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Old 04-02-2018, 06:53 PM
Jim Gries Jim Gries is offline
Join Date: Mar 2013
Location: Tucson, Arizona
Posts: 4

Regarding this SB, can anyone explain why the carbs need to be sync'd once the new springs are installed? The springs can be installed without missing with the throttle cables.
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Old 04-02-2018, 07:21 PM
DHeal DHeal is online now
Join Date: Sep 2006
Location: Windsor, California
Posts: 738

OK, here's my take on it: One of the functions of the carb springs is to maintain a consistent tension on the two throttle cables -- this serves to reduce any free-play or slack in the two cables thus making dual carb action more consistent and smoother. The new coiled springs likely exert a slightly different force on the cables than the former tension springs. The rebalancing of the carbs (both mechanical and pneumatic) reestablishes the appropriate cable tensions for the new-style coiled springs.
David Heal - Windsor, CA (near Santa Rosa)
EAA #23982 - EAA Technical Counselor and Flight Advisor; CFI - A&I
RV-12 E-LSA #120496 (SV w/ AP and ADS-B) - N124DH flying since March 2014 - 700+ hours (as of Nov 2018)!
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Old 04-02-2018, 08:49 PM
rvbuilder2002 rvbuilder2002 is offline
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 7,873

Nothing to do with spring force......

The carb sync adjustments, when doing it precisely, ends with very small adjustments being made to the cables.

The fit of the arms to the throttle shafts on the carburetors is not close tolerance by any means. The length of the lever arm multiplies the slop in the arm/shaft interface.
Since the arms have to be removed from the shafts to install the new springs, it is highly unlikely that the arms can be just reinstalled and fly.

It is a standard practice when doing carb inspections to remove the throttle and choke lever arms from the carb. so that the carb can be removed without disturbing the cable adjustment (this is taught in the Line Maintenance class).
It is also standard practice that the carb sync must be checked (and usually adjusted) when the carb's are reinstalled.
Any opinions expressed in this message are my own and not necessarily those of my employer.

Scott McDaniels
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
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Old 04-03-2018, 09:39 AM
Jim Gries Jim Gries is offline
Join Date: Mar 2013
Location: Tucson, Arizona
Posts: 4

Thanks for the response. After installing the new springs I ran the engine and discovered the idle RPM had changed. I would agree, tolerance at the linkage attach point is enough to require a carb re-sync.
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Old 04-03-2018, 04:22 PM
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scottmillhouse scottmillhouse is offline
Join Date: Apr 2007
Location: Madison, AL
Posts: 234

This is an easy SB to do. Once cowl was off probably only took 1/2 an hour. The carb sync always takes me awhile with getting up to temperature and then trial and error in and out of the plane. Thought tension was too weak to pull to full throttle until I tried it out in flight. Works great.
Scott- 2018 dues paid
Got medical back, now building a 7A; N579RV reserved, new engine installed, doing FWF, instruments & finish painting.
RV-12 #120515, N319RV 1st flight 12/2015 now over 270 hours.
RV-9A, 536 hours sold, RV-7 sold, Kitfox sold
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Old 04-03-2018, 05:08 PM
mwardle7 mwardle7 is offline
Join Date: Oct 2009
Location: Utah
Posts: 62

I couldn’t find the torque spec for the throttle arm nut in the latest Rotax Illustrared Parts Catalog. I did, however, find a spec in an older German version of the manual - 44 in lbs.
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Old 04-14-2018, 09:55 PM
Gandalf Gandalf is offline
Join Date: Apr 2008
Location: Medford, OR
Posts: 279

Originally Posted by bob1393 View Post
Looking at the revised instructions on 50-05, it appears the new spring is put on differently than the old type.

Yes. Those of us with older plans will need to download and print out 50-5 Revision 5.

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