bf40 Honda, converting to bf50


Old 06-29-17, 10:47 PM
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bf40 Honda, converting to bf50

A quick google search shows me that this is a common question and nobody really knows the answer. My goal is to find out. I have a 2005 honda bf40 4 stroke. I want to convert it to a bf50 for the 25% power gain. The motors are the same. I went through an illustrated parts list for both and the engines are the same, from the bore and stroke, the camshaft, the valves, everything internal, all the same.

I checked the CDI, both take the same unit. The intakes are the same, carb plates are the same, the only difference I found is the carb numbers are different. The exhaust is even the same. Now all I can assume is that the venturi size on the 40hp engines must be smaller than the venturi on the 50 in order to less air/fuel mixture through the same carb plate and intake manifold. I think the carb is choking off the potential for this engine and a set of carbs from a 50 will be the only thing I need to make the change. Any input?

If anyone is in the know on this, please speak up before I spend money to find out I'm wrong the hard way. Thanks!
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Old 06-30-17, 04:24 AM
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If the Honda rating relates to displacement is it possible the larger engine has a different head with a slight dome that would create more displacement.

I solved a similar question when I purchased a small outboard motor for our freighter canoe.

The choice I had was between a 2.4 or 3.5 hp four stroke Mercury outboard engine.
The specifications relating to bore and stroke and the part numbers for the carburetor are identical between the two motors.
The difference is that the 2.5 hp is rated in the range of 4500-5500 rpm and the 3.5 hp at 5500-6000 rpm!
I suspect the difference in Honda motorcycle is rating is the same.

IOW I believe it's a clever marketing trick which in my case worked.
I thought it was worth a couple of hundred dollars to have the higher HP sticker on the motor and the ability to make the engine scream if I needed that bit of extra power.

Another marketing trick relating to this outboard motor is the fact that Mercury does not make any outboard motor under 30 HP.
I could have saved even more if I had bought the exact same Tohatsu motor who is the manufacturer of Mercury Marine's smaller motors.
Old 06-30-17, 10:28 AM
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Yeah, I had a 9.9 johnson that was the same as the 15 johnson other than carbs and exhaust. The swap to make more power was pretty easy.

The head is the same on both the 40 and 50 hondas that I'm comparing, I can't see any other difference. The 40 won't rev as high as the 50, which likely accounts for the hp difference just like your 2.5 and 3.5 mercury comparison. Some motors have a limiter on the throttle cam but that part is also the same between the two motors I am comparing. I think the bigger carbs are allowing the engine to rev higher on the 50. Right now I am borderline under-powered with a full load, so that little extra might be just the ticket.

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