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.
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.
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 bending metal.
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.
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.
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 throttle stops.
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.
