clicking sound @ max RPM

mpnick · 06 October 2011, 03:33

I have a new Tohatsu 20 hp on a small rib. My first trip out with it was flawless. Three man crew, good sea condition we saw 27mph, with the stock 9 1/4x10x3 prop.

Second trip was with a two man crew. A new prop installed, 9 1/4x9x3 stainless prop. Now at WOT I hear a clicking sound coming from the engine. If a back off the throttle a small amount the sound is gone. I still can hit 27 mph but I have to back off the throttle. I then go back to shore in order to reinstall the stock prop.

Try another run and the sound is almost completly gone. Just a very small click or two after holding WOT and max MPH. I did not have my hand held tach with me so do not know what RPM we are hitting.

I pulled the plugs and they are fine. Good color and no signs of detonation, plus a run the highest octane that is sold here, 93 octane. My thinking is the I am hitting the rev limit. The first trip out was with threre people on board so the boat did not rpm as high? The second trip was with two people and a smaller pitch stainless prop. I just instelled a new 9 1/4x11x3 stainless prop. I will try it out this weekend. What do you people think?

Thanks

Nick

captnjack · 06 October 2011, 05:43

Most outboards don't have a governor limiting RPMs. So I would guess that's not it...

Possibly the alternator changing tone/pitch? It must be fairly loud to hear it at WOT no?

jyasaki · 06 October 2011, 15:29

Quote:

Originally Posted by captnjack

Most outboards don't have a governor limiting RPMs. So I would guess that's not it...

Maybe not smaller engines. Pretty sure my Yam F115 and upwards all do. Usually doesn't make a sound (as I recall); just gets sluggish.

jky

captnjack · 06 October 2011, 15:50

Quote:

Originally Posted by jyasaki

Maybe not smaller engines. Pretty sure my Yam F115 and upwards all do. Usually doesn't make a sound (as I recall); just gets sluggish.

jky

Do you know what its set at? Presumably a little above the 6,000 rpm "max" RPM for a F115? With electronic ignitions I guess smaller engines might have them too, although I would guess the OP doesn't have a tach anyway.

mpnick · 07 October 2011, 04:04

The motor does have a rev limiter for sure. I am new to the marine world, so I am still getting up to speed with it. I my world if you run a CDI box you more then likely have a rev limit of some kind. It is a pull start engine, no stater or alternator to make any clicking sound. Yes you can hear it over the normal engine sound. Almost like a piston knock, hope it is not that. I will see if the bigger pitch prop works.

Thanks

Nick

Leapy · 07 October 2011, 06:01

Quote:

Originally Posted by mpnick

I have a new Tohatsu 20 hp on a small rib...I hear a clicking sound coming from the engine...What do you people think?

It's new...I think you should go back to the dealer

jyasaki · 07 October 2011, 15:35

Quote:

Originally Posted by captnjack

Do you know what its set at? Presumably a little above the 6,000 rpm "max" RPM for a F115?

As I remember, I turned about 6200-6300 rpm with a way-too-short prop. Didn't do that for very long.

jky

mpnick · 09 October 2011, 23:08

Update.

I went back out today with a new 11 pitch stainless prop and just me, no kids in the boat. I set the trim on the third hole from the transom. First try out of the box the GPS shows a new top speed of 32 mph. I tried running from one end to the other end of the bay every way I could. Every time it maxed out at 32 and it gets there and stays there right away. Big gain from the 27 with the stock 10 pitch prop. The clicking sound is amost all gone. If a back off just a small amount of throttle the sound if gone completely.

I then adjust the trim to the second hole from the transom and retry the runs. This time the max topend MPH is 30. Every was across the bay is the same, 30 MPH max. Less topend but the clicking sound is all gone now. I can hold the throttle at WOT the whole way across the bay, no sound at all.

I still do not have my hand held tach to see what the RPMs are. I need to get it on to see where I am. I will also have to see if the kids added weight while running at the third trim hole will keep under the rev limit. I will let you know. The way picked right up makes me think if the boat could handle a 12 pitch prop, if it is light load?

Thanks

Nick

spartacus · 10 October 2011, 08:51

Out of interest - are you borrowing or buying your stainless steel props?

mpnick · 10 October 2011, 14:41

They cost about $128.00 with shipping. Anybody need a used one with less then 2 hours on it?

Thanks

Nick

BTW, cool name, loved the movie.

jyasaki · 10 October 2011, 15:47

$128 for a stainless prop???

My backup aluminum was nearly that much.

jky

mpnick · 11 October 2011, 02:40

Small 20hp Tohatsu 4 strokes, very cheep props.

mpnick · 11 October 2011, 02:53

More good news today. I got to go out with a full boat. Myself and my three kids, I have been told that they are mine anyway. Set the trim to the third hole from the transom. Full throttle all the way across the bay. We hit 29 mph most of the time. There is no sound at all coming from the enigne. The added 275 pound did cost me 3 mph but the engine is not hitting the rev limiter any longer. It does take about 7 seconds to get on full plane with this load. With only one kid it is under 3 seconds. I need to get the tach on it to check the RPM.

Thanks

Nick