Power piston spring. Fully open at idle, dies if pushed closed.

[4evrnyt]

This could likely be a case of me simply not understanding how this thing works but I was under the impression that my power piston should be seated at idle and then will lift up as the vacuum drops under throttle.  However at idle my valve remains fully open at idle and if I push the piston down the engine will die. 

I used the straw test to check the piston position and movement. 

[Cliff Ruggles]

To even begin to help I'd need to know the carburetor number?

What has been done to it, parts used, how it is set-up, etc?

All of the engine specs, CID, compression ratio, cam specs, etc?

Also most specifically what power piston spring is in it?  Most engines make PLENTY of vacuum at idle to pull any spring available down and have the PP seated.......Cliff

[Hillbillyenginering]

Just a FYI.
I had the same issue no long ago. The arms on the power piston were bent outward just a little. Causing the needles to hit the filler block and jets. Just enough resistance to keep it from moving down under vacuum. Just something to look at.

[4evrnyt]

To even begin to help I'd need to know the carburetor number?

What has been done to it, parts used, how it is set-up, etc?

All of the engine specs, CID, compression ratio, cam specs, etc?

Also most specifically what power piston spring is in it?  Most engines make PLENTY of vacuum at idle to pull any spring available down and have the PP seated.......Cliff

4mv  7044513

I rebuilt the carburetor last week using an HP kit.  I used the light blue spring on the Power Piston.

The engine is a stock small block 400.  It was rebuilt 20 years ago by the original owner but no modifications were made.  When I attached a vacuum gauge to the manifold it registers over 15ft. 

It should be noted that when cold the engine is running fine, well in fact.  When it warms up it develops a slight, hesitation when I first dip into the throttle.  It only does it when I first engage the throttle and then the carbs seems to be operating fine through the rest of the RPM range. 

When I was testing it with the straw, if I pushed the piston closed with the engine running not only does it die but fuel bubbles up out of the vent tube as well as the accelerator pump.

Thanks for your help.

[4evrnyt]

Just a FYI.
I had the same issue no long ago. The arms on the power piston were bent outward just a little. Causing the needles to hit the filler block and jets. Just enough resistance to keep it from moving down under vacuum. Just something to look at.

Ok thanks, I'll check that if I have to end up pulling it apart again.

[von]

Sounds like you're actually pushing down on the float instead of the power piston if fuel bubbles up out of the vent. It's an easy mistake to make. BTDT. To get on the power piston IIRC you have to angle the straw a little toward the front (inside). Top of straw toward the rear.

[Cliff Ruggles]

Correct, the engine is being flooded, and 15" vacuum would easily have the PP down with any spring from our kit.....Cliff

[4evrnyt]

Sounds like you're actually pushing down on the float instead of the power piston if fuel bubbles up out of the vent. It's an easy mistake to make. BTDT. To get on the power piston IIRC you have to angle the straw a little toward the front (inside). Top of straw toward the rear.

I figured it was something stupid I was doing.  I'll angle the straw more next time.  Appreciate the help.

[4evrnyt]

Correct, the engine is being flooded, and 15" vacuum would easily have the PP down with any spring from our kit.....Cliff

Ok, thanks.