Vernier Mixture Repair/Adjustment?

[Bob - S50]

Question for the brain trust.

I don't know if it is normal behavior or not, but there is some 'slop' in the vernier action of our mixture control.  That is, for example, on the ground I normally lean until the engine starts to stumble and then enrichen just enough to make it run smoothly.  However, when I switch from leaning to enriching, it usually takes a full twist of the knob or more before it actually starts to have an effect.  Same thing in flight.  If I've been leaning and decide I want to enrichen, it takes a full twist or more before I see the fuel flow increase.

Is there an adjustment that can be made to take the slop out?  Or is it just normal?  Or is it a sign of wear with nothing we can do about it other than replacement?

Thanks,

Bob

[Bob_Belville]

Bob, I replaced all 3 control cables a year or so ago. There is almost zero slop in the mixture cable. It should not take your A&P long to examine and possibly find a clamp on the shield or something else that's moving. It's much quicker if there's a helper to move the know while the mechanic checks out the cable.

If your cables are original, i.e. 40 years old, McFarlane would recommend replacing them.

Recommended Service Life
Due to the criticalness of engine controls and considering their operating environment, the maximum recommended in service time for McFarlane engine controls is equal to the maximum time between engine overhauls as recommended by the engine manufacturer. However, you may find that over time, the friction lock mechanism on your McFarlane throttle control does not seem to hold its position as well as it did in the past. We have a solution for that! To learn more, please read Extend the Life of Your McFarlane Throttle Control by Matt Sharpton, A&P Mechanic.

https://www.mcfarlaneaviation.com/articles/engine-controls/content/

 

[KSMooniac]

I thought I had similar slop with my old setup... Past TBO engine and original control cables with my throttle quadrant. I overhauled the engine, including the FI system, deleted my quadrant and retrofitted a linear control console with brand new cables. The behavior is essentially the same, so I concluded that is the nature of the Bendix fuel injection system.

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[carusoam]

Consider it normal wear...?

The challenge becomes, if you over richen or over lean and you want to go back just a touch...

when is the slop taken up vs. making a slight adjustment. The cycle of over richen, than over lean, just a touch. Enough to be annoying...

This inaccuracy will make you type questions into an Internet forum...  

The whole idea of the vernier control is its accuracy.  So, slop is not normal for such a fine control.

One thing important to consider...  the looseness might be something else loose in the control's system.

Definitely identify where the looseness is coming from.  If the mixture control is slowly coming apart at the engine end, that could lead to a surprise in flight!

@M20Doc always has some good mechanical insight on this stuff...

A call to these guys could be interesting to see if an adjustment is possible...  https://www.mcfarlaneaviation.com/contact/

PP thoughts only, not a mechanic.

Best regards,

-a-

[Guest]

I have never cut a vernier control cable in half to see exactly what is inside.  But some wear in the threaded screw and connection to the linear cable would take place over years of use, how much is too much is a good question.  I would move the mixture cable from the engine end to see just how much linear play there is, and decide if you're comfortable with it.

Clarence

[N201MKTurbo]

There is an easy way to tell where the slop is. Just push and pull on the knob without twisting it or pushing the button. This will show how much play there is in the Vernier screw. If there is none then the play is in the rod end at the end of the cable or the attach point at the end of the cable.

[Bob - S50]

Thanks Turbo!