I got a question

[omaha]

There are 4 different springs for the power piston. (that come in the kits) Approximately at what vac do they "kick in" at? If you got 18 inches of vac. , at what point of vac should the piston start to actuate. (i'm gonna guess at around 14?) or is it more trial and error with the rods and springs. My buds got a little bit of a flat spot on this carb that I will be going through, the carb came off a late 70's olds motor and is now on a 70 pontiac 350. I;m guessing that the olds was set up fairly lean. I'm thinking I should put in a stiffer spring. The off idle is great but the primary acceleration is a bit soft. I'm not sure if I should mess with the main air bleeds. THe idle seems perfect and The cruise is fine (good fuel mileage!) so the jetting is ok I think. I guess all that would be left is the spring. Maybe raise the rod with the apt just a little, How about more of a pump shot with longer duration? Like I said, it's not right at the off idle but after you step into it a bit. Like right where you would  think the power piston should come on at.

[Jim Elliott]

Many years ago I read that stock from the factory 11 inches and the rods activated, I've replaced mine in the motor home (454) to 6 inches cuz of the weight dragging up the hills and much lower vacuum available.

Jim

[Cliff Ruggles]

Depends on where the "flat spot" is at.  The power piston is designed to richen up the primary main system when the engine sees heavy load, and typically does NOT come it during any light throttle application.  During "normal" driving, gently leaving stop lights, easing into the throttle to accellerate gently, etc, the power piston is NOT a player.

From what I've seen with those later model Old's units, the metering rods are too fat (K series, stepped with a .005" taper on the upper section) and the tuner doesn't have full control of the A/F with the APT screw right to start with.  Replace them with the metering rods that we sell on the website, they taper from .044" all the way down to .026".

The accl pump also comes into play with any quick/heavy application of the throttle.  One must also consider that the vacuum advance (if used) retards the timing with quick/heavy throttle movements.  This requires the additional fuel from the carbs accl pump, and the main system.

One must also consider that by design, additional fuel is added from the main system due to the pressure differential above and below the venturi area.  This is why we are able to run straight off the jets and not even use a power piston right to start with.  In other words, fuel delivery is increased with throttle opening, and lower engine vacuum.

Now back to the power piston spring(s).  They are set up from lightest to strongest by color, orange, light blue, green, dark blue.  The cut in points are approximate, 5", 7", 9", 11".  The spring "rates" are difficult to map out in terms of opening speed and vacuum.  About all we can predict is the opening point. 

Folks call in and email all the time wanting to know which one to use.  Any of them will work with most set-ups, unless your engine is really heavily cammed and has less than about 10-11" vacuum at idle.  This still doesn't mean you can't use a stronger spring, since the vacuum the engine produces right off idle, or cruising at light throttle will always be considerably greater than the strongest spring.

In any and all cases, the carb builder and tuner should experiment to find out which spring works best for their particular set-up.

We made the long green spring so it can be custom trimmed if/as needed for custom tuning.  It is not recomended to clip the shorter springs as they will not raise the PP to it's full height if too many coils are removed from it......Cliff

[omaha]

Ok, now I am getting the picture. Power piston only applies with some serious pedal input. During  the normal acceleration, the transition on the jets is where I am going lean.  so use a rod with more taper so the apt can be adjusted into range, and maybe work a bit with the accel pump.  going down the road at a steady cruise is fine, no surging at all.  Secondaries work fine too, a bit slow but this is not a race car. We drive it conservatively as to not destroy  the 40 year old motor. (still with the original bearings!)
I think I will check the vac advance to see if it is operating correctly. He says the fuel mpg is not as good as it used to be so that might indicate a bad advance unit. THX  Jim and Cliff!