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Honda BF50A & BF40A comparison

bmousser

bmousser

Member
Hi,

I wanted to ask any expert available about comparing the BF50a & BF40A differences in the cylinder head. I spoke to several mechanics here and basically got that the only difference they know of between both of these motors were the jet size. Is this true? When exploring on Boats.net (which I like their exploded view of the parts!) I couldn't help but notice that some of the Honda part numbers are identical (by Honda part #) between the 40 & 50. Using my common sense thinking this would be practical in terms of manufacturing and cost. However, I notice that the intake valve part # were different between the 40 and 50. This would mean that the intake valve between the 40 and 50 are different (why would they give an identical part 2 different part #?).

So I'm thinking that the cylinder head between these 2 motors are not inter-changeable given the fact the intake valve part #'s differ from each other and probably have different specs (diameter I'm thinking). It makes sense to when you think in terms of horsepower. Would anyone care to elaborate. Hondadude, pleas educate me................
 
bmousser,

There are a few things that differ between the 40 and 50....

The intake valves
The cam assembly
The carburators, associated jets, and idle mixture screw
The dashpot check valve on the carburator assembly
The CDI unit

As you have surmised, the carbs and intake valves feed a little more fuel to the cylinders.
Most likely, the change in cdi unit changes the timing or duration or both of the spark (just a guess on this)

I would not use them interchangeably. I can not be sure what the result of the additional intake valve action without the additional fuel coming in or the "adjusted" spark. Maybe nothing. If I do not know, I do not take a chance.

Jimmy D may have some theory on this. He usually has some good words of wisdom especially when it comes to operation theory.

Hope this helps.

Mike
 
Thanks you guys. I thought so as well by just researching and comparing parts. Thank GOD I'm not left here in Hawaii all by myself. I'm truly thankful for forums like this that brings the world together!

Thanks again.
 
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