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Old 18 March 2014, 17:47   #1
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Country: UK - Wales
Town: monmouth
Boat name: Bobalong
Make: Aquapro sport
Length: 3m +
Engine: Honda 9.9 4 stroke
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Honda 4 stoke 9.9hp, more power please!

I've been saving up some money to upgrade to a 15hp for the small rib we have. Unfortunately house hold running cost have meant that this is not now possible and the money has been spent on boring things! I've read lots about getting more power from this engine but don't know what to believe. Has anybody got more power from this unit. If so how and what parts does it need.
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Old 18 March 2014, 18:59   #2
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Country: Other
Town: Lima-Peru
Boat name: Nautile
Make: Sea Rider Boats
Length: 4m +
Engine: Tohatsu 18 /30 HP
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Posts: 1,706
A 9.9 HP is a 9.9 HP how can you bump more HP to it. You can check if 15 HP has same CC compared to a 9.9, if so, probably will neeed to change carb, make timing adjustments. If not, start saving again for a larger HP engine.

That's probably same detuned 15 HP model. A matter to check with parts manual for parts compatibility between both HP models.

BTW, whay do you need more power, poor planing issues, too large family, overall poor water performance ? Which type pf boat is it ?

Happy Boating
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Old 30 March 2014, 18:36   #3
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Make: humber destroyer 5m
Length: 5m +
Engine: 90 yamaha
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After a very quick google of your engine;-
According to hondas website a 9.9 is 222 cc
and a 15 hp is 350 cc (or thereabouts)
so they are completely different engines.
You are unlikely to be able to get any meaningful extra power out of yours without spending more money on it than it would cost to sell it and buy a bigger one.
(In my humble opinion for what its worth)
However described and listed well and presented in clean condition on ebay in May or June
you might easily get as much or more for your engine than you can buy a badly listed 15 hp for !
I find it best to buy outboards in January and sell them in June.
I went from a 40 mariner to a newer 90 yam like that with a seventy evinrude in between for about 500.
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Old 08 April 2014, 19:35   #4
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Country: UK - England
Make: F-RIB 360
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Jim, owning the same BF9.9 as you got me thinking just what exactly are the differences between this BF9.9 and BF15 motor?
After going through the Honda workshop manual and checking the Honda website I can confirm the following:
Both have a total piston displacement of 280cm and bore and stroke of 58X53mm.
All the components including camshafts, valves, carb, ignition system, etc are all identical and both units weigh exactly the same as far as I can tell.
The only differences I'm able to ascertain is the BF15 uses a different main jet within the carb and uses a different spark plug.
The BF15 can achieve a max 6,200rpm with a #108 main jet using a NGK DR-6HS spark plug.
The BF9.9 can achieve a max 5,000rpm with a #102 main jet using a NGK DR-5HS spark plug.
Am I wrong to assume providing the extra fuel=extra enrichment at higher rpm's=the extra HP achieved=the 'harder' spark plug required?
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Old 09 April 2014, 15:59   #5
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Bear in mind that it's usually not a larger carb jet swap, but a complete carb swap itself. The larger HP carb has wider internal fuel passages which lets more fuel in. If you change just larger main jets, engine will bog down when giving throttle. Check if both engines have same carb code for larger jet swap, if not, to each his own carb model.

With respect to plugs difference 5 being colder, 6 being slight hotter than 5, but both next to each other on the thermal range.

Happy Boating
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Old 12 April 2014, 12:16   #6
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Thanks for your input Locozodiac.
Went to talk with a chap that has worked on these engines for many years and he confirmed the carb body to be the same. He told me that fitment of the larger jet might improve hp by 1-2 max but you'd have to replace the ECM/CDI unit for the type fitted to the BF15 to allow the engine to gain the increased 1200 rpm required to achieve the 15bhp rating.
Pity as I had a new 175 ECM/CDI unit fitted a while ago and could've 'up-graded'.
Hopefully this info is beneficial to Jim and others.
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