]

I have to "guess" along with ddd.  Load, boat attitude, wind direction, chop or slick water, and especially motor trim can all have an effect on top speed and rpm.  A boat will actually run faster on a small chop than it will on slick water because there is less surface contact.  I realize you said you could stop and rev it a little and it would do fine.  If that's the case then I have no clue.

As for oil, the opinions I have seen and heard say run the most expensive like Yamalube or whatever 'Rude is selling now.  I'm not sure how much difference it makes.