Mixture Control - not enough throw

An A&P looked under the cowl while we had it off to do other things and he said he noticed that while the mixture control would take the mixture to Idle Cutoff, that it wouldn't get it to full rich. He adjusted it so that we get to full rich, but now we don't get back to idle cutoff.

While I understand the importance of getting to full rich, I hate the fact that I can't get it back to ICO. So I won't fly the plane until this is resolved.

I'm using a standard quadrant - similar to if not an actual CT83F

My questions are:

1) Has anyone seen this?

2) I would have thought that the factory hole in the mixture control would have been placed far enough from the pivot as to give more than necessary throw. Is this not always the case?

3) Is there any other reason the throw might not be sufficient? Possible flex in the cable?

4) If I calculate a distance necessary to get me the additional throw and if I can drill a hole in the lever far enough away from the pivot to get it could I be weakening the integrity of the lever? (obviously if it's a thousandth away form the original hole that's bad).

Thanks!

Vic just covered this in one of his webinar examples the other night.
You may have to drill another hole in the actuator arm, or get a different arm (I had to get an offset arm from Wentworth)

Increasing Throw

If you drill a new hole closer to the pivot point you increase the angular throw. That should not weaken anything.

Don

cfiidon said:

If you drill a new hole closer to the pivot point you increase the angular throw. That should not weaken anything.

Don

Click to expand...

I would think you'd have to drill it further away.

Don had it right.

Dave

David Paule said:

Don had it right.

Dave

Click to expand...

Gonna have to think about that one.....

It would seem to me that what you want is more distance moved by the hole per degree angle movement. You don't get that by drilling the hole closer. Take it to the limit...if the hole was at the pivot point the cable wouldn't slide at all. C = 2*pi*r and all that..the longer the R the greater the C for a given chunk of angle.

I'm assuming that what you want is to increase the distance the end of the cable moves. Is that incorrect?

I recently discovered the same issue on my 8- throttle and mixture had both lost 3/8" of full forward throw. Replacing both cables fixed the issue. My theory was heat had limited the forward throw because the governor cable was ok. The throttle cable was fire sleeved only on the section close to the exhaust pipe heat shield under the oil pan; fire sleeve was added all the way to the engine mount, along with reflective fiberglass tape over the fire sleeve. The mixture cable had no fire sleeve, so fire sleeve and reflective tape was added to past the engine mount. No time flown on the fix yet, so can't report if it solved the issue. The only way to detect this while flying is to drill new quadrant clevis holes for full forward throw, or mark the full forward position on the quadrant.

First, unhook the control from the arm and measure the "throw". This is the distance the cable can go from full closed to full open on the panel. If the throw of the mixture or throttle arm is less than that, it should be able to hit both of the stops. If NOT, then reduce the length of the arm (shorter radius on the arm will reduce the throw distance) or get a cable with more throw.

I had the same issue with my O-360.

In the end, it took a lot of messing around to get everything adjusted right so it would go the stops on both ends.

I had the same issue with the throttle lever. (I have a center mount DJM throttle quadrant. One of the best custom touches I made!)

Saville said:

Gonna have to think about that one.....

It would seem to me that what you want is more distance moved by the hole per degree angle movement. You don't get that by drilling the hole closer. Take it to the limit...if the hole was at the pivot point the cable wouldn't slide at all. C = 2*pi*r and all that..the longer the R the greater the C for a given chunk of angle.

I'm assuming that what you want is to increase the distance the end of the cable moves. Is that incorrect?

Click to expand...

This is a semantics problem. "Actuator arm" can mean the arm on the thing being controlled (which is what Don was saying, I think), or it can mean the arm on the thing doing the actuating (the throttle/prop/mixture control arm in the cockpit), which is was some others were saying.

Hole further from pivot on the user/cockpit end -> more distance (up to the limit of what the cable will physically do).
Hole closer to pivot on the engine end -> more angular travel per linear throw of the cable.

RV7A Flyer said:

This is a semantics problem. "Actuator arm" can mean the arm on the thing being controlled (which is what Don was saying, I think), or it can mean the arm on the thing doing the actuating (the throttle/prop/mixture control arm in the cockpit), which is was some others were saying.

Hole further from pivot on the user/cockpit end -> more distance (up to the limit of what the cable will physically do).
Hole closer to pivot on the engine end -> more angular travel per linear throw of the cable.

Click to expand...

Ah ok that makes sense. I was thinking only of the control lever arm in the cockpit.

But yes if you drill a hole closer to the pivot on the engine mixture control arm (not the quadrant arm) you'd drill closer.

Thanks for clearing that up.