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Old 06 August 2019, 09:50   #1
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Outboard knock on full throttle, any ideas?

So ,I recently purchased a Honda BF5A 2009 but have noticed that it makes a knocking sound when Iím planning at full throttle so I have to back off a tad. It also seems slightly underpowered, it seems to only just get on plane with me (11 st) on my zodiac 275 RIB.

Iíve been running with new fuel and a new plug.

Any ideas would be greatly appreciated.

Cheers in advance,

Dan
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Old 06 August 2019, 11:12   #2
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A 5hp is right on that will it won't it plane situation so yes it is low powered. But exactly what is your Zodiac... a GRP hull RIB or an air floor SIB?? And what HP is it rated for?

Regarding the knock it shouldn't do that... but are you sure it's the powerhead or gearbox or just something ancillary rattling? For example my outboard lock can rattle at certain speeds to the point you'd say the engine bearings had gone... but just put your hand on it to damp the sound and it's quiet.
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Old 06 August 2019, 11:29   #3
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You need to check it out...but, I have a Honda 2.2hp outboard and a Honda generator and they both knock like buggery on full throttle and full load.
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Old 06 August 2019, 12:43   #4
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Quote:
Originally Posted by Fenlander View Post
A 5hp is right on that will it won't it plane situation so yes it is low powered. But exactly what is your Zodiac... a GRP hull RIB or an air floor SIB?? And what HP is it rated for?

Regarding the knock it shouldn't do that... but are you sure it's the powerhead or gearbox or just something ancillary rattling? For example my outboard lock can rattle at certain speeds to the point you'd say the engine bearings had gone... but just put your hand on it to damp the sound and it's quiet.
Hi mate, yeah itís a proper rib with v keel so should be easy to plane I would imagine. Re the motor, it sounds like itís from the motor itself, its a consistent knock when Iím full throttle planning but I guess it could be something else, annoying thing is it only does it when it planning (should of mentioned that before!) so I canít even get near the engine to listen for it as I have to be up the front of the rib.

Cheers
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Old 06 August 2019, 12:44   #5
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Cheers Jwalker, atleast I’m not the only one with this issue haha hopefully one of us find a solution/ reason for it happening.
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Old 06 August 2019, 14:23   #6
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.. hopefully one of us find a solution/ reason for it happening.
IIRC both my Honda engines have a fixed ignition timing of over 20 degrees so I think they're a bit advanced for full load and full throttle, if I back off the throttle slightly they soften up noticeably. When starting them I have to be very deliberate and not hesitate on the cord pull cos they fire early and will rip the cord out of my fingers and it fekin hurts!
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Old 06 August 2019, 14:28   #7
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I'm shocked... see para 4 on page 47 of this instruction book...

https://cdn.powerequipment.honda.com...s/31ZV1634.pdf

I'd really not want to operate an OB at sea that does this and I'm amazed Honda think it's "normal".

Have you tried Super Unleaded?

What's the RIB rated to... 8hp... 10hp? Might be nice to look towards a slightly larger motor.
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Old 06 August 2019, 14:54   #8
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Can’t seem to open that link

I did wonder about super unleaded, I will give it a go! Also been told esso supreme is the best because it’s ethanol free.

The RIBs rated at 8hp but it’s really old and only a 2.7m so I wonder if they mean a 8hp 2 stroke which would be much lighter.
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Old 06 August 2019, 14:56   #9
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Interesting! I’m not very engine savvy tbh but I’ll have a read on that. Mines the same, if I back off a tad the knocking stops.
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Old 06 August 2019, 15:02   #10
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Have you managed to open it now. Even if I find it weird it does seem to say it is "normal" for that engine.
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Old 06 August 2019, 15:42   #11
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Just had a read! Doesn’t sound great does it! I’ve ordered a new fuel pump and will get some fresh decent fuel + clean the carb. Will let you know how it goes
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Old 06 August 2019, 16:17   #12
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If the sound at wot is normal for that motor and need to plane faster with a shorter size Rib need to prop it right for motor to run to its full wot rpm range factory stated.

For that venture will need to install an induction tach and make 2 water tests. One lightly loaded solo boating, the other with max 2 up and check achieved wot rpm for both scenarios, will know how many wot revs the motor is revving up with one and two up and how far off the motor is from ideal max wot rpm range in each boating scenario.

I run a 380 Mtr Sib with a 2 strokes Tohatsu 5 HP motor, with factory delivered prop was mission impossible to plane with just me, I'm a 100 kilo boater, installed an OEM 7 pitch prop from current 8 pitch and now the combo planes fast at 6000 rpm, keeps on plane at 3/4 throttle. With 2 heavy guys won't plane, but will achieve a nice fast horizontal displacement speed without straining the motor heavily at 1/2 throttle. If throttling more the combo will achieve a up bow attitude, increase hull drag exponentially and motor sound which is useless.

Happy Boating
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Old 06 August 2019, 18:40   #13
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Just had a read! Doesnít sound great does it! Iíve ordered a new fuel pump and will get some fresh decent fuel + clean the carb. Will let you know how it goes
Just use it, it's what they do. However, flat out full throttle for 100% of the time is probably not the wisest way to go.
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Old 07 August 2019, 05:35   #14
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>>> However, flat out full throttle for 100% of the time is probably not the wisest way to go.

Yep I guess that was my feeling... if that 5hp needs to be driven at the point of knocking for a large proportion of a trip then going up in HP is probably wise... but if a fuel change or backing off a fraction without falling off the plane sorts it then fine.
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Old 07 August 2019, 14:50   #15
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Thanks so much for this info! I’ve been watching YouTube videos today explaining prop pitch etc and it’s really interesting and almost definitely where I am going wrong. Sounds like my motor is just hitting the limiter on WOT so I think I’m going to look at a higher pitch prop. Will test with tach too, looks like I can get a cheap one from eBay.

My current prop is 7 7/8 x 6 3/4 and available props are:

7 1/4 x 4 3/4 (58130-ZV1-841ZB)
7 7/8 X 6 2/3 (58130-ZV1-940ZB)
7 7/8 X 6 3/4 (58130-ZV1-941ZB)
7 7/8 X 7 1/2 (58130-ZV1-840ZB)
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Old 07 August 2019, 14:51   #16
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Yeah will let you know how it goes Fenlander, I’ve just cleaned the carb jet, ordered a new fuel filter and will try with different fuel.
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Old 27 August 2019, 06:14   #17
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Mine does the same; Mariner 15 hp fourstroke. A tad less throttle and it's gone, so that's the way to go.
However, with 2 people aboard it doesn't knock. So I think it has to do with overspeed or something. I'm not going to reprop for the toy boat.
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