Question Stiff throttle assembly - Carbon Cub FX-3

[Neal]

I'll put this out there to get opinions as I started my condition inspection and it's time to figure this out. Since delivery of my FX-3 nearly two years ago the throttle travel has friction, even with the friction knob completely in the loose position. It's not like my prior plane where with loose friction there was a much easier travel of the throttle lever. I asked this question not long after delivery and it was suggested it's the nut/bolt too tight which joins the front and rear throttle assemblies. The other option is something in the throttle body itself but I will take the lower risk approach of removing the left interior panel (not fun) and check the assembly.

I'm open to any suggestions as to what could cause, or most likely be the cause, of this friction situation. I'd like to have it very loose with friction knob backed off and then be able to adjust my desired friction using the friction adjustment.

 

[Neal]

Finishing up the condition inspection and I believe this friction is coming from the throttle body and not anything behind the left interior panel. After 3 days of condition inspection work I don't have a desire to really pull that panel to find out it's not there and will just live with it and hope it loosens up over time.

 

[Cactus Charlie]

The throttle cable attaches to the front throttle lever and the front and rear throttle levers are connected by a link rod. There will likely be a small amount of play in each of these three connections unless the attach bolt were over tightened.

It may be possible to gently feel for the play at each lever. That may tell you what part of the system is tight.

There is an exploded view of the front quadrant in the maintenance manual Section 76-00 Fig 2. An unexploded view is shown as figure 4.

Look at the drawings, see where the free play would be, then try to feel for it.

Over tightening any of the hinge/pivot bolts in either quadrant could cause throttle stiffness. That problem is not likely to get worse. However, a bad throttle cable is quite likely to get worse.

 

[Neal]

That was the guidance from Dave Embry as well pointing to the bolts at the connecting rods. I just have to decide if it’s worth the hassle of the left panel removal. We’ll see. One more chore tomorrow which is Beringer brake dual caliper work, installing Beringers new style brake pads and bleed them. It would be nice to not have to keep the friction knob as far counter clockwise as I can get it.

 

[Cactus Charlie]

You don't have to remove anything to feel for the play in each joint.

Exposing the quadrants requires removal of three side panels! (Done it twice, don't want to do it again.)

 

[Neal]

I don’t feel any play. It’s all stiff and secure. I unscrewed the plates at the base of the levers today and looked in and put a little LPS 2 on the connections. I also LPS 2’d the throttle body connections. It’s not that bad but I’d like it looser so that a little friction knob force is applied.

 

[Cactus Charlie]

"It’s all stiff and secure. "

Then you may have identified the problem.

 

[Cactus Charlie]

I checked the throttle levers of my FX-3 and found the expected low friction movement between the front and rear levers and also between the front lever and the prop cable clevis.

The movement is small but easily felt and seen. It is not noticeable when operating the throttle in flight.

The lack of any free motion in these parts either means the assembly was built with much tighter than normal tolerance on the holes or, more likely, the nuts on the pivot bolts are too tight.

I suppose the good news is that the over tight parts will wear and the stiffness will reduce in time.

Edit to add - I have dismantled and re-assembled all these parts. The free motion on my FX-3 may not not be typical of all FX-3.

 

[Neal]

Mine isn't that bad but the friction knob is of no value. As I'm completing my condition inspection I want to get an official borescope as I did one myself, but I want local maintenance shop to do that and my transponder 24 month re-cert. With that said, that's a week away and I have to remain de-cowled so I have nothing but time now so I guess I'm going to remove the left panel and take a look. Should be done every other year or so anyways to inspect for fuel selector leaks which was a big issue on my prior FX-3 so I'm a little sensitive to this item. So I'll see what I find later this week with that.

 

[Neal]

Got the left interior panel off today to inspect this throttle friction. I could take a flat head screwdriver and rotate almost all of the connecting rod screws freely except for one or two of the forward ones, they were definitely a little stiffer. However, there's no way I'm getting on the back side of that nut to get the pin out much less get another one in, it's just not worth dealing with it. However, I did put a rag behind the assemblies and hit all of the areas with LPS-2 including the throttle cable and that seems to have helped. The entire setup with friction knob etc. just seems odd but it's all lubricated now and better than when I started.

Final task is to consider permanently mounting the cargo nets but I have to get the structure behind the panel properly located onto the panel so I can ensure the cargo net holes aren't going to interfere with the structure going on behind the panel. I'll assess the situation tomorrow and then reassemble.

 

[Cactus Charlie]

However, there's no way I'm getting on the back side of that nut to get the pin out much less get another one in, it's just not worth dealing with it.

I have had these pins and nuts off (and back on) more than once. I learned to place a sheet of cardboard between the quadrants and the fabric to protect against damage from a slipping tool. I also stuff the gap between the floor pan and the fabric to stop anything dropping into the belly.

If you take those precautions it's a bit fiddly but not impossible.