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RUSS9999

RPM Question

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Anyone know max. RPM on a 2001 Johnson 90hp. Was turning 5800 to 5900 I looked down at the tack Friday night and it was 6300 so I backed out of the throttle. When I came to a stop at idle it was 900rpm. That seams high to me too.   Its on a Lowe 180W, the prop is a 14x17. Going back out tomorrow morning  to see what it does.  If it's the same then going to the boat shop next.

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I recommend getting yourself a photo tach. I am pretty sure Harbor Freight still has them on sale for like $40. Anyway take the engine cowling off and run the engine on a garden hose and ear muffs.Test the engines rpm and then compare it to your boats tach. Your boats tach may be going bad.  I would do it first if the photo tach reads correctly then replace the tach yourself. It is cheaper than the marine mechanic. If it reads the same then take her to the mechanic.

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Pretty sure 4500-5500 is your WOT range. Idle should be 6-750(ish). Disconnect the battery and Pull the tach off the dash. Make sure all the connections are clean of corossion (especially the grey wire). Theres a knob on the back of the tack that will have a flat head groove to move it. Move it back and forth between the different cylinder settings(2,4,6,etc) and the put it back to you engines correct cylinder setting. Sometimes this setting can get bumped off and you need to toggle it around and then back. Put it back in the water to test it. Starting it on muffs will give you a false rpm reading. On muffs, your RPMs will be higher by about 2-300. 

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iabass8 I will give that a try. I forgot about them having different cylinder setting's

7 hours ago, iabass8 said:

Pretty sure 4500-5500 is your WOT range. Idle should be 6-750(ish). Disconnect the battery and Pull the tach off the dash. Make sure all the connections are clean of corossion (especially the grey wire). Theres a knob on the back of the tack that will have a flat head groove to move it. Move it back and forth between the different cylinder settings(2,4,6,etc) and the put it back to you engines correct cylinder setting. Sometimes this setting can get bumped off and you need to toggle it around and then back. Put it back in the water to test it. Starting it on muffs will give you a false rpm reading. On muffs, your RPMs will be higher by about 2-300. 

I did take it out today and it was the same

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Actually, the tach reads off the rectifier and  the numbers on the back of the tach relate to the number of poles on the alternator (stator) on the charging system, and is set to half the number of poles.  Your motor should have a 12 pole stator so the number on the tach is set on 6. 

Bad and no readings on the tach can be caused by a bad rectifier/regulator since that's what provides the tach signal.  If the motor seems to be charging fine, checking the output voltage of the rec/reg, then I would suspect the tach.  I'm not sure what the rev limiter in that motor is set for, but if it was really hitting 6,300 rpm, it would probably be bumping it.  That motor has a max rpm rating of 5,500 and it performs best if it's propped to turn 5,500 and would not hurt it to turn another 100 or so more but not 800 more.

I have my suspicions that tach is reading wrong, or you have a bad prop on the motor.

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My mistake. I've been corrected on pole setting 100 times and I still get it wrong when referring to the numbers on the back of tach. I believe your gray wire on the back of the tach runs to your regulator and rectifier on the engine.

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When I took it out yesterday. If I brought it up to 5500 on the tack it didn't sound like it was turning any RPM's. I think the tack is off. But need to know. Until I can find out just not going to let it go over 5500. Thanks for the help! 

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