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Author Topic:   Strange readings on tachometer 170 Montauk
erik selis posted 08-10-2004 04:38 AM ET (US)   Profile for erik selis   Send Email to erik selis  
I was out yesterday with my 170 Montauk, 90-hp 2-stroke. After cruising for 20 minutes at approx. 4000 RPM I had to overtake a small tug-boat. This tug produced a fairly large wake and overtaking it, at the speed I did, produced a lot of air under the boat. The boat pounded down a couple of times and after this I saw that my tachometer was not showing the correct RPM's. It was showing 5000 RPM instead of 4000 RPM according to the speed I was reading and the approx. position of the throttle. This lasted for about 10 minutes. I wanted to see if the tachometer would go all the way to maximum range if I tried going up to 5000 RPM. It didn't. I gave her WOT and the tach remained at 5400 RPM. Going back down to lower RPM's the tach now seemed to be showing the correct value and it remained correct for the rest of the day.

Has anyone experienced this before? What could have caused this?


bsmotril posted 08-10-2004 08:19 AM ET (US)     Profile for bsmotril  Send Email to bsmotril     
Did the tone/pitch of the engine change indicative of higher rpm also? If it did, then the prop was ventilating which it will do when run too close to the surface momentatrily. Reducing RPM to let it "hook up" again and then speeding up is how you fix that.

If the motor did not change pitch, then the tach is flaky. Outboard tachs are often use to cover multiple models of motors that may have 2, 3, 4, or 6 cylinders. There are switches inside the tach to set it for the proper configuration. Eventually, you get moisture inside of any instrument, and that can short those switches. My 6 yr old tachs frequently will read high early in the early morning until the heat of the day and sun drives the moisture out of them. Bills

AQUANUT posted 08-10-2004 10:08 AM ET (US)     Profile for AQUANUT  Send Email to AQUANUT     
what year is your motor?

first of all concerning the tachometer, it only has 1 selector switch on the backside..with 6 diffferent positions.
position 6 for 4strokes, the others are for various engine manufacturers and horesepower settings,

in your post you state that the tach was reading 5000 rpm then you ran it up to 5000 rpm????????

I personally don't buy into the flaky tach idea...its either broken or its not. since the tach is working correctly now...I would inspect the gray wire on engine electrical. inspect the connections for possible short or poor connection. you stated that you jumped a large wake and this condition resulted...look for a physical electrical

LabRtvr posted 08-10-2004 11:05 AM ET (US)     Profile for LabRtvr    
The tach on my Montauk 170 with the 90 2s has done the same thing. I haven't been able to pin it down as to a common circumstance nor has it happen long enough to show the problem to my dealer. I have wondered if the condensation that builds up is the cause??
erik selis posted 08-10-2004 11:16 AM ET (US)     Profile for erik selis  Send Email to erik selis     
Thanks for your replies guys.


I was making approx 4000rpm and the tach was showing 5000rpm. I could see I was doing roughly 4000rpm by looking at my speed on the Lowrance and the position of my throttle. I went up to 5000rpm (real rpm's, again judging by speed and throttle position) to see if my tach would show i.e. 6000rpm. It didn't go higher than 5400rpm and then it went down to it's normal value when I slowed down.

My motor is a 2003 model built in 2002.


I didn't hear any difference in tone/pitch indicating a higher RPM. I was thinking the same at the time. Funny though that the problem was (momentarily?)solved by doing exactly as you stated.

This happened in the early afternoon and the boat had been in the blistering sun all day. I don't think it would be a moisture issue.

I did happen after jumping the big wake which would indicate an electrical-contact problem IMO.


AQUANUT posted 08-10-2004 09:57 PM ET (US)     Profile for AQUANUT  Send Email to AQUANUT     
ya know everyday I run outboards and inboards, I am a rigger at a b/w dealership, we sell yamaha/mercury/honda engines. yet I have not with this expierence of running motors daily, never have I achieved the ability to say...this or that motor is running at 4000 rpm without using test equipment or a tach.

my point is without a proper piece of test equipment or functioning can't judge rpm...
a person does over time in the saddle,. know when things don't sound the same or right.

I suggest you take it to a dealer, they tank, or lake test,
to duplicate .

good luck!

erik selis posted 08-11-2004 04:23 AM ET (US)     Profile for erik selis  Send Email to erik selis     
You know Aquanut, all due respect for your experience as a marine rigging technician (as you continuously point out), but you sound like many mechanics that I have come across over the years...making it sound as if the driver, operator or helmsman doesn't have a clue of what he's talking about.
If you have been travelling at 29mph running 4000rpm for 20 minutes and after a while you are still travelling 29mph and you don't hear any change in tone/pitch, but you are now running at 5000rpm, I think it's fair to assume you are still running at 4000rpm. No? I don't need any testing equipment to tell me that. The conditions were exactly the same: wind, current, weight...

I appreciate you trying to help me out here but don't make it sound as if I don't know what I'm talking about.


AQUANUT posted 08-11-2004 10:27 AM ET (US)     Profile for AQUANUT  Send Email to AQUANUT     
in regards to respect,
you opinion is valued, and I realize "you aint no rookie"

I would certainly like to apologise for any inference of lack of knowledge dirrected at you ..
in all honesty, { I usually can listen to an engine running in our "cememt pond" and guess the rpm within 100pm or less ]

irt the not being able to to guestimate rpm at or near wot,
I stand firm... my opinion...I stand firm on it.

quote from my last post this thread:

my point is without a proper piece of test equipment or functioning can't judge rpm...
a person does over time in the saddle,. know when things don't sound the same or right.


erik selis posted 08-11-2004 11:56 AM ET (US)     Profile for erik selis  Send Email to erik selis     

Your point has been taken. Thank you.

You too have a nice day.


sumnerdav posted 08-11-2004 12:51 PM ET (US)     Profile for sumnerdav  Send Email to sumnerdav     

My set up is the same as your's. I have given up on relying on my tach for anything, as it will change reading often as you described. It even pegs the needle intermitently while cruising at 3000 rpm. When you resolve your issue let us know. My dealer told me to bring the boat in and leave it for 2-3 weeks and they would get to it when they can. I decided to wait until winter.


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