Complying With Rotax’s 5 Year Time Limit For Rubber Coolant Hoses

In section 05-10-00 of the Rotax 912 Line Maintenance Manual there are considerations for time limits for various Rotax engine components … most notably, the table for all the rubber components which Rotax suggests replacing every 5 years. As can be seen below, Rotax wants a lot of stuff replaced.

The Rotax 5 year time limit table for rubber parts.

Highlights of some of the items that require replacement are: All the rubber hoses used in the cooling system, fuel and oil hoses that are not Teflon, carburetor mounting sockets, carburetor diaphragms, carburetor and fuel pump vent hoses, plus the fuel pump.

This post only covers replacement of the rubber coolant hoses. The decision was made to follow the path quite a few RV-12 owners have taken … which is to NOT replace the rubber coolant hoses using rubber hoses from Rotax. Instead, silicone hose will replace the rubber hose because, unlike rubber hose, silicone hose does not harden or crack and will last a very very long time … so 5 years from now, the silicone hoses will not need to be replaced. (As a side note for fellow RV-12 owners, silicone hose is great for coolant … but definitely should NOT be used for fuel or oil applications). I was able to locate some high quality silicone hose in a green color which color coordinates nicely with the green paint on the engine’s valve covers and the green color I panted the fiberglass air shroud on top of the engine. The silicone hose was purchased through HPCouplers (.com) and is Flexfab 5521-062 for the 5/8" hose going to and from the cylinder heads. Flexfab 5521-038 was purchased for the 3/8" hose going to the expansion bottle mounted on the firewall. Flexfab 5521-100 was purchased for the two 1" hoses to and from the radiator.

Work begins by draining the coolant from the cooling system. The Rotax manual suggests draining the engine coolant by removing the lower bolt from the water pump. This is much easier said than done on a completed RV-12 … and I can tell you with all honestly, I will NEVER remove that bolt again to drain the engine coolant. It is just very difficult to get onto with a wrench … and once you do get a wrench on it, there really is not enough room to turn the bolt. After fighting with the lack of access and trying various combinations of tools to remove the Allan head bolt, I got to the point where I could break the bolt lose but not able to do much with it after that. I finally gave up and waited to remove it when the engine was jacked up while completing the nose gear leg replacement service bulletin … (which was being completed at around the same time). The only tool combination that seemed to work well for removing the lower water pump bolt was using a wrench slid over the shaft of a hex socket as shown in the photo below.  Hindsight being 20/20, it is truly far easier to just pop off one of the lowest two coolant hoses going to either cylinder #1 or cylinder #2 … or just remove the lower hose on the radiator.

While there was more clearance from having the engine hoisted and the lower radiator hose removed, a metric hex socket and a wrench was used to remove the lower water pump bolt to drain the engine coolant. Prior to hoisting the engine and removing the lower radiator hose I figured this bolt was a no go because access to the bolt just plain sucks on a RV-12.

The single ply rubber hoses leading to and from the cylinder heads on the Rotax 912ULS are 17mm … good luck finding that size in a silicone hose here in the USA. The closest fitting silicone heater hose is 5/8" which equates to 15.88mm … so the silicone hose is stretched out a little to fit onto the existing coolant fittings. Also of note, the outer diameter of the 5/8" silicone heater hose is slightly more than that of the 17mm rubber hose Rotax uses … this creates challenges, because the Rotax constant tension hose clamps just barely slide over the silicone hose once the hose is slid onto the coolant fittings attached to the Rotax 912’s engine. Some owners have opted to order slightly larger hose clamps from McMaster-Carr … but if you are really patient and lube the hose with coolant, the Rotax clamps will get the job done.

The coolant hose replacement began in earnest by replacing all the lower coolant supply hoses between the water pump and the cylinder heads. For easier access, the muffler was removed from Rotax 912ULS to gain unrestricted access (that’s laughable) to the lower coolant hoses. Prior to removing the old coolant hose, a red Sharpe was used to mark the current position of each coolant hose on the cylinder head fittings so the new silicone hose could be placed in the same position on the cylinder head coolant fittings.  Next, each rubber hose was carefully removed and used as a template to measure the replacement silicone hose.

The tool used to make all the necessary hose cuts was a Craftsman Handi-Cut. It makes nice, clean, smooth cuts. If you have access to a Handi-Cut or similar tubing cutter, use it.

Photo of the Craftsman Handi-Cut used to cut the silicone coolant hose.

Reinstalling the silicone hoses on the right side of the engine went fairly smooth. The inside of the silicone hose and the outside of the coolant pipe fittings were wet down with a little coolant to make them a little slick so the silicone hose could be positioned onto the fittings easier. Once the silicone hose was in position on the fittings, the constant tension clamps were slid into position on the hose. Discovered it also helps to wet the outside of the silicone hose with a few drops of coolant, so the constant tension clamps slide into position easier … this is because the silicone hose is slightly thicker, so the clamps tend to drag over the hose, especially at the bead end of the fittings.

Replacement lower silicone coolant supply hoses for cylinders #1 & #3 attached to the water pump.

Replacement lower silicone coolant supply hoses for cylinders #1 & #3 attached to the cylinder head coolant pipes.

Replacement lower silicone coolant supply hoses for cylinders #2 & #4 attached to the water pump.

Replacement lower silicone coolant supply hoses for cylinders #2 & #4 attached to the cylinder head coolant pipes.

A few years back, the DOG Aviation procurement department purchased a special locking cable tool made for the type of constant tension hose clamps used by Rotax. This tool works well for the most part … just as long as there is plenty of clearance to use it. However, there are a few locations where there just simply was not enough room to use it. For the really tight locations I needed to contact Chad, the local A&P mechanic at the airport, who graciously allowed me to borrow his Tool Aid hose clamp pliers. Thanks Chad!! After using Chad’s tool, I immediately ordered a set for myself.

The special locking cable tool made for constant tension hose clamps works well to expand a constant tension hose clamp and hold it expanded, as can be seen here … just so long as there is plenty of access room. The head of the tool takes up quite a lot of room, so there are numerous locations where it just didn’t work well.

This is one of the two Tool Aid 19750 hand tools for constant tension clamps borrowed from Chad. This tool was used at locations that did not allow good access to use the cable tool … it has cups on the end of the tool that capture the clamps. The cups rotate, so the tool can be used either horizontally or vertically … or any angle in between.

After using Chad’s clamp pliers, the DOG Aviation procurement department ordered the Tool Aid 19750 hose clamp pliers set. The pliers with the large cups worked great for the Rotax constant tension clamps on the DOG Aviation RV-12. Have also recently used the other pliers on smaller hose clamps.

The Tool Aid 19750 hose clamp pliers set. The upper pliers in the photo worked quite well on the Rotax hose clamps and the lower pliers work well on smaller sized hose clamps.

All and all changing the four lower water hoses was not too big of a deal … other than receiving a NASTY DEEP cut on my thumb (darn that hurt!) when the hose clamp pliers slid off one of the clamps when my hand was over the pliers (dumb) guiding the clamp between the lower engine mount triangle and onto the cylinder #2 water pump fitting.

Moving on to the upper hoses … NOTE: I have two suggestions for those about to replace the upper cylinder head return coolant hoses. First, because this is a mandatory 5 year rubber replacement, the rubber carb sockets will also need to be replaced … meaning the carbs will need to be removed also. So, make life easy for yourself and remove the four hex bolts on each intake manifold so the intake manifolds can be removed to allow for easy access to the hose clamps under them for the #3 & #4 cylinders (don’t forget to order 4 new O-Rings for the manifolds, Rotax part number 230-910). Second, my other suggestion is to remove the water tank, attach the four new silicone hoses while at the work bench, then fish the tank back into position … trying to slide the hoses onto the tank when it is on the engine is very difficult because there is very little room for hands and tools. Just remove the tank, you will be glad you did.

As work began on the upper hoses, things got a lot more interesting and much more difficult. While working on replacing the four upper coolant return hoses that connect the cylinder heads to the water tank, I ran into two roadblocks … the first being Rotax uses a close to 90° pre-molded rubber coolant hose between the coolant tank and cylinder #3. So, the DOG Aviation procurement department purchased a pre-molded 5/8" 90° silicone hose to use at this location. Decided to begin the fun by changing the #3 cylinder’s hose first figuring the pre-molded hose will set the position of the coolant tank. For starters, it was very difficult to slide the pre-molded hose onto the coolant fitting on the #3 cylinder head and it got worse from there. Even after removing the fitting from the cylinder head so I could hold the fitting in my hand, it was still a fight to slide the hose all the way onto the fitting … but eventually it did go on.  OK … so far so good. This is where the wheels totally fell off …. the pre-molded 90° hose that was purchased has very little stretch because of the 4 layers of reinforcement fibers used internally, and it absolutely just REFUSED to slide onto the coolant tank. I measured the beads on the tubes coming out of the coolant tank and they measure 20mm … obviously beyond the stretching ability of the four ply pre-molded hose. 

I tried all the tricks too, removed the coolant tank from the engine so I could get more leverage on it and lubricated the pre-molded 90° hose and tube with coolant … nope. Lubed the hose with liquid BoeLube … nope. Tried slippery CorrosionX .. nope. Even brought the coolant tank and the pre-molded silicone hose home so I could put the hose into boiling water in the hopes of getting a little more flex … nope. That four ply pre-molded 90° hose was just not going to slide onto the coolant tank pipe … period. Sadly, being furious about all the screwing around with the 90° pre-molded silicone hose, it was tossed into the trash before I realized no photos were taken. Can't say for sure but suppose if the 90° pre-molded hose had been one or two ply, it may have worked out OK.

Moving on, after checking the actual bend radius of the Rotax pre-molded hose and comparing that to the specifications for maximum bend radius of the silicone hose, discovered the necessary bend was well within the bend radius limits given for the 5/8" Flexfab hose … so just installed a straight hose to cylinder #3. As an insurance policy, although not necessary, a Goodyear E-Z Coil was also installed over the #3 cylinder’s hose as an insurance policy to prevent the hose from kinking … and a bed of silicone was placed under the E-Z coil to prevent it from wearing into the air shroud under it.

The second roadblock I discovered involved trying to slide the Rotax constant tension clamps over the 20mm bead on the end of the coolant tank pipes. The Rotax clamps were slid onto the hose far back from where the pipe bead will end up … then the hose was slid onto the tank’s pipe. Even with the Rotax clamps expanded as far as they would go, the clamps refused to slide over the beads on the coolant tank. The very time-consuming solution was to place the clamps onto the coolant tank’s pipes first then slide the hose over the bead up to the clamps and then slowly work the clamps over the hose in stages … slide hose on a little, slide clamp up a little and repeat until the hose was fully seated and the clamp in its proper position. I was doing this by myself so it took FOREVER … but, had I had a helper, it would only have taken a few minutes. This chore REQUIRES a helper if you want it to go smoothly using the Rotax clamps. Slightly larger hose clamps are available from McMaster-Carr and this is one area where they may have been the way to go … but I really wanted to use the Rotax clamps to keep the parts original except for the hose itself.

Coolant tank with all the new silicone coolant hoses installed ready to be fished back into position on top of the engine. Note the use of the Goodyear E-Z Coil to prevent the hose from kinking when it takes the 90° bend to cylinder #3.

To add another level of difficulty, the cylinder head coolant return pipes for cylinders #3 & #4 reside directly under the intake manifolds. Getting good access to those hose clamps requires removing the carburetor/ intake manifold assemblies … which is what was done. Not saying that the manifolds must be removed … however removal is easy, so why not just remove the four easy to access hex bolts and the manifold is off in a minute (don’t forget to replace the two rubber O rings under each manifold Rotax # 230-910). It is hard to photograph this area and get a good photo … but as an example, looking closely at the photo below of the intake manifold, one can see the new green silicone hose and black clamp tucked under the manifold.

Looking closely at this photo, one can see that the coolant return hose runs under the intake manifold. Getting to the hose clamp is tough so removing the intake manifolds makes accessing the hose clamps for cylinders #3 & #4 much easier.

Looking closely, the Goodyear E-Z Coil can be seen installed over the coolant hose for the #3 cylinder. The coil will prevent the silicone hose from kinking along the bend.

To complete the coolant hose replacement job, both the upper and lower radiator hoses were replaced with 1" ID Green Flexfab 5521-100. After both original hoses were removed from the radiator, the coil springs inside each rubber hose need to be extracted and inserted inside the new 1" silicone hoses (lower hose is 30" long and the upper is 20" long). The hose between the coolant tank and the expansion tank mounted on the firewall was also replaced with 3/8" Flexfab 5521-038. I elected not to use traditional radiator hose clamps on the 1" silicone hose because they tent to cut into hose. Instead, a form of constant tension hose clamp was used that is smooth on the inside along the entire area where the clamp rides on the hose and it has Belleville washers that allow the clamp to expand and contract with radiator temperature while keeping a constant tension on the hose … the clamps were purchased from McMaster-Carr.

Close-up photo of the new constant tension hose clamps with the Belleville washers used on the 1" silicone hose. The Belleville washers allow the clamp tension to remain the same as the clamp expands and contracts with coolant temperature changes.

Because the nose wheel gear leg assembly was replaced with Van's newly redesigned stronger version, the mounting for the lower radiator hose is done differently. The original gear leg normally has a hole drilled into it where a single Adel clamp is mounted to support the lower radiator hose. The new nose wheel gear leg assembly no longer has that mounting hole ... now the lower radiator hose is supported via two Adel clamps, as shown in the following photo.

As can be seen in this photo, the new Van’s nose wheel gear leg supports the lower radiator hose via two Adel clamps.

The coolant used to refill the cooling system was Dex-Cool mixed 50/50 with distilled water. Dex-Cool is on the Rotax list of recommended coolant for the Rotax 912ULS engine. All and all except for the nasty gash to my thumb, replacing all the coolant hoses was not all that hard once all the challenging aspects of the task were figured out … but it was a royal pain in the derriere to be sure.

Completing Service Bulletin SB-00023 WD-1221 Engine Mount Standoff Inspection/Replacement

Service Bulletin SB-00023 involves inspection/replacement of the RV-12’s WD-1221 upper engine mount standoff. There have been a few RV-12 aircraft that have developed cracks near weldments on the WD-1221 upper engine mount standoff discovered during inspections. Van’s took quick action and redesigned the WD-1221 using tubing double the thickness thereby “theoretically” making the areas where the cracking occurs much more robust.

Service Bulletin SB-00023 requires inspections of the WD-1221 upper engine mount standoff be conducted every 100 hours and ongoing log book entries be made unless a crack is found … which, if discovered, requires a mandatory replacement using the new WD-01221-1 upper engine mount standoff. Figured while I still had access to the engine hoist I borrowed from Tom (a fellow RV builder at the airport putting the finishing touches on his RV-10 for his first flight), may just as well go ahead and replace the WD-1221 standoff (even though mine is not cracked) and just be done with it.

All and all, with the exception of dealing with the two sets of double Adel clamps that attach onto the WD-1221 upper engine mount standoff, the process of changing out the standoff was not all that difficult. Fortunately, no fuel, oil lines or wiring needed to be removed (all those items were installed following Van’s suggested path). It was tight but there was enough room to shoehorn the old mount out and install the new mount without jumping through a lot of hoops.

Design and outward visual appearance of the two upper engine mount standoffs seem identical ….however, the new standoff appears to have a tiny hole on the endcap. The new part number for the redesigned WD-1221 upper engine mount standoff is now WD-01221-1. The following  photo shows the new WD-01221-1 standoff and the next two photos show the existing WD-1221 standoff and Adel clamp placement.

As one can see from this photo of the new WD-01221-1 upper engine mount standoff it visually looks identical to the original, which can be viewed in the two Adel clamp photos bellow.

Photo of the pair of Adel clamps used to secure the main firewall forward wiring harness to the WD-1221 upper engine mount standoff. Reattaching these Adel clamps after the upper engine mount has been changed out will be a challenge. Fortunately, there are tricks that help ease the pain … but it still presents a challenge because there is not a lot of room for good access.

This photo is of the second pair of Adel clamps attached to the WD-1221 standoff … these clamps are used to secure the throttle and choke cables to the WD-1221 upper engine standoff.

Unfortunately, accessing the bolts that secure the WD-1221 upper engine mount standoff to the firewall requires removing the F-1240 upper forward fuselage skin. Once the F-1240 upper forward fuselage skin is removed, closing the canopy will allow great access to the WD-1221 standoff’s mounting bolt heads … as such, this can be done as a one person operation. There was one unexpected complication involving removal of the two mounting bolts that created a little extra work. The bolts could not be completely removed because they contacted the F-00055 com supports.  Solving this issue required placing a sawhorse under the tail cone and jacking the engine until the tail cone contacts the sawhorse … then jacking just a little more causing the isolation mounts to separate from the WD-1221 standoff so the isolation mounts could be removed to garner the enough room to slide the WD-1221 standoff forward far enough to clear the bolts protruding the firewall. Guess I could have bent the lower ear on the com supports … but really did not want to do that.

This photo shows how the F-00055 com supports prevent the mounting bolts from being completely removed.

This photo shows when the bolt was pulled aft to the point of touching the com support, there was still quite a bit of the bolt extending beyond the firewall.

With the WD-1221 upper engine mount standoff rotated upward, it can easily be fished out towards the left side of the aircraft.

The new WD-01221-1 upper  engine mount standoff is slid in from the pilot's side of the aircraft following the same path used to remove the old mount,  then it is rotated down and slid aft over the protruding bolts. Next the isolation mounts are reinstalled. Also of note: Rotax recommends replacing the rubber isolation mounts as part of their 5 year rubber replacement protocol …  so all of the engine’s isolation mounts have been replaced (forgot to mention that in the last posting). After all the bolts are inserted, the castle nuts are torqued to 160 in lbs … after torqueing to 160 in lbs, if a castellation is not yet aligned with the hole in the bolt, the castle nut is tightened more until a cotter pin can slide into the hole in the mounting bolt. The upper limit for the torque is 190 in lbs. so there should be enough margin to allow a castellation to align with the hole in the mounting bolt without over torqueing.

The new WD-01221-1 upper engine mount standoff installed and ready for the two pairs of double Adel clamps to be attached.

The new WD-01221-1 upper engine mount standoff completely installed … the new mount looks identical to the old WD-1221 standoff with the exception of the tiny hole in the endcap.

The last item left to do is reinstalling the two pairs of double Adel clamps … this is a challenge. Adel clamps are very springy and want to spring open so even if there is plenty of room to get your fingers on them, they are a challenge. That difficulty is upped dramatically when reinstalling the two pairs of Adel clamps onto the new WD-01221-1 upper engine mount standoff. There just is not much room for fingers and tools …. especially when reinstalling the two Adel clamps that support the firewall forward main wiring harness. Even using the tricks I know, reinstalling these Adel clamps was an exercise of patience.

Best photo I could get showing both double sets of Adel clamps attached to the new WD-01221-1 upper engine mount standoff.

Builder tip: The easiest way I have found to install double Adel clamps is to use Adel clamp pliers in conjunction with safety wire. First squeeze each Adel clamp separately and wrap a thin safety wire around the clamp to keep the clamp from fully opening when the pliers are removed. Then align the two clamps together and place the Adel pliers over the two clamps, tighten the pliers, then slide the alignment awl through the bolt holes in the clamps. When the holes are aligned, quickly remove the awl and insert the bolt. Here is where it gets a little dicey …. usually, the bolt is not long enough to allow a nut to be installed because the thickness of the Adel pliers takes up a lot of room. So what usually works well is to press down on the head of the bolt, then slide out the pliers and reposition pliers on top of the bolt head and tighten the pliers again. This should leave enough threads beyond the pliers to get a nut threaded onto the bolt. Once the nut has caught a few threads, remove Adel pliers and thread the nut on the bolt a little more. Next remove the safety wire, final position the Adel clamps, then tighten the mounting bolt. Below is a photo sequence of the steps involved.

Adel clamp pliers and the awl that comes with the pliers.

Using Adel clamp pliers to squeeze the clamp’s ears together so thin safety wire can be wrapped around the clamp. Leave the ends of the wire long so the wire can be unwrapped by easily by hand.

Example of how the awl that comes with the Adel clamp pliers is inserted through the holes of both Adel clamps to align the holes so a bolt can pass through the clamps.

As can be seen here, typically an AN3-3 bolt is not long enough to allow a nut to be threaded onto the bolt because of the thickness of the pliers.

Once the AN3-3 bolt is inserted through the pliers and Adel clamps,  forcefully press down on the bolt head then remove the pliers. Quickly reposition the Adel pliers to capture the head of the bolt …. this allows the threads of the bolt to extend beyond the pliers just enough to thread on a nut.

This photo shows how repositioning the Adel clamp pliers to press on the head of the bolt will allow enough threads extend beyond the pliers to allow a nut to be threaded onto the AN3-3 bolt.

As one can see, clamping the bolt head with the Adel clamp pliers allows a nut to be threaded onto an AN3-3 bolt. Now remove the safety wire, final position the Adel clamps and finish tightening the nut and bolt.

As mentioned previously, completing Service Bulletin SB-00023 was not that difficult … but dealing with the two sets of double Adel clamps located in tight quarters is actually the most challenging aspect of replacing the WD-1221 upper engine mount standoff with the new WD-01221-1 standoff.

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.