Sticky throttle after new carburetor install? Accelerator pump adjustment?

[gjh22]

My RV-6 came back from condition inspection with a new MA-4SPA carb. On flying it with a friend yesterday we noticed that the throttle was really sticky, almost unmanipulable, around idle, only when the engine was running - perfectly normal immediately after shutting down and the problem reproduces immediately after starting up. To reproduce - start the engine, and rev it quickly while near idle, like you might do on short final in gusty weather.

I have some suspicion of the throttle control itself, but can I exclude something weird going on in the accelerator pump assembly? If that pump builds up pressure, that would cause this symptom as well.

• Pulled the intake filter and verified that the accelerator pump works normally when the engine is off but the boost pump is on - straight squirt of fuel into the throttle valve
• Accelerator pump nozzle looks normal
• Accelerator link is in the "medium" position (ref this diagram)
• The plug where the accelerator pump discharge check valve is looks weird - my previous carb and every picture of the MA-4SPA I can find has an extended plug with a hex adjustment screw (pictured circled in blue), but a picture of my new one is below (circled in red):

Anyone installed a new MA-4SPA recently? Did it have the adjustment screw on the location for the pump discharge check valve? Any other ideas?

 

[gjh22]

Marvel-Schebler has a service bulletin to the effect of the pump discharge check valve having been revised in 2017, which matches the before/after photos. So that may not be it.

 

[lr172]

Accelerator pump reservoir is not pressurized, it just draws fuel from the bowl. The output side of the pump IS pressurized. While a blockage in the acc pump circuit could cause a stiff throttle, it would be the same running or not as long as the bowl is full. When the bowl is empty, then there is no fuel to restrict, so no resistance. Each time you go in and out with the throttle with the engine off, the accel pump is moving fuel from the bowl to your air cleaner. Without the engine running, or boost pump on, the bowl doesn’t get replenished. If you shut down and then push the throttle, this should give identical acell pump action as with the engine running. Each successive attempt changes the bowl level.

In summary, acc pump action should be identical with engine on or off as long as bowl is full. There is nothing inside the accell pump circuit that is impacted by anything the engine is doing.

I would be looking for something in the linkage or cable that is reacting to all the vibration of a running engine. The engine sits a bit low when off. With the prop spinning, the thrust acts against gravity and lifts up a bit. Precession helps to hold it there, as it also removes the impact of gravity.

 

[mahlon_r]

If the throttle arm attachment nut was slightly overtightened when lining up the cotter pin hole it will pull the shaft outward from the carb and cause the throttle plate to bind on the inside of the carb throat. It causes a stiff spot at idle. If the control checks out, and the carb is stiff when moving the arm off idle, try loosening the nut holding the throttle arm onto the carb and tapping the shaft to push the shaft back into the carb and re- centering the throttle plate in the throat. Good Luck, Mahlon

 

[gjh22]

I ended up replacing the throttle linkage with the McFarlane RV-10 one and found there was a little bit of binding between the carb-side rod end and its retaining washer. I fixed that and we're back in business. The McF RV-10 linkage is an ok fit for an O-320 - it's marked as 47" long; optimal would probably be 50-51".