While reviewing some DOG Aviation photos for a fellow builder, I ran across some photos regarding replacement of the Rotax 912ULS throttle springs that I forgot to add to the Blog last fall.
Van’s Aircraft issued a service bulletin about two years ago switching
to a newly designed throttle return spring for the Rotax 912ULS which will
hopefully solve the throttle return spring issues that have been a nuisance for
the RV-12 fleet.
Before diving into the latest throttle spring service bulletin (I think
there have been at least two prior) … first a little back story regarding the throttle
return springs. Rotax has designed the 912ULS engine’s carburetors go to full
power in the event of a throttle cable failure. On older RV-12 aircraft the
throttle return springs supplied by Rotax for the 912ULS engine were VERY
strong … so strong, in fact, the throttle would constantly need to be adjusted
and readjusted during flight because the strong springs would cause the
throttle lever inside the cockpit to constantly creep towards full power. Another
issue plaguing the Rotax 912ULS throttle lever return springs is, over time the
throttle return springs were also prone to breaking.
In an effort to eliminate the throttle creep, new weaker springs were
developed … but they did not totally solve the issue with throttle creep plus
spring breakage remained an issue. While the DOG Aviation RV-12 was under
construction, Van’s began supplying a vernier-assist throttle lever manufactured
by McFarlane (a nice throttle unit), which became the standard offering. The
McFarlane vernier-assist throttle is accompanied by weaker throttle springs
supplied by McFarlane …. a step in the right direction, however, throttle
return spring breakage remained an issue.
Van’s has now totally redesigned the throttle return spring and made it
a helical torsion spring as opposed to the typical stretch spring. From a design
aspect, I think this is a much better approach and should totally eliminate throttle
lever return spring breakage.
Van’s Aircraft issued service bulletin 18-03-06 which covers removing of
the old style throttle return stretch spring from the Rotax 912ULS engine’s carburetors
and replacing the springs with the newly designed helical torsion springs. The
service bulletin refers the installer of the springs to follow the procedure laid
out in Section 50 of the plans. The Van’s part number for the new throttle
return spring kit is SPRING-00002-1 2 PACK. That part number will provide two
springs, one for the left carburetor and one for the right carburetor. Note: The
spring for the left carburetor has an ink marking to denote it from the right
spring. Below is a photo of the old style
throttle spring compared to the new style helical torsion spring.
The spring on the left is the old style throttle return spring … the
spring on the right is the newly designed helical torsion throttle return spring.
As one can see in the following photo, the standard Rotax 912ULS
throttle lever return spring is stretched between a hole in the throttle lever
and a bracket attached to the body of the carburetor. I suspect, being
stretched between two points and under constant engine vibrations, the throttle
return springs are more susceptible to fatigue cracking.
My finger is pointing to the old design throttle return spring. The
upper portion of the spring is connected to a hole in the throttle lever and
the lower portion of the spring is connected to a hole in a bracket attached to
the body of the carburetor.
Instillation of the new throttle return helical torsion springs is quite
easy. First the old style throttle return spring is removed from the throttle
lever. Then the hex nut and spring washer that secures the throttle lever and
throttle stop onto the throttle shaft is removed. Probably unnecessary, but I
used a red sharpie pen to mark the position of the throttle lever prior to
removing the throttle shaft hex nut. Use caution when removing the throttle
lever … I placed a wrench on the throttle shaft and another on the nut then
twisted the throttle shaft to make sure the throttle shaft was in the center of
its normal range of movement … then proceeded to remove the hex nut. Making
sure the throttle shaft is in the center if its range of motion assures that
the force applied to remove the nut will not be applied to the stops … possibly
This photo shows the red sharpie marks placed on the throttle lever (actually
not necessary). At this point, the throttle shaft hex nut and spring washer
have been removed from throttle shaft. As a note, the stop lever can be seen
quite well in this photo, it sits on the throttle shaft directly behind the
throttle lever … it will also be removed from the throttle shaft.
After removing the throttle shaft hex nut, the throttle lever is
carefully slid off the throttle shaft. There is no need to loosen or remove the
throttle cable to get the throttle lever off the throttle shaft. Behind the throttle
lever resides the throttle stop, it also needs to be slid off the throttle
shaft as can be seen in the next photo.
Here one can see the throttle lever and throttle stop have been slid off
the throttle shaft. Once the throttle stop is removed one can see two Philips screws
… my finger is pointing to the upper Philips screw that will capture one end of
the throttle return helical torsion spring.
Instillation of the new throttle return spring is quick, simple and easy
to accomplish procedure. The new spring slides over the carburetor’s throttle
shaft … the inboard end of the spring will sit under the head of the upper Philips
screw (the screw I’m pointing to in the above photo) and the outboard end of
the spring will rest on the throttle stop. Instillation of the new spring, task
wise, is not difficult. That said, however, finding the right tool for the job
proved difficult. I tried a couple of varieties of spring tools I had in the
shop, but they all seemed to have clearance issues. I did not want to use a
small screwdriver to push on the spring (as most probably do) for fear of
creating small scratches that, over time and vibrations, may possibly create
stress fractures in the spring. After lots of pondering and playing around with
various tools an idea occurred to me …. perhaps a piece of waxed string will work
to tension the spring. That idea worked like a charm!!! I slid the throttle
return spring partway onto the throttle shaft and slid the throttle stop onto
the throttle shaft positioning the outboard end of the spring so it is captured
by the throttle stop. Next I looped a piece of waxed cord over the inboard end
of the spring and slid the assembly further onto the throttle shaft. As the
assembly got close to the Philips head screw, I pulled on the waxed cord to
tension the spring enough so the inboard portion of the spring could be
positioned under the head of the Philips head screw. Worked slick … as documented
in the following three photos.
In this photo, one can see how the inboard end of the new throttle
return spring will be captured under the head of the upper Philips head screw
when the new spring is in its final inboard position.
As one can easily see here, a piece of waxed cord was used to capture
the inboard end of the spring so it can be tensioned by pulling on the string.
Looking closely one can see how the outboard end of the throttle return spring is
captured by the throttle stop. All that is left to do is pull down on the waxed
cord so the inboard end of the spring clears the Philips head screw and push
the assembly in the remaining 1/8” so the inboard end of the spring sits under
the head of the Philips head screw.
This photo shows the final position of the new throttle return spring …
the inboard portion of the spring is captured under the head of the upper
Philips head screw and the outboard portion of the spring is captured by the
throttle stop. Using a waxed cord to tension the spring makes this task truly a
piece of cake.
Once the throttle return spring and throttle stop are fully seated on
the throttle shaft, the throttle lever is positioned back onto the throttle
shaft and the assembly is secured on the throttle shaft by the spring washer
and hex nut …the hex nut is tightened to 44 inch pounds. I accomplished that by
using a crows foot wrench attached to my torque wrench and holding onto the
throttle shaft with another wrench … here again, making sure the throttle shaft
was positioned in its center of motion so no force would be applied to the
Completed reassembly of the throttle lever on the throttle shaft. Unfortunately,
once in position, the new throttle return springs are hidden from view by the
Service bulletin 18-03-06 is a very easy service bulletin to complete
(especially if one uses my trick of using waxed cord to tension the spring) and
having helical torsion throttle return springs should put an end to the broken throttle
return spring issue.