me thinks my tachometer isn't reading right at upper RPM??

[joetheis]

This is on a 1991 Evunrude 70 HP VRO hooked up, 4 blade prop.
First time I REALLY had boat out this year was Saturday on the St. Lawrence near Clayton N.Y.
Water was glass, early A.M.
I SWEAR at 4,000 RPM , (the rpm I like to run my motor at, not WOT, but, it's happy there), last year I was at 20 mph on my bike gps and my 2 chart/fish finders).
This year I'm at 18 mph?
Just seems like the the tach is off a few hundred rpm, maybe even 1000 rpm.
Idle rpm is fine, just where it was the last 3 years.
Now, I changed from a rectifer last fall to a voltage regulator, (so the the voltage is always under 15 volts. Had a problem with the Ray marine chart/fish shutting off, over voltage).
When I FIRST fired it up after that, the tach was screwy for a few starts and then was perfect from like Aug. on to Oct..
I was going to see what WOT was, (last year it was 5200), but the weather blew up, and we had to get off).
How can I check this?
Thoughts?
Ideas?
Jokes?
Joe
Why do weird things happen to me??

 

[emdsapmgr]

Before you get into any engine stuff, check to be sure the tach is set correctly. There should be an arrow pointer on the back of the tach. For your engine, that arrow should be set/pointint to the number "6". That setting sends the correct signal to your tach so that it counts rpm's correctly. If it's already on the setting 6, then it's either the tach or something on the engine.

 

[flyingscott]

Your full throttle rpm should be closer to 6000 with a light load not 5200. I agree with the above

 

[WernerF]

Others have laughed at me suggesting this before. But the very quickest way to check the tach is to compare the engine sound tone with the tach reading.

5550 rpm F#
5239 rpm F
4944 rpm E
4667 rpm D#
4405 rpm D
4158 rpm C#

 

[interalian]

Others have laughed at me suggesting this before. But the very quickest way to check the tach is to compare the engine sound tone with the tach reading.

5550 rpm F#
5239 rpm F
4944 rpm E
4667 rpm D#
4405 rpm D
4158 rpm C#

Love it!! Just take your guitar tuner along for your next jaunt. Does it depend on the number of cylinders?

 

[Chinewalker]

This is on a 1991 Evunrude 70 HP VRO hooked up, 4 blade prop.
I was going to see what WOT was, (last year it was 5200), but the weather blew up, and we had to get off).

I'll say! It blew all weekend! I was going to launch my 18-footer this weekend but didn't feel like worrying about it bouncing around in the slip so I left it in the slings.

As for your issue, if the battery is charging and the tach is set on the correct setting, I'd think the issue is the tach itself. I had one go out on me two seasons ago. Just quit working... And, I agree. You should be able to spin it up into the 5600-6000 range at wide open with a light load. If 5200 is the most you can get out of it, you're likely running too much prop. It's akin to driving uphill while towing a trailer, while still in top gear. Drop down some pitch and bring the RPM up...

 

[WernerF]

Love it!! Just take your guitar tuner along for your next jaunt. Does it depend on the number of cylinders?

In theory, I3 and V6-engines may have a fundamental tone at the quint above that, if the 3/6 cylinder's waveforms are VERY similar. But I didn't find that on my 1989 60hp, the tones I heared were just the ones from the table. Now I have a new 3 cyl. 90hp E-Tec and it's the same with that. Two or four cylinders result in octave shifts, so no problem at all with them.

 

[oldboat1]

I wonder if tach operation might change if fish finders were disconnected -- might be worth a try.

[wind and snow here in the Finger Lakes yesterday, but boat is cozy, all buttoned down and double tied in the slip]

 

[joetheis]

I will try my WOT to see what the top, reed bending rpm is, perhaps it is closer to 6 grand, as that was 3 years ago, I did it once (glass water on the St. Lawrence is rare).
The engine RPM sounded low to me, but, after 8 months of not listening to it, I thought it was me!
Being a Gee-tar player, I will try my tuner!!
Batteries were topped off, As first thing I do when we open the place up is drag the boat over and plug it in to charge, (I also have 2 solar panels and regulators to trickle charge them til we get there).
I am over propped. I changed to a 4 blade and dropped pitch 1", added a foil when I first got her.
I had 4 A-dults in the boat, 3 standing aft, and the little 70 just doesn't have enough "umph" to push us on plane, so I dropped and added.
Since THAT trip, it very rare I get more than the wife n me in the boat, so I might go up (this is a 13 3/4 X 15 prop to maybe 13 1/4 X 16 or 17).
( I do like it when it LEAPS on plane though)!!
Getting there FAST, isn't my biggest concern, getting there AND back is!
I check out the tack position, top RPM next time I get north, (I'm sure it set, as the P.O. was non mechanical, and he didn't do squat but gas/oil, but..... never know how it roll off the shelf!
It may be that after market volt reg. I installed.
Thanks!
As always you guys clear the fog and lead me down the right path!!
Joe