Stromberg NA-Y9E carburetor

[zedzil]

On SNJ-5 with NA-Y9E carburetor:
When engine not running (cold), throttle movement is smooth throughout its travel. When engine is running and have reached its normal operating temps., when throttle lever is moved about 1/4 to 1/2 inch from idle the throttle "hangs" (feels like binding) and then, upon being forcefully pushed towards the open position, the RPM jumps to 1100. The carburetor has just returned from having its accelerator pump repaired and was bench tested and calibrated. Throttle operating linkage system was checked and no binding was discovered and the operation is smooth.
Any ideas as to what may be happening?

[bdk]

First thing to check is whatever was touched. Are you sure the throttle arm on the carb is in the right orientation?

[zedzil]

First thing to check is whatever was touched. Are you sure the throttle arm on the carb is in the right orientation?

Checked throttle arm position and it's OK; checked the whole throttle lever linkage from cockpit to carburetor and found per specs (drawings.) Disconnected the throttle linkage from the carburetor and ran engine while operating the throttle arm on the carburetor manually and the problem persists.

[bdk]

Disconnected the throttle linkage from the carburetor and ran engine while operating the throttle arm on the carburetor manually and the problem persists.

Sounds like you have the source then. Maybe the accelerator pump piston (seal) is getting hung up in the accelerator pump cylinder (assuming it isn't a diaphragm type pump)?

At idle there is a lot of suction force on the butterfly so there might be some problem there that you can't feel statically. If the overhauler removed the butterfly from the shaft it may have been replaced in a slightly differently location than it was before (tolerance between the screws and the butterfly) and could be dragging on the housing, but only noticeable under vacuum due to wear between the shaft and the bushings.

[zedzil]

Disconnected the throttle linkage from the carburetor and ran engine while operating the throttle arm on the carburetor manually and the problem persists.

Sounds like you have the source then. Maybe the accelerator pump piston (seal) is getting hung up in the accelerator pump cylinder (assuming it isn't a diaphragm type pump)?

At idle there is a lot of suction force on the butterfly so there might be some problem there that you can't feel statically. If the overhauler removed the butterfly from the shaft it may have been replaced in a slightly differently location than it was before (tolerance between the screws and the butterfly) and could be dragging on the housing, but only noticeable under vacuum due to wear between the shaft and the bushings.

The carburetor was removed and sent to a shop. We'll see what the findings are. I am only afraid that the overhaul shop will not be able to operationally test it and thus, after all the work of installing it, I might have the problem again. Hope not.

[jdankos]

Do you have any updates on the carb issue?