TACHOMETER ACCURACY

[phx5kegsman]

I have a 1978 Johnson 115hp V4 crossflow. I'm having issues with my Tachometer. It seems to be accurate up to around 2000 rpm. But, when I push the throttle down tach climbs to around 7200rpms, and I'm not even at WOT yet! I think WOT should be around 5200 on my motor. Obviously, my tachometer is not accurate. Any suggestions WHY?

 

[Berdink]

I know you know your tach may be off, but to check it you can do a loan a tool from AutoZone, or other local auto parts store, and see if they have a timing light with the included tach.
Then at least you'd know how far off it is.
.
In my searching for a tach, I read older ones can be off.
A cheapo inductive one is said to even be more accurate than older factory tachs.

 

[saltchuckmatt]

I have a 1978 Johnson 115hp V4 crossflow. I'm having issues with my Tachometer. It seems to be accurate up to around 2000 rpm. But, when I push the throttle down tach climbs to around 7200rpms, and I'm not even at WOT yet! I think WOT should be around 5200 on my motor. Obviously, my tachometer is not accurate. Any suggestions WHY?

Is this a "new to you" tach? Pulse settings correct?

 

[Chris1956]

Is your prop slipping? Buy a new tach and try it. They do not last forever.

 

[Chris1956]

It probably makes sense to check and clean your wiring to the tach, especially the ground.

 

[phx5kegsman]

The tachometer is 4-5 years old. The brand is Faria. I did buy a handheld tach, but it's useless out on the water. You have to put a piece of reflective tape on the flywheel and aim the tach at the tape from less than 12 inches away. It's useless while driving the boat. I have a timing light, but it has no tach. I have yet to figure out how to check my timing, although, I'm sure that I should. I have replaced ALL of the ignition components over the 6 years I've owned this motor. The manual says to check timing with the engine speed "set to 4300-4600 rpm (in gear)". I have yet to figure out how to accomplish that by myself. I have seen some small digital tachometers online. I think one brand name is TINY TACH. Has anyone had any experience with one of these?

 

[saltchuckmatt]

The tachometer is 4-5 years old. The brand is Faria. I did buy a handheld tach, but it's useless out on the water. You have to put a piece of reflective tape on the flywheel and aim the tach at the tape from less than 12 inches away. It's useless while driving the boat. I have a timing light, but it has no tach. I have yet to figure out how to check my timing, although, I'm sure that I should. I have replaced ALL of the ignition components over the 6 years I've owned this motor. The manual says to check timing with the engine speed "set to 4300-4600 rpm (in gear)". I have yet to figure out how to accomplish that by myself. I have seen some small digital tachometers online. I think one brand name is TINY TACH. Has anyone had any experience with one of these?

You need a test tank and prop to do that accurately.

 

[phx5kegsman]

I have a tank. What type of prop?

 

[phx5kegsman]

Is your prop slipping? Buy a new tach and try it. They do not last forever.

I want to bypass the wiring, if possible. Hopefully to eliminate the wiring as the cause

 

[saltchuckmatt]

You need an old school test prop. The tanks are fairly large and have a vent fan.

 

[saltchuckmatt]

Just cus I have to ask, it's not overrevving from a bad prop or anything? Otherwise, tachs die, rectifiers on you engine go bad and what got me recently...a bad ground on my tach.

 

[Chris1956]

Joe Reeves published a timing method for Johnnyrudes, on this forum I believe it did not involve actually running the motor at high RPM to set the max spark advance.