pod filters and jets

[Pat Conlon]

  "....... When it speeds up, it creates a higher vacuum which initially picks up more fuel (kind of like an acceleration pump on a Holley)......"  

Hey David, just a quick comment: I'm not sure that is a good analogy.
The Holley accelerator pump is a mechanical pump which (for a double pumper) squirts fuel down the primary and secondary barrels.
It does not use vacuum. You can tell when the engine is off (no vacuum) look down the throat and quickly open the linkage.
See the 4 little squirts of gas coming from the center of the carb? That's the accelerator pump. It's mechanical and works with out vacuum. That's why you can flood your engine by pumping the gas pedal, all those little pesky squirts...

The Mikuni TMR and Keihin FCR flatslide carbs have accelerator pumps. Crank the throttle a couple of times on those puppies and you can flood your engine real quick. The Mikuni FS and CV carbs do not have pumps.

Thank you for your continued explaniation on the functions of our CV carbs. Now I know why they call them "Constant Velocity".
I hope you don't mind but I saved this explaination in our carb files.  Pat

[SlowOldGuy]

Pat,
The accelerator pump analogy was a comparison of the effect rather than the actual mechanism.  In a Holley (or Rochester, or Quadrajet, or Carter, I've worked on them all), yes the accelerator pump is a mechanical driven pump.  It adds fuel for smooth acceleration because the typical auto carb does not have the advantage of a variable venturi (the diaphragm slide). 

When you abruptly open the throttle on an auto carb, you get a quick in-rush of air that the fuel flow can not respond to which creates a very lean condition that is not conducive to smooth acceleration.  Thus the use of an accelerator pump to prevent an accel bog.  In a CV carb, the slide inertia helps prevent the sudden in-rush of air be forcing the air through a small throat that keeps the velocity up and the fuel flowing to prevent an acceleration stumble.  DJ kits include stronger slide springs to help this effect.

This is related to Dad's question about manual carbs where the slide is directly controlling the air flow and there is no separate throttle plate.  This setup is very similar to an auto carb.  Snatch a big handful of throttle and bog the crap out of the motor with a big gulp of lean air/fuel ratio.  That's why the "good" flatslides usually have an accelerator pump, to compensate for the lean acceleration. 

Running a manual carb without an air filter will affect the mixture because the fuel flow is a function of the velocity induced vacuum just like a CV carb.  Manual carbs are just a sensitive to intake restriction and jetting as a CV carb.  Change an intake component on a manual carb and you will affect the air/fuel mixture.  Re-jet accordingly.

Oh yeah, the auto carb accelerator pump is a great means to check your idle mixture setting.  Gently tap on the pump mechanism to add just a touch of fuel and see if the idle quality improves.  If it does, the idle mixture is too lean, richen up the idle.  If the engine stumbles when given the extra shot, then you're either okay or rich on the idle mixture.  Of course, fuel injection has taken all the fun out of this.  :-)

DavidR.

[moonrunnah]

okay so if i read this right more air flow = less vacuum like in a car with a turbo at idle you have vacuum but with the turbo spooled you have pressure?
so with less vacuum to move the jets your have to manually move them via a spacer or jet kit?

im pretty sure i understand 

[Pat Conlon]

Pat,
The accelerator pump analogy was a comparison of the effect rather than the actual mechanism.

Ok, got it, thanks.

Moonrunnah, you don't move the jets anywhere. You move the jet needle (up) via a shim (washer) or via the clip on the needle, if you have a adjustable needle.

[moonrunnah]

yes that was what i meant
is the stock fjs adjustable if so how i would like to do a 4-1 exhaust with the uni pods 

[andyb]

The stock FJ doesn't have a true adjustable needle, but it can still be shimmed up if needed.  A true adjustable needle will have mutiple slots in it for a wider range of adjustment.  The other common adjustment is the idle mix screws, though they are behind the pilot jets, so you're replacing brass if you really want to make big adjustments.

Though, with a 4-1 pipe, you would also need to replace the main jets, and quite possibly the pilots.

Jets aren't moved as such, they're simply replaced.  The needle can be moved, arguably it's a needle jet, but that's just arguing definitions.

[moonrunnah]

well how much is involved to change the jets for a 4-1

[andyb]

Not much.  There's more involved that shimming needles, but that's only because you can do those without taking the carbs off.  Take the bowls off the carbs and you'll be able to see both the pilots and the mains. 

[moonrunnah]

if i take the carbs out to change them to i screw up the sync of the carbs