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Choke Control

 
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victorbravo(at)sbcglobal.
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PostPosted: Tue May 16, 2017 11:26 am    Post subject: Choke Control Reply with quote

Happy Tuesday Kolbers, I'm trying to hook up the throttle and choke cables for my HKS installation.

Because I'm an "airplane guy" instead of an "ultralight guy" it's been rather frustrating, to say the least. Hooking up one way "pull-only" controls that rely on a return spring on the carburetor arm is counter-intuitive to me. There was almost no friction in the entire throttle system, including the cable "splitter", until I hooked up the throttle arm return spring. As soon as I hooked it up the entire system had way way more resistance to pulling the cable. Seemed like way too much resistance to movement.

Now I understand why the Kolb has a very tall throttle lever.

Finally I managed to get the throttle mostly done except for final adjustment. Now the choke is becoming a difficult problem.

I moved the choke lever from behind the pilot's left shoulder (factory position) to the right side of the cockpit next to the pilot's right thigh. This seemed safer to me, because I can always see where the choke lever is, instead of forgetting it because it was out of my sight behind my head. So I ran a cable and sheath from the choke "splitter" down to the right side of the cockpit. I mounted the OEM choke lever (a piece of twisted aluminum sheet metal) using an Adel airframe clamp on one of the fuselage tubes.

The problem is that the choke lever doesn't exert enough force on the cable to overcome the friction in the system when the (really strong) return springs are hooked up on the carburetor. I don't feel good about using weaker springs, because they may not allow the choke to spring back to the OFF position.

I believe that these cable friction problems are "built in" because of how many bends and changes in direction that the throttle and choke cables go through on a Kolb installation. But since there have been hundreds or thousands of Kolbs flying for years with return spring type systems... I want to figure out whether I am doing something wrong, or whether everybody else has the same problems and complaints about their throttle and choke systems.

The cable "splitter" units are smooth, I took them apart and lubricated them.I used new throttle cable and sheath for most of it. I used brand new brass elbow fittings screwed in to the tops of the carburetors (Bing). I verified that the throttle and choke controls worked smoothly and with little friction when I disconnected the return springs.

Can anybody guess or predict if or what I'm doing wrong here?
Bill Berle
HKS Firestar


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stuart(at)harnerfarm.net
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PostPosted: Tue May 16, 2017 2:57 pm    Post subject: Choke Control Reply with quote

Bill,

Possibly what you are experiencing is just normal friction plus return spring resistance.

Some thoughts:

The return spring for the choke needs to hold the choke open against airflow and also should be able to supply enough force to pull a broken cable through the cable housing. Suppose the cable broke at the lever end while it was pulled all the way out, you would need enough spring to overcome whatever friction could be introduced to the system if that should happen. In reality that is a very remote possibility at best because normally you would shut the choke off while still on the ground and are not likely to touch it again until the next startup.

Now, the throttle is a different matter. If something should break in the linkage somewhere, what do you want the carburetor to do? Go full power as the default or go to idle? The usual choice is go to idle. In most cases that is considered the lesser of two evils. One would hope that proper pre-flight inspections would prevent any such event from ever happening. But you must always consider "what if".

So, how to tell how much resistance is too much. (remember, "resistance is futile").

Here is a little test. You can probably judge it by feel alone, but if you use a small spring scale you can quantify it for your satisfaction.

1) Disconnect the cables at the carburetor and pull each control with the proper spring installed. Note the force required.
2) With the cable still disconnected (but routed the way you have them) pull from each direction and note the force required. I would think it should be the same going both ways.
3) Connect the cables to the carburetor and pull again. Forces should be the sum of those measured independently or just slightly more.

Now, if you think the test # 2 has too much friction, remove the cable and lay it out straight and re-test. The difference will be what your routing added to the mix. Usually the only time this is a factor is if you have lots of turns or turns with too small of a radius.

All of this assumes you have not tightened the control levers too tight and introduced a lot of friction there. For my Firefly, I played with different washer thicknesses and tensions until I found the combination that introduced as little friction as possible but still would hold the throttle against the return spring. As things have worn a little it will creep down just a little at cruise. Someday I will have to tighten it a little to stop the creeping.

I had to manufacture some cable ends for the control lever side of things as none were available for purchase when I needed them. You can check it out here: http://harnerfarm.net/serenity/engine.html

If I didn't explain this very well, let me know and I will try and re-word it. Sometimes what is in my head doesn't translate to the keyboard very well.

Stuart
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rickofudall



Joined: 19 Sep 2009
Posts: 1392
Location: Udall, KS, USA

PostPosted: Tue May 16, 2017 8:57 pm    Post subject: Choke Control Reply with quote

Bill, The Bing 64 carburetor has no choke. What it does have is a starting carburetor that is jetted richer than the 64. It only works below about 3,000 RPM so leaving it open won't be noticed until you pull power for approach and notice an unusually high idle.One thing that has always bothered me about the HKS installation of the 64 is that it is like a motorcycle, if the cable breaks the carburetor goes to idle. I'd love to change it out to the Rotax system. Like a regular airplane engine if the cable breaks the engine goes to full throttle so only a very light spring is needed. The cable pushes the throttle open and pulls it closed. McFarlane Aviation Products; http://www.mcfarlaneaviation.com/ has a dual cable throttle for the 912 that requires no mixer and uses 1 X 19 cable that easily handles the push/pull requirements.
The Rotax 912 IPC has a drawing of the way their throttle works.
Rick Girard
On Tue, May 16, 2017 at 5:56 PM, Stuart Harner <stuart(at)harnerfarm.net (stuart(at)harnerfarm.net)> wrote:
[quote]--> Kolb-List message posted by: "Stuart Harner" <stuart(at)harnerfarm.net (stuart(at)harnerfarm.net)>

Bill,

Possibly what you are experiencing is just normal friction plus return spring resistance.

Some thoughts:

The return spring for the choke needs to hold the choke open against airflow and also should be able to supply enough force to pull a broken cable through the cable housing. Suppose the cable broke at the lever end while it was pulled all the way out, you would need enough spring to overcome whatever friction could be introduced to the system if that should happen. In reality that is a very remote possibility at best because normally you would shut the choke off while still on the ground and are not likely to touch it again until the next startup.

Now, the throttle is a different matter. If something should break in the linkage somewhere, what do you want the carburetor to do? Go full power as the default or go to idle? The usual choice is go to idle. In most cases that is considered the lesser of two evils. One would hope that proper pre-flight inspections would prevent any such event from ever happening. But you must always consider "what if".

So, how to tell how much resistance is too much. (remember, "resistance is futile").

Here is a little test. You can probably judge it by feel alone, but if you use a small spring scale you can quantify it for your satisfaction.

1) Disconnect the cables at the carburetor and pull each control with the proper spring installed. Note the force required.
2) With the cable still disconnected (but routed the way you have them) pull from each direction and note the force required. I would think it should be the same going both ways.
3) Connect the cables to the carburetor and pull again. Forces should be the sum of those measured independently or just slightly more.

Now, if you think the test # 2 has too much friction, remove the cable and lay it out straight and re-test. The difference will be what your routing added to the mix. Usually the only time this is a factor is if you have lots of turns or turns with too small of a radius.

All of this assumes you have not tightened the control levers too tight and introduced a lot of friction there. For my Firefly, I played with different washer thicknesses and tensions until I found the combination that introduced as little friction as possible but still would hold the throttle against the return spring. As things have worn a little it will creep down just a little at cruise. Someday I will have to tighten it a little to stop the creeping.

I had to manufacture some cable ends for the control lever side of things as none were available for purchase when I needed them. You can check it out here: http://harnerfarm.net/serenity/engine.html

If I didn't explain this very well, let me know and I will try and re-word it. Sometimes what is in my head doesn't translate to the keyboard very well.

Stuart


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