M40C Tachometer install weird readings

[keny01]

I installed a hardline tachometer on a m40c. I set the tachometer to the 2p1r setting and did 4 wraps around the middle of the spark plug lead. The tachometer is still reading 2000rpm at idle. Also when I increase the throttle It revs upto to 6000 then it drops to 3000 while I'm still increasing engine rpms then starts going up to 4-5000rpm depending how hard im running it it's like it is running at 4p1r and then changing to 2p1r after 3000rpms or so

 

[dingbat]

I use the same tach to set idle on all my small engines. Works like a charm

The problem is the pulse settings are not clearly defined.
Typically:
1Pr= 2 stroke
2Pr = 4 stroke

Given that your running a two stroke, your only get 1 pulse per revolution, not 2 pulses as you currently have it setup

 

[Mc Tool]

I use the same tach to set idle on all my small engines. Works like a charm

The problem is the pulse settings are not clearly defined.
Typically:
1Pr= 2 stroke
2Pr = 4 stroke

Given that your running a two stroke, your only get 1 pulse per revolution, not 2 pulses as you currently have it setup

unless the ignition is the "wasted spark " .... maybe ?

 

[dingbat]

unless the ignition is the "wasted spark " .... maybe ?

Motor is a 2 stroke.
“Wasted spark” is a 4 stroke

 

[Mc Tool]

Motor is a 2 stroke.
“Wasted spark” is a 4 stroke

Not necessarily , MY Tohatsu M18d is a fine example Two cylinder 2 / one coil , two plug leads , fires both pots every 180deg , one pot at tdc and the other at bdc .... ergo .... wasted spark ...........so there

 

[dingbat]

…

 

[keny01]

Not necessarily , MY Tohatsu M18d is a fine example Two cylinder 2 / one coil , two plug leads , fires both pots every 180deg , one pot at tdc and the other at bdc .... ergo .... wasted spark ...........so thereView attachment 405854

Yes I have two leads coming out of my coil like yours so that would mean I have 2pulses per one rotation ?

 

[keny01]

I use the same tach to set idle on all my small engines. Works like a charm

The problem is the pulse settings are not clearly defined.
Typically:
1Pr= 2 stroke
2Pr = 4 stroke

Given that your running a two stroke, your only get 1 pulse per revolution, not 2 pulses as you currently have it setup

If I set it to 1 pulse per revolution it says its doing 4000rpm at idle wich definitely isn't right

 

[keny01]

I installed a hardline tachometer on a m40c. I set the tachometer to the 2p1r setting and did 4 wraps around the middle of the spark plug lead. The tachometer is still reading 2000rpm at idle. Also when I increase the throttle It revs upto to 6000 then it drops to 3000 while I'm still increasing engine rpms then starts going up to 4-5000rpm depending how hard im running it it's like it is running at 4p1r and then changing to 2p1r after 3000rpms or so

I have switched it onto different settings on the tachometer can't get it to work right seams like it's most accurate on 2p1r and I just have to half the rpm value its showing until it reaches 6k

 

[Mc Tool]

Yes I have two leads coming out of my coil like yours so that would mean I have 2pulses per one rotation ?

yes . if its a bit erratic you could try wrapping a couple more turns around the plug lead .
I have hour meters triggered off the plug wire and I also wrapped a bit of insulation tape to sorta bind the pick up wire to the plug lead .

 

[cyclops222]

Defective tach or wrong type.

 

[dingbat]

The tachometer is still reading 2000rpm at idle. Also when I increase the throttle It revs upto to 6000 then it drops to 3000 while I'm still increasing engine rpms then starts going up to 4-5000rpm depending how hard im running it it's like it is running at 4p1r and then changing to 2p1r after 3000rpms or so

Looking though the manual, the unit is rated up to 16,000 rpm at 1 pulse per revolution which is 266.6 Hz

If your getting 4 pulses per rev as it sounds, your exceeding the 266.6 Hz read rate of the unit at 4,000 rpm, thus the error noted at higher rpm

 

[Mc Tool]

I can see why an engine could have 2 pulses per rev ( 2 stroke twin wasted spark ) but 4 ? How would you get 4 ?
I was thinking tho that maybe it auto ranges if the pulse rate goes out of range for the setting its on .
And anyhow dingbat what are you doing reading the instructions

 

[dingbat]

I can see why an engine could have 2 pulses per rev ( 2 stroke twin wasted spark ) but 4 ? How would you get 4 ?

Scratching my head as well…..

Makes no sense unless he’s picking up some kind of induction “fly back” off the coil.

I use a similar unit to adjust carbs on 2 cylinder, both 2 and 4 stroke, motors w/o issues.

 

[keny01]

Defective tach or wrong type.

It's the right type for a 2 cyclinder 2 stroke I'm thinking it must be defective