Service Bulletin 18-03-06 Carburetor Throttle Return Spring Replacement

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 bending metal.

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 throttle stops.

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 throttle lever.


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.

Completing Service Bulletin 19-03-22 Replacement Of #2 Exhaust Pipe

Last fall during the condition inspection, I completed a few service bulletins and am just now getting around to documenting them on the DOG Aviation blog. From a safety aspect, the most important service bulletin I completed is Service Bulletin 19-03-22 ... which involves replacement of the #2 cylinder’s exhaust pipe due to some of the RV-12 fleet experiencing cracking at one of the welds and in a couple of cases, a  complete separation of the #2 exhaust pipe.


First a little history about the RV-12’s exhaust system. The exhaust system on early RV-12’s placed the muffler very close to the RV-12’s oil cooler which contributes to high oil temperatures … especially for those in really hot climates.  In an effort to move the muffler a little further aft from the oil cooler, Van’s changed the shape of the Rotax 912ULS cylinder’s exhaust pipes (which are custom welded to begin with) allowing the muffler to be positioned a little further aft. The original four exhaust pipes are part numbers EXH-1201 through EXH-1204 and are not affected by the service bulletin. As a side note, on the original RV-12 exhaust system, the muffler’s exit pipe protrudes through the lower cowling at an aft angle.


The DOG Aviation RV-12 received one of the first shipments of the redesigned exhaust system … which has exhaust pipes numbered EX-00015 through EX-00018. One of the identifying features of this exhaust system is the muffler is positioned a little further aft and the muffler’s exit pipe protrudes straight down through the lower cowling. Also of note, the #2 exhaust pipe is positioned close to the lower cowling and one of the springs is very very close to the lower cowling. The cracking #2 exhaust pipe has a part number of EX-00017.


The cracking or complete separation of the EX-00017 #2 exhaust pipe is a serious safety problem which needs to be taken seriously. In addition to the obvious threat of carbon monoxide, the #2 exhaust is so close to the lower cowling that a separated exhaust pipe could have the potential to easily start a fire.

My finger is pointing to the weld on the EX-00017 exhaust pipe that is cracking/separating.

The old EX-00017 #2 exhaust pipe is on the left and the new EX-00017-1 #2 exhaust pipe is on the right. Looking closely, one can see the new EX-00017-1 exhaust pipe has a few obvious differences …. a doubler plate is added over the weld that was cracking for added strength, the overall shape of the pipe is a little different and the welds are larger.


Not wanting to take any chances, decided it best to not rely on inspections and just go ahead and replace the #2 exhaust pipe with the newly revised EX-00017-1 exhaust pipe along with new copper flange nuts and new springs where the #2 exhaust pipe meets the muffler. Instillation of the #2 exhaust was simple and done in a few minutes … replacing the safety wire I use through the springs is another matter.


After installing the new EX-00017-1 #2 exhaust pipe and attaching the lower cowl, I immediately noticed the new shape of the EX-00017-1 exhaust pipe created more clearance between the #2 exhaust pipe and the lower cowling … which I was glad to see. Unfortunately, I did not have my camera with me the day I reinstalled the lower cowling, so did not get a photo of the completed instillation with the lower cowl in place … will have to try to remember to do that and place the photo here.

RV-12 Rudder Pedal Block Extensions

Thus far, I have been very pleased with flying the DOG Aviation RV-12 … it is truly a nice flying airplane. My only minor complaint stems from the rudder pedal configuration, in that, I find it difficult to make large rudder inputs with my feet on the rudder pedals without also pressing on the brakes unless I keep the balls of my feet pulled back in an unnatural position. Of course, for most ground operations or while in the air, this is not an issue … but on short final or during takeoff roll it requires a cognizant effort to pull the tips of the toes aft to insure large rudder inputs are not accompanied with unwanted brake inputs.

Sliding the feet higher on the rudder pedals would help but then the tips of my shoes (size 12) run into interference with the firewall shelf. Admittedly, it is not a big deal … just a few moments of uncomfortableness while pulling the toes aft to keep them off the brakes during takeoff and landing.

That got me thinking ... Gee, if the F-1290 pedal blocks were a little thicker, it might put my feet in more of a natural position on the rudder pedals and  help keep them off the brakes. With all the buzz about 3D printing, I decided to see if I couldn’t make a set of pedal blocks that are exact copies of the Van’s F-1290 pedal block … just a little thicker so the feet are in more of a natural position when on the rudder pedals. But that posed a problem … I have never used any type of 3D modeling software, so where do I begin?

EAA (Experimental Aircraft Association) to the rescue. I was talking to some friends at the airport about wanting to 3D print a set of thicker rudder pedal blocks, but didn’t have access to 3D software. I was informed that as an EAA member it was possible to obtain a free full featured (minus stress analysis) version of Solidworks Educational Premium that college student’s use. The EAA has formed a partnership with Solidworks allowing EAA members to download Solidworks Educational Premium for free … along with a 1 year license agreement which can be renewed as long as EAA membership is valid.

Knowing nothing about Solidworks (or any other drafting program), it seemed like learning to draw the thicker pedal blocks would be a daunting task. When first launching Solidworks, my eyes immediately glazed over … the sheer number of menus, options and submenu items available is truly daunting for a beginner. Fortunately, within Solidworks (under the help menu) there is a very good built in tutorial that starts out simple and builds on previous lessons.

After playing with the tutorials for a few evenings, I was able to acquire enough knowledge about Solidworks to successfully draft up a thicker rudder pedal block by carefully measuring the F-1290 pedal block and entering those dimensions into Solidworks then adding an extra ½" to the overall height of the petal block. I did not want to change the mounting bolt hardware so a recess was made for the mounting bolt to drop into. With a little bit of tutorial practice and some patience, making an accurate drawing for a thicker pedal block was not that hard. Below are a few screen shots of the finalized design drawing.

Top view of the thicker rudder pedal blocks drawn using Solidworks.

Bottom view of the thicker rudder pedal blocks.

Side view of the thicker rudder pedal blocks.

 End view of the thicker rudder pedal blocks.

A couple of years ago Tom, a friend who is building a RV-10, mentioned he bought a 3D printer and made the offer should I ever want to have something printed, to let him know. I told Tom about my using Solidworks to create a drawing for thicker rudder pedal blocks and he said he had plenty of blue HDPE filament … so Tom printed two blocks for me that are hollow inside with a crosshatch pattern for strength to save on material, since they are just for proof of concept. Thanks Tom!!!

The blue RV-12 rudder pedal block on the left was printed by Tom using the Solidworks file I made.

Side by side one can see the blue pedal block is the same as the black Van’s pedal block on the right … just a ½" thicker.

Bottom view of both rudder pedal blocks. Have to say, as a first time effort at making a 3D drawing and then having a part printed from the drawing, the thicker rudder pedal block looks great.


The big question now is how well will the thicker rudder pedal blocks actually work out? Only one way to find that out … install them.

The thicker proof of concept rudder pedal blocks installed on the pilot side of the DOG Aviation RV-12.

For comparison, these are the original Van’s F-1290 rudder pedal blocks on the passenger side.

So you are probably wondering … was it worth the effort … absolutely! My feet are now in a much more natural position when placed on the rudder pedals. Although the blocks could even be a tad thicker, the positioning of my feet on the rudder pedals is much more comfortable while moving the rudder pedals and staying off the brakes ... so I’m calling this effort a success.

Now that I know the Solidworks drawing I’ve made will produce a functional part, the plan is to make up four rudder pedal blocks from a tougher black material such as NylonX or CarbonX. Unfortunately, Tom’s 3D printer does not have the proper type of printer nozzle required for those filaments, so I will need to have the final pedal blocks printed elsewhere.


Return from the future:


As previously mentioned, the printer nozzle on Tom’s 3D printer is not suitable for printing stronger filament plastics like CarbonX or NylonX … so a few months back, I contacted Steve at rvplasticparts.com who is a fellow RV owner on the Van’s Aircraft Forums (VAF) that has a cottage business of 3D printing accessories for owners of various models of Van’s aircraft. I told Steve I've made a drawing of a taller rudder pedal block and had already printed a prototype which fit nicely. However, I wanted to have a set printed using a stronger material such as CarbonX or NylonX … Steve decided NylonX would be a good choice. I told Steve he could have my drawing to use if he wanted to add the thicker rudder pedal blocks to his product line and he decided to take on the project.


Steve tweaked my drawing a tad and will be offering the taller rudder pedal blocks in an assortment of thicknesses. The prototype Steve sent me is 3/16" taller than my original prototype … which should be perfect since my prototype blocks felt good … but, as mentioned earlier in this post, could stand to be a little thicker. Another change Steve made was increasing the thickness of material under the bolt head from ½" to ¾" feeling the end result will be much stronger. The original rudder block from Van’s was ½" so I made my drawing using the same thickness which created a surprise when I tried to test fit the new taller NylonX rudder pedal blocks Steve sent me …. the original AN4-15A bolts were not long enough, so I needed to order longer AN4-17A bolts. Below is a photo showing the new NylonX rudder pedal block on the right, my proof of concept prototype in the center and Van’s stock rudder pedal block on the left.

As can be seen in this photo, the NylonX rudder pedal block on the right is 3/16" taller than my original blue prototype in the center which is 1/2"taller than the stock Van's rudder pedal block on the left.


As previously mentioned, my blue prototype shown above felt good and is a vast improvement over the stock Van’s rudder pedal block … but I felt the block could stand to be even a tad taller. When the longer AN4-17A bolts arrived today, I headed to the hangar to test the fit of the new NylonX rudder pedal blocks Steve printed for me. At first, I only replaced one block so I could compare the feel of my prototype with Steve’s 3/16" taller rudder pedal blocks. The overall feel was about the same … but I noticed Steve’s NylonX rudder blocks allowed larger rudder inputs before the foot actually began pressing on the brake pedal. So overall, I’m very happy with the thicker NylonX rudder pedal blocks Steve printed and will order another set for the right side of the airplane.

The new taller NylonX rudder pedal blocks installed. Note to fellow RV-12 owners, changing to these rudder pedal blocks will require longer AN4-17A bolts to replace the AN4-15A used to install the stock Van’s rudder pedal blocks.


A big thanks to Steve at rvplasticparts.com  for taking on my project and sending me a prototype to test. I think this is one modification other RV-12 owners will want to look into … especially if keeping your foot off the brake during rudder inputs feels slightly uncomfortable for your foot.


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The photos below are just stored here so I can post them on the forums. Basically, kicking around an idea here. Background: The owner of a RV-12 has a grade in front of his hangar to traverse and is trying to figure out a good way to get his airplane into the hangar without holding onto the tow bar so he can hold a winch controller or pull a rope connected to pulleys.

I suggested reversing the tow bar and pull it with a winch or rope/pulley setup connected to the reversed tow bar. However, it was pointed that without locking the nose wheel somehow, should the tail begin to track off center a little, the castering nose wheel will make matters worse and quickly get the airplane off track … true enough.

So that’s had me thinking for a few days and I think I’ve come up with an idea that may solve the guy’s problem. During the last unexpected warm day we had here I slipped up to the airport to take a few photos I will post on the forums. The idea is to use the RV-12’s steps to support a board traversing the bottom of the fuselage that the tow bar can rest on and be secured between blocks. The blocks will prevent the tow bar from moving side to side thus keeping the nose wheel locked straight while the tow bar is being hauled aft into the hangar by a winch or rope/pulley arrangement. Below are proof of concept mockup photos using a yard stick so the RV-12 owner can see my vision in a picture form.

The tow bar is placed on the nose wheel reversed as shown here and will sit on horizontal piece of wood or metal traversing the belly of the airplane. The areas where the blue tape is placed around the tow bar will need blocks attached to the horizontal piece to capture the tow bar. The blocks will capture the tow bar preventing side to side movement thus keeping the nose wheel held straight as the airplane is pulled backwards into the hangar.

The horizontal piece needs to be connected to the RV-12’s steps using a vertical piece that has wooden pins which will insert snugly into the step’s tubing as shown in this photo … or the vertical piece could slide over the step’s tubing, builders choice. The vertical piece will likely need some gussets where it meets the horizontal piece to stiffen the assembly, so the fixture has no side to side play. The steps will lock the whole structure preventing ant side to side movement and also support the tow bar as the airplane is pulled aft into the hangar.

As can be seen in this photo, the Van’s tow bar ends well forward of the com antenna.

Looking aft, As can almost be seen in this photo (bad focus), the com antenna is biased to the pilot side of the RV-12’s bottom fuselage … so a cable or rope connected to the center of the tow bar handle will easily pass by the com antenna.


I  believe my idea will likely work well, however not having an incline to traverse to get inside my hangar, won’t be constructing a working version to prove the idea actually works.