Hey guys, new plane owner (1 year) so still some rocks I'm overturning here and there, and came across this issue which I believe is new. 

I have what I believe to be the McFarland throttle control with vernier assist. Black knob with white writing indicating direction of turn for micro adjustments. I need to jump back in the library of records on this plane to confirm that, but its what it looks like I have. I didn't build this plane so not sure yet. 


  • When I unscrew the friction nut the black control nob jumps out and has SIGNIFICANT push against my hand which obviously causes the engine RPM to increase.
  • This is not a "throttle creep" which is smaller increases over time, this kind of slams into my palm like it has air pressure behind it.
  • If I continue to unscrew the friction component and don't catch it, I believe it would go full throttlle/RPM entirely on its own. 

The only way to have it hold RPM is to select, hold, tighten friction component, then sit and watch and worry its going to jump out at me again. 

  • Is this broken?
  • If so, where is the pressure derived that would push with such a force?
  • Is this how throttles are supposed to work (I don't believe it should jump to full RPM as its default but maybe wrong)?
  • Have I never unscrewed the friction component to allow this to happen (I strongly disagree with my own last hypothesis here as I believe I have unscrewed this in the past and this is a new issue, but I'm open to anything)?

Anyone seen this problem in the past? 


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I have a Jab 3300 on my STOL 750 with the McFarlane "Vernier Assist" throttle control (however, I have the Sonex/AeroConversion's "AeroInjector" TBI carb.

What you describe is a bit confusing ... normal throttle motion is that when a throttle goes farther out, it should decrease the engine rpm, not increase it!  The McFarlane Vernier Assist is not spring loaded - it sounds like you might have a spring on the carb to bias it to go to full throttle should the throttle cable break - but again, the spring should pull the throttle in and not out for full throttle.

Does the Jab have a stock (Bing) carb?  If it had a TBI carb like the AeroInjector, then pulling the throttle out would increase the rpm rather than decrease it.  You have to use either a throttle-reversing bell crank or some other mechanism to reverse the cable direction to the carb to get normal throttle-in rpm increase throttle-out rpm decrease.

If you're not sure, a photo of your carb set-up would be most helpful!



John - I have a friend who is an F-35 pilot and as smart and capable as those guys are, they have a saying that goes "You lose 20 IQ points when you put on your helmet". 

So...safe to say I lost far more than 20 when I had my headset on.

1. You are correct, and I didnt explain this correctly. My throttle operates the same way yours does. Push forward, go fast - - pull back, slow down. But I still get a slap in the palm before I get full throttle. I will try to get some photos asap once I open the cowling. 

This seems fairly extreme compared to what I have seen, but all Bing Carburetors have a spring that pulls the Carburetor to full power if the cable breaks or come loose.  This is a safety feature that keeps you with full power until you can get to the airport and kill the engine and make a power off landing.  The Bing is used on Jabiru and Rotax with the same spring.  On my airplane the spring is not very strong and I have a fair amount of friction in my cable without the friction lock so it does not even creep open.  I wonder if someone replaced your spring with a heavier one or if the air flow is forcing the butterfly open.  I will try to take a picture of my spring to get you some reference. 

Hi John, ok this is really good to hear, it actually means the default setting is to increase power. Good safety feature. Interesting. Now in the scenario you described, should I be able to pull throttle back? I am able to pull throttle back. So Im just curious if the throttle cable is broken it should only be in full throttle, correct? Im going to pull the cowling in the next couple days and get some photos. 

Hi John Burns & John Austin, thank you with your help thus far. Ok I was able to get some pictures and video last night. Looking at the pictures, I don't believe anything is wrong with the carb. I don't believe any spring is missing. Comparing the pictures to the diagram (both below) it doesn't appear anything has fallen off or broken. I could be wrong, but all my external springs are where they should be.

Do we agree on that? Asking because maybe I am missing an internal spring somewhere that would counterbalance the external spring? 

Now if everything is as it should be, I believe either:

  • My friction lock was simply never loosened enough for the carburetor "full throttle safety spring" to do its work. 
  • Over time my throttle cable, which is new-ish, has broken in enough to slide with ease and allow the spring to do its work, thereby making the friction lock do even more work. 

Now the big question:

  • Additionally If I am to rely on the friction lock as a counter-balance to the safety spring, wont I burn up the friction and have to replace the entire throttle cable relatively often?
  • If I remove the full throttle spring then wont I have a normally operated throttle cable? 
  • What do people do who didnt purchase the friction lock style of throttle cable? Do they fight the spring their entire flight? 

I know nobody has "the" answer as I know everyone will say "they did it that way for a reason", its more of an open discussion. 

Looks like that spring on the Bing is fairly strong - perhaps John B or someone can send you a comparison picture to make sure you've got the right spring.  You can call McFarlane and get the spec on the max number of pounds the Vernier Assist friction lock is supposed to resist - tech support mentioned it one time to me but I don't recall the exact number now!

John A

I was finishing a 25 hour oil change today and took some pictures. 

I am not sure if this helps any, but this is what my setup looks like.  As you can see I have a solid cable (wire) throttle.  It is the one that came with the FWF kit from Arion Aircraft.  I have dual sticks so the throttle is in the center and after it curves through the firewall to get to the carburetor in a straight line around the nose wheel strut there is too much drag in the assembly to do what you are seeing.  I don't have to use the friction lock at all except in cruise.  Of course I also have a long time habit of hand on the throttle except in cruise so I might not be noticing some light spring return.  Apologies for the camera angles, but it was the only way to get a picture of the spring. 

Ok this is great! This means my carb looks like yours and looks the way it should! So I guess either a softer spring or use the friction lock. It would seem no mechanical issue! Thank you so much for getting some pictures! Big help! 

I have exactly the same setup on my 601.  Your throttle looks fine.  Adjust the tensioner so that you can move the throttle fairly easily but keep enough tension so that it doesn't creep.  Once you are settled down in flight, you might make it slightly tighter to keep it in position.  You can now make minor adjustments to your rpms by turning the knob.  I find that I have to give it a few turns to get any noticeable changes.



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