Difference between carb'd BF75/BF90 models?

Hi all, I was told by my engineer a while ago that as far as he knew the only differences between the carb running BF75 and BF90 engines is the carbs, and the stickers on the engine cover!

He was keen to point out that he could state that as absolute fact but was what he believed to be the case, and I've never dealt with a more thorough and knowledgeable man than him, so I put a lot of merit on what he says.

I have a 1996 BF75 and would like to keep my eyes open for a set of BF90 carbs if this is true, to give me a bit more grunt, but would prefer not to have to fork out for them if fitting the 90 carbs is only part of the picture, or would make it run worse without other modifications etc.

Not wanting to 'tune' the engine as such as I will stick to cars and bikes for poor reliability as it were, but if the carbs is all it needs to make it basically a factory spec 90 I would really love to do that.

Would be really grateful if anyone is able to confirm or debunk this?

From memory, I think the more & stroke of the two engines is the same so it would go a little way to supporting the idea, but I'm not sure if they run different ignition in any way etc.

Many thanks!

Hello Cretster. I read your other post regarding the differences between the BF75 and BF90 carbs with great interest. I’ve just dismantled my BF75A (European 1998 model) carbs to do some cleaning and might do some modifications towards the BF90A range.
My carbs are marked 35AA-MK27. I measured the venturi and its 24mm, which is very small for a 1600cc Engine. 24mm venturi is actually pretty small for 35mm carb. I’m thinking of widening the venturi to 25-26mm and increase the main jet. Looking at the carbs design, this should not directly affect the slow jetting. I not afraid to give it a try, but I rather win than fail.
Do have the venturi dimensions of your BF90 carbs? Any detailed pictures? I’m happy to share the progress with the forum if I do these modifications. Thanks for any more input regarding the BF90A carbs!

Crester,

It may be old age, but did you not already go through all this? http://www.marineengine.com/boat-fo...BF90-models-basic-info-needed-please-re-carbs

Mike

have no idea, but hope you find the good answer.：O型

I'm with hondadude, deja vu...all over again. Didn't it come down to 4 primary orifices vs. 3?

Hi all. Yes I read the other threads. If it waseasy/possible to modify the BF75A carbs into BF90A carbs that would probably bethe safest way to go. We now know that Cretsters BF75Acarbs is different from his BF90A carbs. Personally I’m preparedto gamble a bit with my BF75A carbs. These carbs are quite simple in theirdesign. The idle/slow jet ports are close to the butterfly valveand the main jet port at the fixed venturi. On my carbs I've got 4+3 primaryports in the top of the carb. These primary ports should not be affectedif I make the venturi larger. It's not safe anyhow, I could get introuble in the midrange, where the slow jets are maxing out and the main jet iskicking in. These Hondas are jetted on lean side. My sparkplugsare white, not light Brown, indicating they are running hot and lean.The BF75A service manual has a interesting modification point in the"shop manual news" chapter, that is changing the Main Jetfrom #128 to #135 and the Primary Jet from #38 to #40. After these modificationsthe pilot screw should be set to 2-3/8 out instead of 1-7/8. Those, by Honda recommended,modifications should make it run richer and perhaps give it a couple of moreHP. I found the service manual here:
http://boatinfo.no/lib/honda/manuals/bf75bf90.html#/346