Converting a Tach Reading

[glengold]

OK, this is starting to make a lot of sense, I just want to clarify a few things...

I checked my tach and it was set to 6, which would be the correct setting for my original '97 50.

When I repowered with a rebuilt '77 55hp I didn't change the setting on the tach, and was disappointed with the purchase bc it wouldn't rev over 5 grand.

From what I understand the '77 is a 10 point, 5 pulse system, meaning that the true RPM's would be equal to (6/5)*(tach reading).

This would put me over 6000 RPM WOT, and at risk of blowing the thing up.

I am using a thirteen pitch prop I bought for wakeboarding, because i trashed the original 17P.

I felt that if I can barley turn 5 grand with a thirteen pitch prop moving up in pitch would be over proping. Now, based on my math, I need to reprop because I am currently overreving the motor.

Does all this make sense? I can't get to the lake till the weekend and would like to get a replacement prop ordered today. I think a 17 P is in order, as operating range is 5-6 grand, and currently I am turning (6/5)*(5000)=6000 RPMs

 

[SparkieBoat]

Re: Converting a Tach Reading

your tach should have had a manual that told you how to set it..maybe a tach maker will have a manual you could download..I am not sure of the setting for a 77..but if you are right then set it to 5 then prop it to run at around 5700-5800 RPMs WOT with no load. so if you are turning 6000 with a 13 prop get a 15...a 17 would put you down too much..about 200 decrease in rpm with each pitch up..a 17 would put you down to around 5200..that is a little on the low side specially when you load the boat may go under 5000. a 15 will get you down to about 5600 and around 5300 loaded..that would be perfect...also SS props run a little lower RPMs than aluminum..so if you are going from aluminum to SS go with a 14 pitch prop. of course I am assuming your motor is in good running condition...

 

[boobie]

Re: Converting a Tach Reading

For the mtr you have now, the tach should be set on # 5 position. If you know a good dlr that has something like a Merc-O-Tronic shop tach your tach rpm could be verified.

 

[chris94]

Re: Converting a Tach Reading

You may be able to verify your tach with a Digital Contact Tachometer on your fly wheel like this one at harbor freight. harbor freight

 

[glengold]

Re: Converting a Tach Reading

shouldn't switching the tach from 6 to 5 make it read a higher WOT RPM?

 

[SparkieBoat]

Re: Converting a Tach Reading

I would think so..why don't you give it a try and see

 

[glengold]

Re: Converting a Tach Reading

i wish i could the weather sucks and i'll be lucky to get out on sunday.... I ordered a 17P prop bc i have a hydrofoil installed and typically only fish with one other person.

 

[Joe Reeves]

Re: Converting a Tach Reading

What make is that tachometer?

 

[glengold]

Re: Converting a Tach Reading

I don't know the make, although I'm pretty sure by its condition that it's original to the boat.

I bought the boat, '97 welcraft 16 with the original '97 'rude 50 and "upgraded" to a rebuilt '77 55hp. I never touched the tach, with the '97 it'd read ~6000WOT and with the '77 it reads ~5000WOT. Here is a pic of the setting knob on its current setting. I tried 5P and it was reading ~4000 at idle, can't be right.

** Looking closely at this photo I took for the post I discovered I had been reading the switch backwards, there is a faint arrow point I couldn't see when sticking my head up under the console, so when I thought it was on 5P it really was on 2P.

Hopefully someone else will learn from this thread....

http://s1200.photobucket.com/albums/bb329/glengold/

Based on this I need it set to 5 pulse: (contains tach settings for most motors)

http://www.faria-instruments.com/site_manuals/IS0086p.pdf

 

[boobie]

Re: Converting a Tach Reading

Or set it on 5 pulse as was recommended earlier in this thread.

 

[glengold]

Re: Converting a Tach Reading

Yeah I thought I tried 5P but after taking the pic and going through the process of posting to this site I realize I had been reading it backwards!!! Look at the picture, there's a very faint arrowhead to the opposite side from the groove.

My 17 pitch prop came in the mail today and I was half tempted to drive to the lake and give it a whirl, but that will have to wait until this weekend. I'm glad I caught this sooner than later because WOT had to be over 6 grand.

 

[HighTrim]

Re: Converting a Tach Reading

Off the path question, why would you swap motors?

 

[glengold]

Re: Converting a Tach Reading

Because the '97 has a really weak LU. It was hard to find any replacement unless I spent 1500 on a rebuilt LU. Plus I had just bought the rig from a guy who didn't really take care of it, so I didn't want to spend a ton of money on a LU then have the PH fail! These older OMC's are a dime a dozen on cl/ebay.... I try to keep it cheap!