tiny tach accuracy

Mercury 20 FourStroke Specifications
HP @ Prop 20
kW @ Prop 14.9
Max RPM (WOT) 5400-6100
Cylinders 2 (in-line)
Displacement (CID/cc) 21.4/351
Bore & Stroke (in) 2.40 x 2.36
Bore & Stroke (mm) 61 x 60
Cooling System Water-cooled w/thermostat
Ignition System CDI w/ electric spark advance
Starting Manual or electric
Gear Ratio 2.15:1
Gear Shift F-N-R (Shift-in-Handle Tiller or Remote Control)
Steering Tiller or remote
Alternator Amp 12 Volt / 11 Amp
Alternator Watt 139
Trim Positions 6 or power tilt
Shallow Water Drive Standard
Exhaust System Through prop
Lubrication System Wet sump
Recommended Oil Mercury 4-Stroke Outboard Oil
Oil Capacity 1.0 L / 1.1 Qt.
Fuel Induction System 2-valve single overhead cam
Remote Fuel Tank Standard
Remote Fuel Tank (US Gal) 6.6
Remote Fuel Tank (L) 25
Shaft Length (inches) 15/20
Shaft Length (mm) 381/508
Dry Weight (lbs.) 115
Dry Weight (kg.) 52
Operator Warning System Low oil pressure / Over--rev
Available Propellers Standard equipment
CARB Star Rating 3

Sounds like you're getting the right speed but the wrong rpm's based on the engine specs.

Which model tach did you get?
Appears to be reading half of the true rpms.
A tach designed for a 1 cylinder engine
and being used on a 2 cylinder engine
might read only half the rpms.

 

Splice another chunk on, its an induction coil.
The short length of the wire added shouldn't affect the electrical signal produced.

 

Get your tach working right first.
If you are in the power band 5400-6100 rpm
then no need to change.

 

I just got mine yesterday from the factory. I ordered the TT226R-2C because that's what I thought I needed from reading the descriptions on their site. I received the same model as yours, the TT226R-2X with the hand-written "L" next to it, with a note on the packing slip that it was sent because it was the right one for my motor. I have a Johnson 15 2-stroke. I described my motor when I ordered it. I don't know what the difference is, but assume the factory knows best. Going to install it this weekend and try it out. Glad I saw this post, so I won't be trimming anything off until after I test it. I'll let you know how it works.

Kemo

 

I've never seen a Tiny Tach installation but if it is sensing spark plug firings then it shouldn't be the same part number for a 2 stroke and a 4 stoke - right?

Tom W

 

Some engines fire the plug for the cylinder on every rotation of the flywheel.
Others fire it on every other rotation, so half the number of rpm's would be read.

 

Sounds like the tach factory thinks his 4-stroke fires every time. I'd question the motor factory on what that particular 4-stroke does.

Kemo

 

Proper idle rpm, hmmm, how do you know? By ear?
500 rpm or a 1000 rpm? Idle speed, I dunno...

 

In My Opinion I would go with an 11 Pitch But I am Basing that on my 15 Horse Yamaha I run a 10.5 or 11 but by X what I would have to check the prop box

Dave

At Least i can hear when the Yamaha hits the rev Limiter ... Then go up one Pitch LOL

 

Sorry, didn't get a chance to do my TT yet.  Sometimes I get a chance to do other stuff (for money) and I have to take advantage of it.  Hope to get back to the boat tomorrow.

Kemo   

 

I wonder how you'd check your tach-uracy?    :-?


found it...

http://www.cadvision.com/blanchas/54pontiac/tach.html