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  #1  
Old 05-01-2021, 10:41 PM
chevytoo chevytoo is offline
 
Join Date: May 2011
Location: Langley B.C.
Posts: 29
Default Sticky throttle at idle

The last 2 flights I have had a very sticky throttle near the idle position on approach to land. I have an O-320 with Vetterman exhaust and Robins carb heat muff on one crossover. The aircraft has 15 hours total. I applied carb heat before I reduced power for landing. I went to add some power and the throttle was stuck but more force broke it loose. I tried to reduce power just before touchdown and the first time could not. The second time I used more strength and the throttle went to idle. After roll-out and clear of the active, the throttle was smooth as ever. Procedure here at CYXX, inbound aircraft to be at 2000 ft. Field elevation is 200 ft. Altitude to be maintained until altitude restriction cancelled or on final approach. I suspect my problem is carburetor ice. I removed the cowls. I can see nothing that would explain the problem. I do not get much of a manifold pressure drop when I check the carb heat on run-up. I get a substantial drop if I check carb heat in cruise.
Outside temp was about 70F. Did not check the dew point but there was mist and reduced visibility. Now the throttle is smooth like a new one and does not get sticky with friction applied. Anyone else had this problem?
Ray
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  #2  
Old 05-02-2021, 06:20 AM
plehrke's Avatar
plehrke plehrke is offline
 
Join Date: Sep 2006
Location: Defiance, MO
Posts: 1,781
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It may not be carb ice. Maybe you are getting some throttle linkage binding that only happens in flight. The throttle linkages can move a bit during flight conditions since the mounting bracket can be a bit flimsy if not supported well.
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  #3  
Old 05-02-2021, 06:57 AM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
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can't really rule out carb ice, as prime conditions for it, but seems a bit unlikely that you would have enough ice to keep your throttle from moving, yet have no noticeable symptoms such as RPM drop or over rich or lean.

That said, I could see how a bit of ice built up on the butterfly that was not enough to impact performance, yet made it difficult to fully close it. Eventually it broke off and closed.

I also would double check for something that could be binding.

Larry
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  #4  
Old 05-02-2021, 07:08 AM
Kyle Boatright Kyle Boatright is offline
 
Join Date: Aug 2005
Location: Atlanta, GA
Posts: 4,542
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I'd be suspect of binding or something going over center (or nearly over-center), and would check that first.

Second, the green cables from Van's are heat sensitive and can develop a lot of resistance to movement. Check to make sure your cables aren't routed extremely close to your exhaust system.
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  #5  
Old 05-02-2021, 03:30 PM
622BH 622BH is offline
 
Join Date: Nov 2011
Location: Albany, OR
Posts: 196
Default Recent Experience...

I've recently had an experience where the Throttle seemed to bind at the idle point and with a push to about throttle it moved smoothly. On inspection I found the cable, after the mounting near the Throttle Body, was not straight. It had a slight bend in it due to my error in installation.
When I straightened the cable by adjusting the mount the Throttle moves smoothly throughout its "throw." Check to see if there's any "bend" in the cable by having someone in the cockpit manipulate the knob while you watch the movement of the cable to the throttle body. It should be as straight as possible for best function.
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  #6  
Old 05-03-2021, 12:40 PM
chevytoo chevytoo is offline
 
Join Date: May 2011
Location: Langley B.C.
Posts: 29
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I did check to see if there was any bend in the throttle. None. The cable is a long way from the exhaust. I will wait until there is a good spread between temp and dew point and go flying again and see if the problem returns.
I remember way back with my /67 Datsun driving down the freeway, took my foot off the throttle and nothing happened. I shut the key off and coasted to a stop on an off ramp. Sat there for 5 minutes , manually turned on the carb heat and all normal again.
Ray

Last edited by chevytoo : 05-03-2021 at 12:46 PM. Reason: Add more info
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  #7  
Old 05-12-2021, 11:52 AM
chevytoo chevytoo is offline
 
Join Date: May 2011
Location: Langley B.C.
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Went flying yesterday. Temp 20, dew point 7. No sticky throttle. I am even more convinced I had carb ice.
Ray
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  #8  
Old 06-10-2021, 01:02 PM
chevytoo chevytoo is offline
 
Join Date: May 2011
Location: Langley B.C.
Posts: 29
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More investigation and Carb ice does not seem to be the problem. Talked to a few people about the problem and one said before I even finished the story that he had seen that before. The clearance between the throttle shaft and bushings is too tight. Carb heat is expanding things enough that the throttle sticks. I guess that is why no sticky throttle when I approach with no carb heat and why no sticky throttle after landing and the carb heat has been returned to cold. I will try to verify this soon.
Ray
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  #9  
Old 06-10-2021, 02:13 PM
DNeufeld DNeufeld is offline
 
Join Date: Aug 2016
Location: Coeur d Alene
Posts: 94
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If it sticks only at idle and you can't find any external binding, you might want to check and see if the throttle plate itself is binding in the throat of the carb. If the throttle plate wasn't aligned in the bore properly the plate will stick. There should be witness marks in the bore if this is the case.
Another possibility. If the idle stop had to be adjusted to bring the idle down because of a vacuum leak the plate could stick in the bore. There would be a lean condition at idle if this is the case. If the idle mixture was lean and had to be adjusted, it might be a good idea to look for a vacuum leak near the carb base where a vacuum leak would effect all cylinders. I know of a situation where a stuck throttle plate was the cause of a crash and loss of life from this very problem.
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  #10  
Old 06-10-2021, 05:45 PM
Steve Melton's Avatar
Steve Melton Steve Melton is offline
 
Join Date: Apr 2011
Location: Cincinnati, OH
Posts: 3,255
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engine not running and have a paper towel and a can of LPS1. push the throttle all the way in and completely loosen the locking screw and pull the throttle all the way out. you will see a plastic bushing in there... lube everything with LPS1 and put it back together. high humidity will cause the binding. lube it from time to time.
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