Mixture Creep?

[Dr. Dave]

Hi all.. I have a 170b with the O-360 Lycoming 180 horse conversion.. Over the last few months
I have noticed my mixture control creeping lean when flying at altitude. (8-11,000 ASL)
The cable does not have a lock or spring on it and there is no obvious way to increase the
friction at the carburetor.. Has anyone had a similar experience and found a simple way to fix it?
All suggestions welcome.
Thanks,

Dave

[cessna170bdriver]

Dave,

My mixture control didn't creep, but when the cable broke several years ago, I had a vernier control from Aircraft Spruce installed. A fairly easy fix, and I've been very happy with it.

Miles

[Dr. Dave]

Thanks Miles,

I like the idea of a vernier control for acuracy! Any idea what it cost you?



Dave

[doug8082a]

I wouldn't install it strictly for accuracy's sake. When I refurb'd my '52 I switched from the vernier that had been installed back to the original mixture control (for originality's sake) and experienced no difference in fuel consumption when leaning. I don't believe that the carbs on these engines are sensitive enough to benefit from a vernier control.

I'm not saying "don't do it". I'm only saying that you shouldn't expect any vast improvements in fuel consumption by leaning with a vernier vs. original mixture control.

[cessna170bdriver]

Dave,

I thought I remembered around $80, but that may have included installation. I'm not sure what kind of connection the O-360 uses at the mixture control, but I see the A-790 using a solid wire end is in the US$50-60 range at Aircraft Spruce. http://www.aircraftspruce.com/pdf/catalog/Cat06156.pdf The A-970 with the 1/4-28 threaded ends are about $10 more. I can't remember the length right off hand but it shouldn't be too hard to measure.

Miles

PS: See you in Kelowna?

[cessna170bdriver]

I'm only saying that you shouldn't expect any vast improvements in fuel consumption by leaning with a vernier vs. original mixture control.

I didn't see any improvement in fuel consumption either, but it did make the mixture easier to set, for me anyway, as the roughness comes on more slowly while leaning, and goes away more gradually going the other way. There is a couple of turns of hysteresis (it takes a couple of turns inward for the roughness to begin to go away), but once you get it headed one way or the other, you can easily set EGT's within a few degrees, for whatever THAT is worth. Also, once you set it, there is absolutely no creep, and as long as you maintain altitude and the OAT doesn't change, EGTs stay rock steady.

I understand not doing it if originality is a concern, but in Dave's case it might not be, considering his Lycoming engine. In any case, it's an easy do-over if you want to go back. Dave, if you do want to stay original, you might try installing the locking mechanism that is supposed to be on the original control. Looking up the numbers for the 3 parts of the mechanism, the Hill Aircraft refers you to 0513173-1, Lock for $60.

In case the cable itself is the problem, I checked Hill Aircraft and the original 0411090-2 for the '52 B-model has been superceded by the -4 and "Your Price" is 95.20. The -10 for the '53 and later B-models isn't listed .

Miles

[Dr. Dave]

Thanks.. mine is a 55 model... I am leaning (no pun intended) towards the vernier for the reasons you gave.. Fuel consumption did not occur to me.. I just like to think I am being kinder to my engine if I can tweak and hold the mixture with some finess?

Dave

[cessna170bdriver]

Dave,

If your old mixture control was creeping toward lean, a vernier (or some other non-creeping) control will be kinder to your engine. If it was creeping toward rich, then it will be kinder to your wallet. For me at least, being able to make finer adjustments is kinder to my frustration level.

Miles

[blueldr]

It is quite simple to put a small kink in the mixture bowden wire just as it enters the housing with the mixture in full rich. The slight bind of the wire in the housing as the mixture is leaned will keep it from creeping.

[blueldr]

In the above suggestion, please understand that the wire should be kinked at the lower, or carburetor end.

[Bruce Fenstermacher]

And the kink should be so small as to be nearly undetectable. A little goes a long way here.

[Romeo Tango]

When I had my annual done last year we disconnected the CHT and Carb Temp gauges as they were unreliable. My mechanic (Vern Miller, sad that we lost him last month) suggested we replace the gauges and probes/thermocouples in next annual cycle, but I cannot recall what brands he favored. He was very dismissive of 1 particular brand, but I cannot recall which he was recommending.

Any input is appreciated. I'll probably not worry about the carb temperature gauge as that's tertiary. And I'm tempted to get a dual EGT/CHT gauge as 100LL goes toward $5/gallon, I want to have that precision leaning.

Is there a clear standout in quality? And some that are suspect?

Thanks -

[blueldr]

Sophisticated temperature instruments for precision leaning are rather wasted on the C-145/O-300 engines equipped with the Marvel Schebler carburetor. Mixture varies too much between the cylinders.
Any time you're operating below 75% power, you cannot lean too much to hurt the engine. The time honored system is to lean to just rough and then enrichen to just smooth. That's about as good as you can do with our induction system.

[GAHorn]

I tend to agree with Doug and blu eldr as regards the bowden cable type of control.
Those A-790 cables are also offered with a "ratchet" mechanism which prevents creep. That's what I have on my airplane and I've enjoyed it for 6 years now. In fact, I intend to install one on my cabin heat control as well. It looks original, but does not use the flat-spring lock the original used. Instead, the rod has a series of notches installed and an internal spring-loaded ball keeps it where you put it.
(If you do decide to have a vernier, and want to save some money and can wait til I return from the convention, I have a used one in excellent condition I'll sell you for half-price.)

[Dr. Dave]

Thanks.. what days do you think you will be at the convention? I am planning to be there late Monday afternoon and then leaving again early Tuesday evening. Perhaps you could bring the table with you and we could meet? What about the cable length? Is that likely to be an issue? I can get an approximate length by measuring the portions of the cable I can access without pulling the cowel.. would that likely be close enough?

Thanks,


Dave