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Old 19 July 2009, 06:46   #11
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Quote:
Originally Posted by chris123 View Post
Is there any possibility that it could be the wrong Tinytach for your outboard? What revs are the Tinytach / boat rev counter showing in neutral at idle? I'm guessing the revs you are getting at WOT are about half what they should be?

My experience with two strokes ended with a Yam powervalve (bike) about 15 years ago, but I would have thought if the required idle is (say) 900rpm, your outboard will either idle like a bag of nails at 500 rpm, or will stall.

If the idle appears to be smooth (well, smooth for a 2 stroke ) and the tacho(s) do indicate 500 rpm, I would still suspect the tacho.

Cheers

Chris
It's a good point-if both are wired to the engine wrong and it's reading half revs, you need to go UP a pitch on the prop.
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Old 19 July 2009, 06:55   #12
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Same problem

Guys,
I recently ripped off my old 30 hp yam top speed 22 Knts and replaced it with a 50 hp and i seem to be topping out at about 31 knts. Would have taught for a SR4 with a 50 i was running a bit slower than normal. Will get a look at the boat later and get the prop spec.
Dont have a rev counter on it yet, thinking of getting a tiny tac... look cool.
Thanks
Donegal Dan
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Old 19 July 2009, 06:59   #13
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Where have you guys got your fuel tanks?
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Old 19 July 2009, 07:04   #14
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Where have you guys got your fuel tanks?
In the console.
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Old 19 July 2009, 07:08   #15
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Check the tacho wiring out then as Chris suggested. Weight distribution makes a fair difference-but the tanks are in the right place.
34 knots at 2750 rpm on a 13" does point to the revcounters both reading wrong.
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Old 19 July 2009, 07:16   #16
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Quote:
Originally Posted by chris123 View Post
Is there any possibility that it could be the wrong Tinytach for your outboard? What revs are the Tinytach / boat rev counter showing in neutral at idle? I'm guessing the revs you are getting at WOT are about half what they should be?

My experience with two strokes ended with a Yam powervalve (bike) about 15 years ago, but I would have thought if the required idle is (say) 900rpm, your outboard will either idle like a bag of nails at 500 rpm, or will stall.

If the idle appears to be smooth (well, smooth for a 2 stroke ) and the tacho(s) do indicate 500 rpm, I would still suspect the tacho.

Cheers

Chris
I have the commercial tiny tach and is only a few months old. Dead simple to set up one lead to the HT lead and one to earth so think i have it set up right.

Engine ticks over at 900 and when i drop it down to 400-500 runs a bit rough.

As i said both gauges mirror each other.
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Old 19 July 2009, 07:30   #17
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Originally Posted by gibbo500 View Post
I have the commercial tiny tach and is only a few months old. Dead simple to set up one lead to the HT lead and one to earth so think i have it set up right.

Engine ticks over at 900 and when i drop it down to 400-500 runs a bit rough.

As i said both gauges mirror each other.
I reckon you need to get an engineer to look at it. It isn't right and internet diagnosis can only go so far without seeing it.
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Old 19 July 2009, 07:37   #18
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Originally Posted by gibbo500 View Post
I have the commercial tiny tach and is only a few months old. Dead simple to set up one lead to the HT lead and one to earth so think i have it set up right.

Engine ticks over at 900 and when i drop it down to 400-500 runs a bit rough.

As i said both gauges mirror each other.
I didn't mean to imply that you had not installed the Tinytach correctly, rather to check that you had the correct model of Tinytach, or (as yours is the commercial version) the correct setting for 'pulse per number of degrees rotation'. I assume this is set to 360?

I take it you are using the throttle stop screw to drop the revs to 400-500?

It just seems a bit odd that you appear to have the right prop but are only 15% down on top speed when you are 50% down on revs.

Cheers

Chris
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Old 21 July 2009, 12:19   #19
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Engine ticks over at 900 and when i drop it down to 400-500 runs a bit rough.
If it's idling at 900, how do you drop it down further?

400 to 500 sounds little low to me. I think my F115 idles around 700rpm (just checked: spec is 750 +/- 50.)

jky
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Old 21 July 2009, 13:22   #20
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34 knots at 2750 rpm on a 13" does point to the revcounters both reading wrong.
Nos is right one of those numbers is wrong.

Engine turning 2750 rpm will turn prop at 1/2 that (roughly without knowing the gearbox ratio but it will be +/- ~15%) so the 13" prop is turning at 1375 revs per min, and therefore in a perfect world with with no slip the boat would be doing less than 15 knots.
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