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SB-18-03-06

Allan Stern

Well Known Member
I am curious about this service bulletin and why Van's has issued it saying to rectify the issue before flight or at next annual? My engine came from Rotax without any springs on the throttle shaft. Checking several other manufacturers with Rotax engines shows no springs on shaft. Secondly when I originally got my engine from Van's we were told to replace the original springs with a heftier one supplied with the engine.
Why hasn't Rotax said anything about throttle shaft springs if these are better than what we currently have.

I have ordered the springs and will look at them and see.

Allan Stern 2018 paid
RV 6A sold
RV 8A for sale
RV 12 out of phase 1
 
Doesn't say before flight...

At or before the next annual condition inspection or upon a part
failure of one of the previous spring types installed on the aircraft.
 
This thread would probably be better in the RV-12 section if a moderator wants to move it. <<Done...ScrollF4>>

A couple of corrections -

- The time of compliance listed in the SB is At or before the next annual condition inspection or upon a part failure of one of the previous spring types installed on the aircraft.

So if you just did a condition inspection last week, you don't have to change until a year from now unless one of the current springs breaks.

- The springs you changed to when you got your engine were actually a lighter/weaker spring, not heavier.

The heavy tension spring originally supplied on the engine makes control of the throttle position more difficult and can cause unwanted throttle creep.
They are a universal / meant for all applications spring that is not necessarily ideal in for all situations (think of an Aircam with 12 foot long throttle cables with multiple bends, vs an RV-12)

The new spring has been introduced to resolve a breakage problem some RV-12's were having because of the low spring tension allowing a vibration mode that caused accelerated fatigue failure.

The new spiral torsion spring can not vibrate in the way the tension spring could , thus improving the longevity.

Having said all that, if someone has an E-LSA RV-12, they can ignore a service bulletin if they don't like it. There is nothing regulator to require you to install the new springs, but I would recommend that you do.

For S-LSA RV-12's, following SB's and safety alerts is required.
 
Last edited by a moderator:
I just received the new throttle springs today from Van's. They are very very different than the ones we have been using. They are very short. Scott - will these stretch enough without breaking to fit? Also, they don't have a loop on either end of the spring - are we to bend the spring ourselves to make the loop so they can be attached?

Bob1393
 
Looking at the revised instructions on 50-05, it appears the new spring is put on differently than the old type.

Bob1393
 
The old spring is an extension spring that is stretched linearly (lengthwise). The new spring is a torsion spring that slips over the throttle shaft. As the throttle shaft rotates to the closed position the torsion spring winds-up to provide the counter force.

Torsion springs are commonly used on motorcycle carburetor throttle plates. Well proven design.
 
Throttle springs

Jim,

I know that a lot of motorcycles use those springs on carburetor so and are a proven design. However if this is so, than why doesn't the Bing carburetor factory or Rotax put them on the engines when they assemble them and ship the engines out? I have a hanger mate building an air-cam. Will be interesting to see if Lockwood aviation does anything?

If I get a SB that telling me to do something "at or Before next annual" then I kind of maybe self interpret "before flight" like that stabilator cracking. Which I took off and fixed along with the AST issue too.
 
Allan,

The stabilator and anti-servo tab SBs both gave a time of compliance of, "Before further flight". That's pretty clear and unambiguous. This one doesn't say that it needs to be done right now, mor like "the next time you're in there anyway". And if you're flying an E-LSA, you don't have to do it at all.

We're going to be doing our CI soon, and will be pulling and disassembling the carbs for their 200 hour inspection, so it's good timing for us. Never mind that we have a set of brand new McFarlane springs on the shelf for backup... maybe I can find another Rotax owner who wants them.
 
I did a little searching and it seems maybe Bing did use extension springs on their carbs and perhaps it was carried over by Rotax.
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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.
 
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.
 
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.
 
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.
 
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.
 
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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