Yamaha 50 Questions

gibbo500 · 18 July 2009, 20:44

Looking at the engine experts for this one.

Took the SR4 out today to put her through her paces however could only manage 34 knots as per GPS. The Yamaha digital tacho was reading 2700-2800 now surely that is way down on the recommended RPM range.

I know Nos had 40 knots out of his so would you say there is a problem with the gauge or is the prop fitted incorrect 11 1/3 x 13? Would 2700 rpm give me 34 knots?

Need some help guys as had a bad day on the water....hit a rock and snapped a chunk of the skeg off.....bugger.

Nos4r2 · 18 July 2009, 21:01

It should be quicker than that-but I don't think those revs are right for that speed either which suggests either a problem with the revcounter or the pulse being sent to it.

gibbo500 · 18 July 2009, 21:17

Quote:

Originally Posted by Nos4r2

It should be quicker than that-but I don't think those revs are right for that speed either which suggests either a problem with the revcounter or the pulse being sent to it.

Thanks Nos,

I have a tiny tach which i could try but if the gauge is ok how is the pulse problem rectified?

Nos4r2 · 19 July 2009, 01:51

Quote:

Originally Posted by gibbo500

Thanks Nos,

I have a tiny tach which i could try but if the gauge is ok how is the pulse problem rectified?

It'll just be a poor contact if the pulse isn't getting through properly.

gibbo500 · 19 July 2009, 06:15

Quote:

Originally Posted by Nos4r2

It'll just be a poor contact if the pulse isn't getting through properly.

Thanks mate, will have a look.

gibbo500 · 19 July 2009, 09:36

Quote:

Originally Posted by Nos4r2

It should be quicker than that-but I don't think those revs are right for that speed either which suggests either a problem with the revcounter or the pulse being sent to it.

Nos,

Just wired up the tiny tach and revs mirror gauge.

Do i need to start playing around with the prop?

Many thanks.

Nos4r2 · 19 July 2009, 09:51

Are you sure you've got the prop pitch correct in your earlier post? It should reach a hell of a lot more RPM with a 13" pitch on there.

How rough was it when you tested it?

gibbo500 · 19 July 2009, 10:23

Quote:

Originally Posted by Nos4r2

Are you sure you've got the prop pitch correct in your earlier post? It should reach a hell of a lot more RPM with a 13" pitch on there.

How rough was it when you tested it?

Just checked and it is stamped 11 1/8 x 13 -G.

It was quite calm and just me that's why i was expecting a bit more.

Nos4r2 · 19 July 2009, 10:35

Most odd.

Might be worth servicing the engine if it's not been done recently. Check it's running on all 3.

chris123 · 19 July 2009, 10:38

Quote:

Originally Posted by gibbo500

Nos,

Just wired up the tiny tach and revs mirror gauge.

Do i need to start playing around with the prop?

Many thanks.

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

Nos4r2 · 19 July 2009, 10:46

Quote:

Originally Posted by chris123

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.

donegaldan · 19 July 2009, 10:55

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

Nos4r2 · 19 July 2009, 10:59

Where have you guys got your fuel tanks?

gibbo500 · 19 July 2009, 11:04

Quote:

Originally Posted by Nos4r2

Where have you guys got your fuel tanks?

In the console.

Nos4r2 · 19 July 2009, 11:08

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.

gibbo500 · 19 July 2009, 11:16

Quote:

Originally Posted by chris123

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.

Nos4r2 · 19 July 2009, 11:30

Quote:

Originally Posted by gibbo500

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.

chris123 · 19 July 2009, 11:37

Quote:

Originally Posted by gibbo500

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

jyasaki · 21 July 2009, 16:19

Quote:

Originally Posted by gibbo500

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

Poly · 21 July 2009, 17:22

Quote:

Originally Posted by Nos4r2

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.