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Old 04 May 2010, 14:53   #1
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Firing degree? Tohatsu 50HP 3-cylinder 2-stroke

Fitting a new Tiny Tach to my Thundercat with a Tohatsu 50HP 3cylinder 2stroke (carb).

Eventhough it's 3 cylinder, is the firing every 180degrees? When I set the TinyTach to 120degrees the TinyTach gives crazy readings. Can anyone confirm that it's every 180degrees that the engine fires?

Thanks folks
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Old 04 May 2010, 15:32   #2
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Surely each plug fires once every 360' ? ( ie one complete rev) - there are just 3 'fires' per rev- one on each cylinder. Does the tiny tach run off just one plug ? If yes then can it be set to 360'.

Or if it was reading everyplug then it would be 120'. So you may see 3x as many revs as you are actually doing .
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Old 04 May 2010, 16:26   #3
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Alt, what model tiny tach is it?

If it's the commercial tiny tach you need it set to 360 degrees.

If it's the standard model:-

Given what it says on the website (note the bold)
Quote:
Originally Posted by Tinytach
New Standard TT2A - TINY TACH™

* RPM reading adjustable from 1 spark per 2 revolutions to 8 sparks per revolution. One TINY-TACH™ now works for most gas engines.
you may only be able to get it to read double the RPM the engine is actually doing as any HT lead on your engine is going to be giving one spark per revolution.
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Old 05 May 2010, 03:52   #4
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Thanks folks,
It's the commercial Tiny Tach

And yes, it only reads off one HT lead.

So it's 360 degree I set the Tiny Tach (Commercial) to then? Interesting!
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Old 05 May 2010, 11:37   #5
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As I recall, there are some 2-stroke engines set to fire each plug every 180 degrees of crank revolution. Don't recall the reason, but they apparently are out there.

I would expect that you'd get correct readings on either 180 or 360 degree settings.

jky
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Old 05 May 2010, 12:00   #6
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anyone know for sure??

Quote:
Originally Posted by jyasaki View Post
As I recall, there are some 2-stroke engines set to fire each plug every 180 degrees of crank revolution. Don't recall the reason, but they apparently are out there.

I would expect that you'd get correct readings on either 180 or 360 degree settings.

jky
would that be on a twin cylinder with 180 crank?
that would make sense, one coil for two cylinders, so one plug gets a neg spark and the other a positive spark.
the "lost" spark on the cylinder at BDC doesn't lose much voltage because theres no compression, so the voltage required is low to jump the gap, leaving all the energy for the more difficult pressurised gap at TDC.
I did think this only worked on 4 strokes though, because whats stopping the lost spark igniting the crank case through the transfer port?
maybe the transfer port is still covered by the piston at 20-30 btdc that the plug fires at?

anyone know for sure, would be interested to know if any 2 cyl 2st engines use one coil.
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Old 05 May 2010, 13:30   #7
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Quote:
Originally Posted by doggypaddle View Post

anyone know for sure, would be interested to know if any 2 cyl 2st engines use one coil.
Never seen one.
It sounds very unlikely as it'd be spraying fuel/air mix towards the plug as it fired.
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Old 05 May 2010, 14:05   #8
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Never seen one.
It sounds very unlikely as it'd be spraying fuel/air mix towards the plug as it fired.
thats what i was thinking.....very common on 4st now though.
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Old 05 May 2010, 14:08   #9
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Quote:
Originally Posted by doggypaddle View Post
thats what i was thinking.....very common on 4st now though.
Yep, but the wasted spark is on the exhaust stroke on a 4 stroke.
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Old 05 May 2010, 14:14   #10
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Yep, but the wasted spark is on the exhaust stroke on a 4 stroke.
technically its on the exhaust stroke on a 2st as well
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