OMC 2-stroke Analog Tach compatible with 4-stroke?

[cyber16]

Hello folks,
Last spring we purchased a second hand rig, after being boat-less for ten years
1994 Alumacraft Competitor 170 tiller
it came with a 1998 Johnson 2-stroke 40hp twin-cylinder
I pulled that motor and re-powered with a Merc/Yamaha 1996 50hp 4-stroke
Then sold the Johnson for +$500 over the 4-stroke cost
The twin cylinder motors just idle so rough, i knew this going into the purchase.

This 96 merc/yam four stroke was on the water at least once and up to three days a week last summer from memorial day to labor day and ran great!

I hooked up the tach to the grey sender wire at the merc motor and it was showing 4k rpm, i know it should have shown around 1k rpm, therefore it was reading 4x fast.
I tried every pole/pulse setting on the back of the tach and that seemed like there
was no change in the 4k reading
I know for a fact the tach worked fine with the 2-stroke Johnson

It is my understanding that this 4-stroke fires spark on both power/exhaust strokes
I think some call this as a waste spark system?
Could this be the problem?

I need to figure this out as i want a working tach, in-addition i maybe about to do another motor upgrade to a 2004 efi 4-stroke Merc 50HP, if it tests out ok

I like to keep an eye on craigslist for low-cost upgrades if it only involves a little of my time/labor and the need of flipping the old.

 

[RogersJetboat454]

Re: OMC 2-stroke Analog Tach compatible with 4-stroke?

Hello folks,
Last spring we purchased a second hand rig, after being boat-less for ten years
1994 Alumacraft Competitor 170 tiller
it came with a 1998 Johnson 2-stroke 40hp twin-cylinder
I pulled that motor and re-powered with a Merc/Yamaha 1996 50hp 4-stroke
Then sold the Johnson for +$500 over the 4-stroke cost
The twin cylinder motors just idle so rough, i knew this going into the purchase.

This 96 merc/yam four stroke was on the water at least once and up to three days a week last summer from memorial day to labor day and ran great!

I hooked up the tach to the grey sender wire at the merc motor and it was showing 4k rpm, i know it should have shown around 1k rpm, therefore it was reading 4x fast.
I tried every pole/pulse setting on the back of the tach and that seemed like there
was no change in the 4k reading
I know for a fact the tach worked fine with the 2-stroke Johnson

It is my understanding that this 4-stroke fires spark on both power/exhaust strokes
I think some call this as a waste spark system?
Could this be the problem?

I need to figure this out as i want a working tach, in-addition i maybe about to do another motor upgrade to a 2004 efi 4-stroke Merc 50HP, if it tests out ok

I like to keep an eye on craigslist for low-cost upgrades if it only involves a little of my time/labor and the need of flipping the old.

Waste spark shouldn't make any difference. That just means that when the plug is fired on one cylinders power stroke, its companion cylinder (cylinder that shares the coil) has it's plug fired during it's exhaust stroke. Manufacturers use a waste spark system to minimize the amount of coils needed (no harm done if the plug is fired during an exhaust stroke).

The problem you are having with your tach is because a four stroke has more crank revolutions to complete a cycle than a two stroke. You need to get a tach that will work with a four stroke engine.

 

[cyber16]

Re: OMC 2-stroke Analog Tach compatible with 4-stroke?

Thank you.
I figured this may be the case.
Yet
I even tried a hand held four stroke engine analyzer and it was also showing 4k rpm when it should have been around 1k
This analyzer was not a marine tool, it was for four stroke auto
Maybe the type of signal pickup was the cause here?

 

[RogersJetboat454]

Re: OMC 2-stroke Analog Tach compatible with 4-stroke?

Thank you.
I figured this may be the case.
Yet
I even tried a hand held four stroke engine analyzer and it was also showing 4k rpm when it should have been around 1k
This analyzer was not a marine tool, it was for four stroke auto
Maybe the type of signal pickup was the cause here?

That may very well be possible.
I would try the dedicated 4 stroke O.B. tach and see how you make out.

 

[Silvertip]

Re: OMC 2-stroke Analog Tach compatible with 4-stroke?

The key issue lies with the where the "tach" signal originates in the engine. On the two stroke, that tach receives its signal not from the ignition system, but rather the charging system. The charging system has a 12 pole alternator that outputs 6 pulses per engine revolution so the tach is NOT dependent on the number of cylinders the engine has -- just the number of poles the alternator has. On a four stroke such as a car or I/O powered boat, the ignition system provides the tach signal which IS dependent on the number of cylinders. Some tachs are universal and have settings for this -- others do not. I don't know off hand where the tach signal originates on Yamaha four strokes. A call to Faria or Teleflex customer service will likely clear up the issue.

 

[cyber16]

Re: OMC 2-stroke Analog Tach compatible with 4-stroke?

Thank you all.
I just re-powered to a 2004 Merc efi 50hp four stroke.
I will check the tach on this one prior to going with a new tach
This efi motor has four coil packs without the waste spark system
This efi also supports the mercury smartcraft, therefore i could always go that route as well
edit:
made it easy on my self, purchased a nice used Mercury SmartCraft SC1000 tach