Carb jet size

[Murf]

I am sure this has been asked loads of time but I have looked through search and can’t find anything exactly like I have.

I am rebuilding Keihin carbs from 75 550K1, the main jets that are fitted are
No 1 115,
No 2 115,
No 3 100,
No 4 100,
Now obviously this isn’t right but for this bike it’s par for the course, nothing surprises me anymore.  However could there be any reason for it??? Apart from POs being dicks. Or maybe living at the top of an enormous hill and using it as a way to compensate for atmospheric pressure changes when going to the bottom for groceries 😄.

The parts book lists size as 98
So I would think the 100s are ok
What size is considered correct these days with modern fuels.

The slow jets are 38 as listed.

For No 2
Slide stamped 2.5  03
Needle number 27304  clip in second to bottom grove ie needle high.
Haven’t done the others yet will look at them tomorrow

[Nurse Julie]

It could have been to compensate for altitude but it's a strange way of doing it. More importantly, are the jets genuine Keihin, with the stylised K logo stamped on them or are they aftermarket? 98's and 38's are standard.

[McCabe-Thiele (Ted)]

There is chart on the subject iirc not sure if K2-K6 posted it.

I found my copy it's an old PDF document with some errors I believe.

[deltarider]

It could have been to but it's a strange way of doing it.

I fail to see how fitting wider jets could ever compensate for altitude...

[Bryanj]

Me 3, i thought higher was less oxygen so less fuel

[Skoti]

Can feel my CB750F1 struggle a bit when over in the high alps.
It feels like the choke is on a bit, probably because the air is thinner up there.

So smaller main jets, not larger jets would probably be the cure for constant high altitude riding.

But when riding my newer BMW with electronic fuel injection management system, the fuel mixture adjusts automatic so no problems there.



     

[taysidedragon]

I would guess that the PO just used what they could find, or was blind as a bat so couldn't read the jet numbers.

[Sesman]

Err..is that correct. The school I went to taught that the atmosphere is composed on 78% nitrogen, 21% oxygen and 1% other gases at all altitudes. The atmospheric density however lessens with altitude, consequently this is why the motor struggles and loses power. The solution is intake compression management (supercharger) and fuel injection.

I suppose a smaller jet might help to maintain the optimum combustion ratio, assuming of course that intake vacuum is accurately compensated for as this lessens as you gain altitude affecting both fuel volume and atomisation. Basically, for any given jet size fuelling will lessen as you gain altitude and power is reduced.

I think that’s correct, but welcome any comments.

[K2-K6]

I usually have to count backwards on my fingers to get my understanding in line with this  a little like my golf, as I total my shots up from how many zig~zag has been involved down one fairway.

Isn't it because air mass drops whereas fuel mass stays the same , causing it to move towards rich as the attitude accumulates. Then needing correction to bring equilibrium in AFR and hence combustion.

[Sesman]

Nearly. Atmospheric density falls and hence vacuum drawing fuel also falls, hence less fuel, less ‘air’ and poorer atomisation resulting in less power. I have no idea if the combustion due to these effects is linear, but I somewhat doubt it due to the dissimilar fluid characteristics of air and fuel….i think🤔

[Sesman]

Basically, lower air pressure requires smaller jets, but power will inevitably be reduced. Eg the air pressure at 4000ft is approximately 85% of sea level pressure. Your ‘starting point’ for a suitable jet size would be one with 15% less orifice area than standard. There are other factors such as temperature and induction dynamics (Fluid mechanics) that will influence the end result.

Regarding the original post. My 1977 550 with slightly modified inlet ports (general cleaning up and intake matching) with a dekelvic  4:1 and standard air box is using 98/38 and runs like a good un. Edit…my carbs are 069A variant.

[Seabeowner]

What are the carbs. You can see the stamped mark on no 3 flange sloping side. Should be 022A I think.

[Murf]

Thanks for all the replies, the atmospheric pressure and big hill were just a humorous dig at the mechanical knowledge and ability of POs, the odd jets fit perfectly with the other  bodges and crap that have been done to this bike.   Just my sense of humour as I am past the point of being shocked.

Anyway it did start a good discussion on altitude, and carb jetting for altitude .

But what I really wanted was info on jet size for standard conditions 😄😄.

Seabeowner, Carbs are stamped 022A .

[McCabe-Thiele (Ted)]

Have you looked at the table on my previous post, the table shows the jets & air screw turns.

Also depends on the model of your bike UK, USA as they were different iirc due to air cleaner systems fitted.

550 K1 should have 022A carbs see the table that shows 100 main jet, 38 slow jet.

[Bryanj]

Jf you download the 550 parts book from Ashs stash it is for the us 550k models and will tell you what is standard, if its not there i can look at my paper copy next week.
Also available is a dvd with Honda manuals and parts books from me for not much plus postage