Problem- After running the motor for a while, when I bring the engine to neatural (say when docking), the motor will stall. If I move the auxillary lever a little it will stay running.
1. I adjusted the idle adjustment, low-speed adjustment, and the auxiliary adjustment knob at the bottom of the linkage.
RESULTS- I could get the motor to not stall. However, it had an impact on the RPMs.
Second Problem- The RPM's on the motor typically are at 5,000 at WOT and have a top speed of either 36mph or 38mph depending on prop.
After the adjustments above, my rpms drop down to between 3,000 and 3,600, but my top speed is still at 36 MPH.
That just seems odd to me. Prop RPM and speed are directly related, to an extent. Speed should be impossible to be the same at such different RPMs. I would get ou a GPS to see if your speedometer is reading correctly (or you tach is nuts.....).
David, I too would go along with the "bad tach" idea. I would not believe it possible that a 2000 rpm WOT difference in your relatively light boat could have the same GPS speed using the same prop.
There should be a very noticeable difference in engine note between 3000 and 5000 rpm. If you don't hear it, it isn't so.
What is the rpm for rated hp? Is it 5000 or is it more ... like 5500? A two inch change in pitch should give approximately 500 rpm WOT difference. And that should result in a speed diff of a few mph as you noted. So it still comes back to the tach IMHO.
Okay, thanks. Is there a recommended way to troubleshoot the tach?
Easy answer: try a different tach.
What I would want to know: first, is the tach on the right setting? What are the choices on the back of your tach, pulses (1,2,4, etc) or maybe cyl or 4/2 cycle? The tach counts pulses from the alternator but you need to know how many that is per rev. If you don't know, it is a case of try one setting and see what you get. What you think is the obvious choice is not always the right one. The wrong setting can look correct at one speed but be wrong at a different speed so check it at a range of speeds. It is easiest to check idle and WOT rpms because those throttle positions are are going to be the same for each test.
Where are you getting power for the tach, through the ignition? Do you notice any electrical issues with instruments or lights that have the same source or ground? I'm thinking a bad connection is a possibility. Do you have a schematic of the engine? Trace the tach lead back to it's source and look for loose connections there.
There may be other ways to troubleshoot this, but those are what came to mind.