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Calling owners of older BF90 models - basic info needed please re' carbs

C

Cretster

Contributing Member
I've just bought a set of BF90 carbs (am unsure of the year engine they are from), but I'm slightly uncertain if one of the carbs is not the correct one for a 90.

I might be wrong, but if it's possible for anyone running a carb (non efi) version of a BF90 to confirm a really easy detail for me I'd be really grateful indeed.

The carbs on these engines all have a number stamped on the top of each carb where it meets the airbox. It's pretty easy to see with the engine cover removed.

Now the numbers on these BF90 carbs that I've bought are as follows (in height order of how they are positioned on the engine):
40AA - NG22
35AA - ML01
40AA - NG22
40AA - NG07

I'm a little surprised to see one of them being a 35AA prefix rather than 40AA prefix, since I sort of understood that 35AA is the carbs used on the BF75 and 40AA are the ones on the BF90.

I think it's normally the case that the bottom carb is different to the other three (or often the case - not sure why really) on these engines, but it just seems odd that the second from top is a 35 series carb.
As I understand it, I think this number represents the throat/venturi size of the carb, so a 35 would be physically more restrictive than a 40 in that case.

If any of you have a carbed BF90 and are able to advise the numbers on the carbs your engine has I'd be really grateful. If even just one person can confirm their own engine is similar to this then it'll put my mind at ease, otherwise I'll be worried that one of these is a smaller carb that might result in a dangerously lean burn on the corresponding cylinder or suchlike.

I know that the carbs are the only difference between the 75 and 90 carb engines, so in theory a 75 carb wouldn't matter too much as far as I can tell, but I don't think it's a good idea to have one different to the others unless that was the original intention.

Honda are useless at confirming this sort of thing. I tried asking them ages ago to confirm some carb information for me and despite getting a few helpful sounding replies from them, they never actually gave me a straight answer to my questions and tried to just pass me around people.:(

Really appreciate any info on this anyway, and the more owners that can reply the better since it might be the case that one person's engine year has all the same carb model, but another year model might not etc.

Cheers!
 
The carb manual says that the 35 = 75 HP and 40 = 90 HP. It appears that you are right...you have one 75HP carb.

Mike
 
Thanks for the reply Mike even if it's really not what I wanted to hear. :(

I can't believe the bad luck I'm having trying to just get a good set of 90 carbs. It's beyond belief!

so the questions I've got now then,
1) will it do harm running it with that setup? After all, that engine is built to run on either carb to determine whether it's 75 or 90 hp. Would this just make it an 86.5 or whatever?!
2) where am I going to get a single used carb that matches the 40aa - NG22 ones if I need it without buying a brand new one?

Bugger :(
 
I mentioned this scenario to a mate and he says that the smaller carb (with less power output) will put an uneven load on the crank. It's a valid point that hadn't immediately occurred to me admittedly, although that would only be the case surely if more than one cylinder fires simultaneously? I don't know if this engine does or not offhand.

So if they fire in pairs, then you'll have 2 cylinders firing with the same power as each other, then the other 2 with uneven power and that would indeed create an uneven load on the crank between them.
If the difference is on the middle 2 bores then of course that's significantly less imbalance, but still there all the same. I guess at some level this would cause uneven shell bearing wear around the crank to some degree. The extent of that would be hard to know without finding out from actually doing it.

If they fire individually though like an older distributor engine would commonly fire 1-3-4-2 order etc, then surely there's no mechanical issue with that, other than a power output that varies ever so slightly between engine revolutions?

Bottom line is that I'm not keen on it having one from a 75 and I don't really intend to run it like that - after all, it's not how the engine is intended to be, but from a theoretical engineering perspective it's interesting to think about.

On another point, certainly when it comes to bike carbs, I'm aware that a lot of people 'bore out' carbs to a bigger size when they have the smaller of a range of carbs that are presumably the same base carb but with less material machined out or whatever at the factory? I have a KTM that uses a Keihin FCR-MX39 carb and a lot of people bore this out to turn it into the FCR-MX41 version quite successfully, so I wonder if that could just be done with this 75 carb, then the jet sizes set appropriately....

Need a real carb expert on the case here. Anyone know any?!
 
Nope. But I will say that with the KTM carb it appears you're altering one 2 millimeters and with your Honda it would be 5mm. Big difference. Attainable? I guess you'll need to find that carb expert.
If you keep haunting Ebay, I bet you can find a replacement for the 35 and may be able to sell it there as well. Kind of a hassle for sure but this old internet thing makes doing stuff like this much easier and doable than in the bad old days.
I live on the west coast and see Honda parts motors all the time. I'll keep a weather eye out for a used one for you if you'd like.
jimmyd
 
Appreciate the link jgmo - thanks!

Not sure if they'd be willing to post to the UK but it'd cost a lot to send that whole set, and the problem I've got really now is that I need to get a single one from somewhere rather than a full set. Can't afford to buy a full second set sadly, so I suspect I'm going to be waiting a very long time to find a single one.

Jimmyd - I think you're probably right on all counts.

Was looking (after posting the original comments) online at KTM keihin carbs that had been modified to see what was being done, and while it might not have been the case for how they're all done, the ones I saw didn't have a lot of meat left on them afterwards. A carb is a pretty complex piece of kit anyway I guess, and unless it's a modification that's frequently done successfully I wouldn't want to be a guinea pig as it were.

I have been looking on ebay for quite about 6 months now at least I think, and the number of 90 carbs that have come up has been not many at all in that time, combined with really bad luck on my part. Normally with ebay I get really good deals on stuff and I've been using it many years now with very few issues but when it comes to getting a good set of these carbs I seem to be finding all the wrong people somehow.

First I found a full set of "90" carbs complete with manifold & linkages so I bought that, and was instantly suspicious when I saw all 4 carbs were 35AA models the same as on my two 75s. I couldn't confirm this though as I couldn't find a reference for these numbers at the time, so I had to check the jet sizes, which confirmed they were definitely for a 75. The guy claimed he'd had them on his 90 for 5 years and so on, but that just means (if true) that he was running a 90 converted to a 75 whether he knew it or not.

Eventually got a refund and missed a 2nd set in the meantime.

Then I found a BF90 being broken and agreed a price for the carbs with the guy. Asked him if he could confirm the number stamped on the carbs to avoid any potential issues like I'd had before and he assured me he'd do this (a 30 second job) plus take photos etc etc, then 2 weeks later proclaimed he didn't have time and if I'd trusted him I'd have had them by now etc etc. Think I actually dodged a bullet there as I know he already had them off the engine and only needed to look at them and tell me the number.

So finally found another set, and it now turns out for reasons unknown that one of them is off a 75 again.

I've got a full spare BF75 so I have plenty of those carbs - why is it so hard to get the 90 ones!!!
I would have just got a 90 in the first place if the option had been there, but there's very limited choice where I am (fairly isolated) so I'm having to try doing this to get the extra power spec (before someone just asks "why didn't you just buy a 90 if you wanted that?").

Don't get me wrong, my boat should push ok with 75 but 90 would definitely be preferable.
So yeah, it's getting pretty frustrating. I usually have better luck so I suppose I shouldn't grumble too much.

I'll stick a wanted ad up on a few forums, but I can't see many people wanting to sell a single carb from a set, plus in the UK there are fewer options without paying a ton of postage for one from overseas. :(
Cheers
Jim
 
Well, the shortage of 90 hp carbs probably stems from the fact that every "bloke" that owns a 75 will snap them up if he has a chance.
 
Could be that I guess, but I'd be surprised if most people running a 75 even realise that's the only difference between the two.
 
Really? I'm surprised that the ones on my pair of 75s are 19", and I think that'll be too much for it to rev properly, but we'll see.
Since I have a complete spare engine I can sell one prop and get a 17" one to try if that's the case.

Handy to have.

Anyway, now that I've spent most of my free time for the last few months getting the boat ready to test the weather has gone bad now for autumn. Typical.

Oh, and don't forget it's not just the carbs after all. It's the stickers on the engine cover too!;)
 
In case it's of any use to anyone in future, you cannot modify the BF75 carbs to turn them into BF90 carbs.
Interestingly, the 90 and 75 carbs appear to share the same fundamental castings/body, but with some notable differences:

The jets
The number of primary ports
The bore through the carb
An air jet pressed into one of the airways
Potentially part of the external fittings of the throttle butterfly shaft (this might have been a variation with my 75 carbs being UK ones and the 90 carbs appearing to be from another region, evident by the shrouding around the mixture screw).

I had someone machine out a 75 carb for me to match the bore dimensions of a 90, then fitted the correct jets etc. All was going well until I realised to my dismay that the 90 has 4 primary ports (pin prick size holes in the bore) whereas the 75 only has 3. Cannot realistically add a 4th port so this would mean less fuel than it should be getting. Also the corresponding air jet inside the top of the carb would not be correct.

Possibly this might help someone else looking up the same info in future.

In any event, I had to get a brand new 90 carb to match the other 3. Have installed and sync'd them, and it appears to run nicely. In a barrel at least!
 
That would have probably bitten me just like that too. OUCH!
Hope the machining didn't cost too much. Thanks for the blow by blow replay!
 
Yes thanks mate, I did end up with one brand new one and 4 old ones but recently bought another set of used ones and it runs way better at idle etc now as they're in better nick and do not have the same wear on the float pins which I think had caused flooding occasionally.

I don't think the 40BA thing really matters much though to be honest as it's probably some very tiny revision and a slightly newer type but potentially improved slightly in some fashion.
If they're ones you don't really need then you should be able to easily sell them to someone with a 75 as an upgrade set. Made a fantastic difference to mine.
 
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