Topic: 1975 cb400f air pods. Why do people change the pilot jet too?

[Ban the Dead]

I bought the bike with air pods installed but stock jets.
Main is 75, i'm told to go to 80
I'm pretty sure when i took it apart my pilots were 40, i heard people usually go down to 38? Or is the reversed? how come they change the pilot jets too though?

No other modifications

[Tim2005]

They do it to make it run clean at light throttle openings. It isn't always necessary to change the pilots  - depends on fuel and the rest of the bike too, for instance mine runs a lot leaner low down with a Marving exhaust than with a Yoshi.

However, I have always found that if you go to pods you do need to change the main jets, to 80s or 85s, otherwise it will run too lean & cause problems as well as reduce the power output.

[TwoTired]

Each fuel jet is essentially in a tube, sort of like a straw with a pinched section.  A straw functions based on a pressure differential.  The volume of material drawn through the straw can be regulated by how much pressure differential is applied to the straw ends, as well as how much the tube is pinched.

PODs shorten the inlet duct, which raises the carb throat pressure at the outlet of the jet tube(straw).  The inlet of the jet tube has the same old pressure applied to it, which is outside atmospheric pressure (the stack of air pulled by gravity from the planet's surface to outer space), both before and after POD installation.  Since the pressure differential applied to the jet tube was changed with PODs, to return to the same jet fuel flow as prior to change, the jet orifice must be enlarged (like reducing the amount of pinch in the soda straw).

The pilot jet has an adjustment (air bleed) that provides some regulation to the outside atmospheric pressure reaching the jet tube outlet.  However, the range of adjustment available may be too narrow to provide full restoration of pilot jet fuel flow after pods are installed.  So, the jet must be changed in this case.  Since these carbs are supposed to run rich at idle to provide throttle response from idle under load, if the bike stumbles in this regime, and the air bleed screw adjustment can't correct it, then a larger pilot jet is indicated.


Another answer is:
Jet changes are punishment for putting cheap ugly exposed filters on your bike, and making the whole process more expensive in time and materials than what was originally on the bike.   

Cheers,

[Ban the Dead]

But don't they go from 40s to 38s? Doesn't that make em smaller and lean it out? or are pilot jets the smaller the number the bigger the hole? Main jets are bigger number = bigger hole yes?

[TwoTired]

For Keihin jets, the marked number relates to the orifice diameter.  100 = 1 mm, 40 = .40 mm.

I have no explanation why some would reduce the pilot jet size for pods on this engine/carb set up.
Perhaps they didn't understand how the air bleed screw worked, or they were struggling to meet some emissions test, or they were somehow trying to compensate for the main needle raised too high or a too large main jet.
My goal would be to make the engine perform its best at all throttle positions and throttle transitions.  While you CAN get there with random jet and adjustment changes, I prefer the predictive and/or measured approach.

Yours is the first post I've heard where smaller jets were desired by POD enthusiasts, and I am unfamiliar with your source of information.  The physics for this mechanical combo do not support your assertion, imo.

Cheers,

[Bodi]

It may sound strange and I can't explain it, but going to pods and a Kerker exhaust (plus some engine work, 466 kit and Yoshimura S1 cam) I had to go down to a 38 pilot jet from the stock 40. This cleaned up the low throttle performance quite a bit. I may have been told to do it or found the rich mixture at low throttle by doing plug chops (it took more than I like to remember to get the jetting to actually work well). Without the 466 kit you may not need it?

[TwoTired]

Upping the displacement does cause a need to reduce the pilot jet size as the engine draws more air (more vacuum) to feed that displacement.

To wit: The CB500 used 40 pilots, while the CB550 used 38.  Both used the same carb design, main jet size, and air induction set up.

Cheers,

[Psychonaut]

The CB350 uses a 38 pilot while a CB400 uses a 40. Both use the same carb. Different mains but same needles.

[TwoTired]

The CB350 uses a 38 pilot while a CB400 uses a 40. Both use the same carb. Different mains but same needles.

...But, do you know why?

There is more to the pilot circuit than just the pilot jets.  There is an air bleed screw tip design and setting.  As well as a pilot air jet orifice size.  In the 500/550 case these were the same between models and just the fuel jet changed orifice size with the displacement increase.  These models also used the same exhaust system and its associated pressures that effect engine fuel requirements.

I'm not quite familiar enough with the 350 to 400 carb particulars to know if the pilot screw tip design changed between models, or the setting, or the pilot circuit air bleed orifice diameter.  I do know the exhaust system changed.  And a simple change in pipe pressure could reduce or increase the fuel requirements for the pilot circuit.

Perhaps Psychonaut can enlighten us regarding all the design particulars for the 350 to 400 transition in order to clarify rather than obfuscate?