Tiny Tach - Is it wrong?

[55Crestliner]

I just installed my new tiny tach, and went to the lake. My motor is a '57 Johnson 18 horse. It is supposed to be 4500 at WOT. I'm seeing 5650. I'm using a 9x9 prop. This is real frustrating, I've been running around at WOT all summer. Is my motor about to explode, or is my tach way off? I know the shops have test wheels, is this the only option to find out what's going on?<br /><br />What should this motor be running at at a very low idle? I'm seeing about 6-700.

 

[rodbolt]

Re: Tiny Tach - Is it wrong?

idle seems about right. I always verify new tach installations against my shop tach, it also gives me a chance to check timing at wot. if your motor is actually turning 5600 you can go up 3 or 4 inches of pitch and pick up some top end . may be worth it to have it tested at a shop with a test wheel and an inductive pick up tach.

 

[Stumpknocker]

Re: Tiny Tach - Is it wrong?

Being digital and very hard to hook up incorrectly,I suspect its right on the money. Other than a momentary lag for it to change readings, it should be quite accurate. Maybe you can get a handheld to check against your idle rpm that the TT is giving you.

 

[Paul Moir]

Re: Tiny Tach - Is it wrong?

It's possible that it's picking up pulses from the other wire - you could try moving the other spark plug wire away from the pickup winding and see if it made any difference.<br />That said, 9" seems pretty shallow for your setup. If that rig is as light as it looks, I wouldn't be suprised if you ended up with a 12" prop or more.<br /><br />PS - if I'm not mistaken, the WOT range for your engine is 4000-5000 RPM. So if the TinyTach is correct, you're just 650 above your max.

 

[mjbrueck]

Re: Tiny Tach - Is it wrong?

WOT should be 4500. I think your tach is working correctly, as I had a similar experience when I installed a Tinytach with a similar setup. I probably had around 5 hours runtime exceeding 4500 rpms before I added the tach, with no "known" consequences.<br /><br />Like I said in your other post, my 9x9 can't even get the boat fully on plane before I reach 4500 rpms. You said your 9-1/4x13 was too much prop, so I'd try to get ahold of a good 9-1/4x11. I'd guess even with that prop you won't be able to go full throttle when you're lightly loaded. <br /><br />If your motor's still running fine, I wouldn't worry too much about damage. If it's any consolation, today's oils are better (I know of at least one person who said he used to use his used motor oil for his outboards).