Need help adapting a 2Cyl automotive tachometer to a newer tecumseh V-Twin engine

[circuitman1989]

I have a 18hp vertical shaft Tecumseh V-Twin lawn tractor engine that I would like to put a two cylinder automotive tachometer on. The problem is the engine has a magneto style electronic ignition system that uses 2 seperate magneto's (one for each cylinder). I want to use this tachometer... **broken link removed** but I dont know how to make it work with my engine. Can some one please help me?

Circuitman1989

 

[4pyros]

It will not work, The tach is made to run off a pluse every time a plug fires from the points in the distributer and is used on 4,6 and 8 cylinder engines. You need a RPM meter that runs off inductive pickup, thay have a small pick up wire that raps around the spark plug wire and can be set for 1,2 or 4 cylinder engines.

 

[Diver300]

I have to disagree with 4pyros here.

I think that it will work, and it would work on a single cylinder lawn mower. One of the settings on the tacho is designed for a 2 cylinder, 4 stroke engine, that fires once per revolution. (Each cylinder fires every second revolution, so with 2 cylinders, one fires each revolution).

The magnetos on a lawnmower are on the crankshaft so there is a wasted spark system and each fires every revolution, so that will be right.

Also their website says "Modern DIS ignition applications are covered by the 2 cylinder setting" which is why that setting is there, rather than for 2 cylinder engines which I am fairly sure are rare in modern cars.

The bigger problem is that you will use about 1/4 of the range of the tacho because the engine only revs to 2400 rpm or so.

 

[circuitman1989]

I'm planing on making a drag racing lawn tractor and I'm using the Tecumseh V-Twin as the engine. I already found a ton of aftermarket performance racing parts for the engine. I know with the proper parts and performance mods to the engine, I can run atleast 7500 RPMs (I've dove research). I just want to monitor the RPMs because racing lawn tractors use a straight pipe exhaust system(No muffler, very loud) and I'll have a hard time trying to figure out the RPMs without a tachometer.

 

[Mosaic]

I don't suppose u want to build your own PIC tacho for the purpose? Or a PIC interface to correct the RPM pulses to 'match' your tacho?

 

[circuitman1989]

I don't suppose u want to build your own PIC tacho for the purpose? Or a PIC interface to correct the RPM pulses to 'match' your tacho?

I would like to build a PIC interface, but I don't have a schematic. Plus don't I have to program a PIC?

 

[crutschow]

I agree with 4pyros. As stated in the manual, the Sunpro tach requires a connection "to the driver wires from the vehicle computer to the ignition module". I don't believe there's a suitable connection on a magneto ignition for this signal. Thus you need one with an inductive pickoff, as he stated, perhaps one like this.

 

[circuitman1989]

can I use **broken link removed** and if I can how do I wire it up because the engine has two separate magnetos (one per cylinder). I've included the PDF of the wiring schematic

 

[crutschow]

can I use **broken link removed** and if I can how do I wire it up because the engine has two separate magnetos (one per cylinder). I've included the PDF of the wiring schematic

Seems rather expensive but, if you have a magneto kill switch that you can access as shown in the PDF file, then I would think it should work.

The kill switch is where it picks up the signal to convert for the tach, and that should work for any number of cylinders.

 

[Rexlan]

Here ya go. I made mine to run the controller on my intake for my spec racer and have used it for several years.

 

[4pyros]

Thare are many digital tachs out thare for small engines IE Welcome To Tiny-Tach. Small and economical tachometers for gas and diesel engines. for under $50. Do you have to have an analog meter?

 

[KeepItSimpleStupid]

This **broken link removed** might work, but cannot find any real info on it except that it has a switch that's used to selct twin and not twin engines. Polarity is dependent on which way the sensor faces.

Who knows what it outputs.