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Idle Screw Adjustment


John Vance

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I'm considering a change (slight increase) in idle speed, and have a question about the use of the idle stop screw adjustment.  My Bowden cable adjust screws cannot be moved any further in the increased-throttle direction, but the idle stop screws have clearance when in idle position.  The line maintenance manual expresses a concern about adjusting the stop screws to the point where there is excessive load on the stop (para. 10.3.1).  After reading through the procedure a couple of times, I think what it's saying is that more than 1/2 turn of the idle screw beyond contact with the stop will create this condition.  Am I interpreting this correctly?  Is it better to not make use of the stops and use only the cable adjustments?

 

John

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The idle stop screws prevent the throttle plate from closing completely. Backing them off means less air, lower idle. Driving them in opens the throttle plate. Putting excessive force on the plate that touches the idle screw is the problem. Having a gap though is bad too, because one cable or another might close the throttle on one carb more than the other.

 

The cables have two adjustments: the bowden cable nuts, which shorten or lengthen the distance from the throttle to the arm attachment, and the arm attachment itself. If you are out of movement on the bowden adjustment, you can move the arm adjustment a 16th of an inch to take up or let out cable.

 

The really hard part is adjusting all this so the throttle lever in the cockpit almost touches the stop. I really wish CTs had an adjustable throttle stop, that would help a lot!

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Making the idle adjustment with the Bowen cables has the potential to change the balance between carbs throughout the entire throttle range, although it seems that if done with the screw adjustments it could be controlled adequately by turning the screws the same on both sides.  However, the idle stops only affect the idle condition, which is why that interests me.  That brings me back to the original question of how the load on the idle stop screw should be controlled.

 

John

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You should be making the bowden cable adjustments FIRST. The bowden cable setup affects everything, including the final idle settings. If you have to make changes to the bowden cable nuts for idle, something is wrong and check the following:

  • Check the security of the nuts on the throttle arm, cable wear, or general security on the throttle lever. A loose throttle nut can cause several inches of mercury of play (guess how I figured that one out!).
  • Your throttle arm position or bowden setting is incorrect. They should be set up for your non-idle settings and synced first. If they are significantly off, you'll have to start from scratch. See the asterisk point below on how to do that the easy way.*
  • Failing the above, you will need to do a carb inspection and/or vacuum seal check.

 

The idle screws should only be used to adjust idle.

 

*If your settings are extremely far off and you need to start from scratch, there's a trick to it.

  1. Spring load throttles to idle.
  2. Set bowden nuts to neutral position.
  3. Set idle screws to around desired idle RPM and in sync. You might have to start and stop the engine during this process. Now we know roughly where the throttle arms will be for idle.
  4. Set throttle lever position to just above where you want it to stop moving at idle. In a CT, typically 1/16th of an inch works fine, this is to account for the slack in the cable once you attach to the carb throttle arm.
  5. Affix throttle cables to throttle arm. There's different tricks to this, and you'll figure them out depending on the kinds of tools you have. Don't try to make the nut super tight (you damage the cable), and don't pull the cable like a banjo string. We accounted for slack in the previous step, and you might move the throttle lever if you pull tight.**
  6. Set the spring back to full power on the throttle arm.
  7. WITH THE ENGINE OFF, pull the throttle lever to idle. Now, go to the carb and feel how tight the cables are. There might be a slight difference in tension, and that's what the bowden cable nuts are for. What we don't want to see is a droopy cable, that means there is a dramatic difference in tension, and you need to fix that!
  8. If it's set well at this point, begin the rest of the syncing procedures.

ALTERNATE METHOD FOR STEP 5:

  1. Pull the throttle cable lightly and put it against the throttle arm attachment point.
  2. Mark the cable on both carbs.
  3. Remove springs on throttle arms so you can move them freely.
  4. Push throttle full forward and pull the cables. Now you can get better access to the screw and nut on the throttle arm, set to the marking on the cable and resume with step 6 above.
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Corey - thanks very much for your responses.  All good information.  At the moment, I believe all's well except that I'd like a higher idle setting, which I'm hoping can be attained solely by adjusting the idle stop screws.  I think the process might be something like this:

 

1) Secure the throttle in idle position

2) Using a feeler gauge (maybe 0.001"?), turn both stop screws clockwise to obtain the same (small) gap between the screws and the stops.

3) Advance both screws the same amount incrementally, testing idle RPM with warm engine until idle RPM is where I want it.

 

Obviously, as a result of this adjustment, when the throttle lever is pulled back all the way, load will be placed on the cables, attachments, arms, and the stops. 

 

The manual warns against making an adjustment that results in an excessive load, but I'm not sure that I've interpreted the details properly. 

 

What I don't know is this:  How far can I turn the idle adjust screws past the point where they contact before this becomes a problem?  1/2 turn past contact??  If I can't reach the idle setting I want before that point, then I'm going to need to make adjustments to the Bowden cables, in which case I'll be referring to your previous response.

 

Thanks

John

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All you need to do is just hook up a synchronizer and drive the screws in. A tiny, tiny amount is all you need. 8th of a turn can be a hundred plus RPM.

 

And to answer your question about throttle stop load: don't rest your hand on the throttle. That's how you put a lot of load on the stop plate.

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When set up properly the idle stop screw should hit its stop at the same time as the throttle lever stop makes contact. Higher or lower rpm setups should always be done with sync gauges in place. You can NOT turn a carb idle screw and hope it stays in sync.

 

Having free space on the throttle lever will allow you to stretch the cables, bend the idle stop plate and wear the cable pullies.

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