How to test tach for accuracy?

[jonny rotten]

My gauge works but I'm not sure how accurate it is. I bought one of those cheap tachometers you wrap around the spark plug wire to compare.
At idle boat tach read 800 RPM. The wire one read 1500
At WOT the boat read around 4800 the wire one read 7000
Motor was definitely screaming at WOT but sounded good. I have to assume I wasn't at 7000 and the spark plug one is way wrong. Plus there is no way I was at 1500 at idle.

Boat tach seems accurate but how do I check to see if it really is.? 1990 32 year old tach
Mercruiser 4.3 WOT 4400-4800
Alpha one gen one
prop 14.25 x 19
39 MPH on phone GPS WOT
Going to try a 14.25 x 21 next trip out

 

[dubs283]

possible you had the setting wrong on the shop tach

an advance timing light has a tachometer setting that i use regularly for troubleshooting

 

[tank1949]

My gauge works but I'm not sure how accurate it is. I bought one of those cheap tachometers you wrap around the spark plug wire to compare.
At idle boat tach read 800 RPM. The wire one read 1500
At WOT the boat read around 4800 the wire one read 7000
Motor was definitely screaming at WOT but sounded good. I have to assume I wasn't at 7000 and the spark plug one is way wrong. Plus there is no way I was at 1500 at idle.

Boat tach seems accurate but how do I check to see if it really is.? 1990 32 year old tach
Mercruiser 4.3 WOT 4400-4800
Alpha one gen one
prop 14.25 x 19
39 MPH on phone GPS WOT
Going to try a 14.25 x 21 next trip out

get your money back from wire tack and buy an extra REAL tack and temporarily install it. I purchased a Welcraft whose tack was way off too. 500 RPS and mid-range.

 

[Scott Danforth]

I would trust your dash tach more than the cheap tiny tach imposter you used.

to turn your 4.3 7000 RPM requires a lot of engine work to prevent the pistons from seeking homes outside of the bores.

However a good shop tach or a photo tach or laser tach would be my first choice vs a single wire cheap tach.

 

[jonny rotten]

I would trust your dash tach more than the cheap tiny tach imposter you used.

to turn your 4.3 7000 RPM requires a lot of engine work to prevent the pistons from seeking homes outside of the bores.

However a good shop tach or a photo tach or laser tach would be my first choice vs a single wire cheap t the photo laser tach

I would trust your dash tach more than the cheap tiny tach imposter you used.

to turn your 4.3 7000 RPM requires a lot of engine work to prevent the pistons from seeking homes outside of the bores.

However a good shop tach or a photo tach or laser tach would be my first choice vs a single wire cheap tach.

It looks like with the photo laser tach you need to hold it in front of the flywheel with reflective tape. I wouldnt feel comfortable having my wife stick her arm in there at WOT with the chop we boat in

 

[dubs283]

I wouldnt feel comfortable having my wife stick her arm in there at WOT with the chop we boat in

She drives

You look

Get an advance timing light, they work wonders

 

[jonny rotten]

The tach with the wire was set to the wrong value. It was set at one spark per revolution and it looks like a 4.3 is 3 sparks per revolution. I'll try it again the next time out.
I never used one of these. Live and learn.
If it still seems out of line I'll pick up an advanced timing light.
Thanks for the suggestions

Edit: Is it more accurate to put it on the main coil wire instead of a plug wire?

 

[dubs283]

Is it more accurate to put it on the main coil wire instead of a plug wire?

On coil wire you'll get pulses per coil firing, not rpm. The value will be 8x + what you want for true engine rpm

Hook the lead to any plug wire. #1 is appropo in the sense you can also check idle/advance timing with the proper tool

 

[Lou C]

Or just get a tach/dwell meter and check it against that. + from the meter goe to - on the coil and - on the meter to ground. Put the boat in the water and see what it reads. I found that my dash tach reads about 150 rpm too high using my tach/dwell.

 

[Scott Danforth]

It looks like with the photo laser tach you need to hold it in front of the flywheel with reflective tape. I wouldnt feel comfortable having my wife stick her arm in there at WOT with the chop we boat in

you can test at any RPM and in neutral to verify your dash tach