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BF75/90 carbs - why aren't all 4 the same?

C

Cretster

Contributing Member
I was looking at the parts diagrams for my engine (96 BF75A) and I notice that it lists one part number for carbs 1, 2, 3, but then a different part number for carb 4.

Does anyone know why this is and if there would be a problem caused by running 4 carbs the same? I'm sure the numbers stamped on my carbs are the same for all 4 (will have to check).
Which one is considered as carb #4 as well? Top or bottom?

Many thanks!
Jim
 
Well, having never worked on one of these, I should probably just keep my big,fat, yap shut....but I can't! That's probably why I'm not rich. Well, that and other character flaws.

But, in my defense, one of the reasons I can't resist taking a whack at the question is that I, just the other day, received my brand new shiny copy of the Honda Carburetion Manual. Yippeee!
Soooo.... I figured I'd just crack that baby open and it would be there waiting on page 12 in black and white.... DOH! Not so! Not page 12 or 15 or 36 or 44 or nowhere! I could not find a pointed, emphasized explanation for the "one different" carburetor on the BF75A and 90 engines.

What I did find was a nice drawing of the carburetor linkage showing how the throttle shaft linkage adapter for the BOTTOM carb was different than the other three above it. The book goes on to explain that the BOTTOM carb is the "BASE" carb and, when performing the synchronization of the intake SYSTEM, it is adjusted first and the others are adjusted to match it. And that's about as extensive an explanation as I could find.

AND,....... that's a bit confusing to me....Because, looking at the parts page on boats.net, they depict the TOP carburetor as being the "different" part number. But, I'm sure there's a reasonable explanation for that but we'll probably need hondadude to tell us what's REALLY going on there.

See......I shoulda kept my big, fat, yap shut!

MIKE! Are you there??
 
Ha ha - cheers! You did better than me then since I also have the carb manual AND the full honda workshop manual (both proper ones I might add) and didn't even spot the details you'd seen!

Useful if even to know I need to start at the bottom when I hook up the gauges to them, cheers. :)
 
Oh, so you have the carb book?! Well then, check out pages 6-34 AND 6-42 where they lay out the 3 cylinder and the 4 cylinder engine carb sets, take a BEAUTIFUL picture of each set, LABEL each set TOP-2-3 and TOP-2-3-4 respectively....and...
...wait for it....wait...for...it...


STILL MAKE NO MENTION OF IT WHATSOEVER!

Well, if you're old, like moi, you know that technical writing for the mechanical trades stopped being a true profession back it the 1970's. I got a pretty decent, college level, education reading shop repair manuals for cars, trucks and buses back in the day because they were so thorough and well written. Today's publications of such things are only shadows of what used to be. Sorta makes me wonder what the up and coming shop mechanic is taking into the "library" when nature calls;~)
 
Today's publications are normally translations from Japanese or some other foreign language.

The bottom carb is different because it has the throttle cam on it, thus the part number will be different. The idle stop screw is also on it.

Internally, they are all the same.

As for carb balancing...the bottom is the reference. You do not set it other than turning the idle stop screw to adjust the idle as it varies during balancing. The other carbs are adjusted to match the bottom one (#4).

By the way, I am glad you are enjoying your carb manual. Lots of good stuff in there!

Mike
 
Ahhh, that all makes sense - thanks very much for the input and for clarifying this. Seems sort of obvious in hindsight! :)
 
jgmo said:
Well, if you're old, like moi, you know that technical writing for the mechanical trades stopped being a true profession back it the 1970's.
Click to expand...

38 (today as it happens) so I've missed out on the quality of stuff that you describe. I was weaned with Haynes manuals as I grew up, until I found much better stuff like proper workshop manuals.
No benefit of a career with mechanical stuff for me alas (it's what I wanted to do really), I'm an IT nerd by trade really.
But ever since my first motorbike and car (long before I could get a license - we used to race them round fields) I've been learning my own mechanical 'skills'.

This year I paid a garage to change the clutch in my car as I needed it done quickly. I think apart from tyres and tracking that's the first time I've paid anyone to ever do work on a car or bike for me in all those years so I must be doing ok I think. Quite a few engine builds etc in that time too, so this is all enjoyable stuff for me luckily. :)
 
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