McFarlane Throttle Cables Attached To Carburetors

Yesterday, was a very late afternoon start and became mostly a cleanup day of sorts egged on by the fact I could not locate the throttle springs or hardware that came with the McFarlane throttle in the engine crate. After about a half hour into a long overdue partial shop cleaning, the springs were located in the general area I had hoped to find them. Once the springs were located, continued cleaning for a while until I got bored with that and decided to get some progress done … so completed hooking up the throttle cables to the carburetors on the Rotax 912ULS engine.


When using the McFarlane Vernier-Assist throttle control, the builder is instructed to remove the throttle springs that come with the carburetors from Rotax and discard them. The McFarlane throttle control assembly is accompanied by a package of hardware which includes two carburetor springs and two cable stop nuts. My suggestion is DO NOT discard the Rotax carburetor springs because the replacement springs have been known to break.

The carburetor springs provided by Rotax are to be removed and replaced with these springs that accompany the McFarlane Vernier-Assist throttle control.


Installing the springs is simple, as can be seen in the above photo of the instructions. However, the use of pliers should be totally avoided if possible so the spring metal does not get scratches that can lead to stress fractures from the vibrations. A rounded spring tool was used to remove the existing springs and install the weaker McFarlane springs.

Using a spring tool to install the McFarlane springs on the left carburetor.

After replacing the throttle springs, the Rotax cable conduit and nuts provided with the carburetors are removed and the hardware that comes with the McFarlane cables is used to attach the cable conduit onto the carburetor’s cable bracket. The cable conduit on the end of the McFarlane cable has two nuts and two star washers that will secure the cable tunnel onto each carburetor’s cable bracket.

Removing the Rotax cable conduit from the cable bracket … these parts will not be used.

Installing the McFarland cable tunnel onto the cable bracket on the left carburetor. The cable tunnel should be installed so an equal amount of threads can be seen on both sides of the nuts.


After the cable tunnel is attached to the cable bracket, the throttle cable is attached to the throttle arm on the carburetor. The carburetor is shipped with a bolt and nut that will be discarded … but the Rotax step washer and flat washer will be reused when installing the throttle cable using a Van’s provided bolt and locknut.

A photo of the assembly instructions. The upper nut and screw my finger is point to was removed from the carburetor’s throttle arm and will not be used. However, the step washer that is slid onto the Van’s provided VA-219 cable bolt and the flat washer below it that my other finger is pointing to will be reused as shown. The cable stop provided by McFarlane in the package will be used as an idle position cable stop.


At this point, the builder is just placing the hardware in a position for a successful engine start. Later there will be a procedure where the carburetors will need to be balanced and the cable hardware will need to be readjusted anyway. Prior to installing the throttle cables onto the throttle arms the McFarlane throttle is to be pulled out away from the lock nut 2 ¾". I made a measuring tool from a tongue depressor to easily set the gap correctly without using a ruler at an angle.

A 2 ¾" strip of tongue depressor was used as a gauge for setting the position of the throttle control in the cockpit for idle.

I was working by myself and the instructions say…  while gently holding the throttle arm on the idle adjusting screw stop and pulling the slack from the throttle cable, tighten the nut to 8-10 inch pounds to secure the cable … two much for two hands. I opted to use a wire tie to hold the position of the throttle arm on the idle set screw and was able to use one hand to hold a wrench on the bolt and pull the slack out of the cable while using the other hand to torque the nut to 10 inch pounds. Caution here: … The cable locking nuts should not be over tightened or it will flatten the cable causing it to fray. A frayed cable will make future fine adjustments very difficult when synchronizing the carburetors … plus, the cable will be weakened and prone to early failure.

Snugging up the VA-219 cable bolt to the point the throttle arm will still slide on the cable, but barely.

After the throttle arm’s VA-219 cable bolt is tightened to 8-10 inch pounds, the cable stop is slid all the way back to the cable tunnel and snugged down. The viewer can see how nicely the cable tie worked for keeping the throttle arm in position on the idle screw (screw with the orange paint on it).

Tightening the idle cable stop against the cable tunnel.

Completed throttle cable instillation … of course, the wire tie needs to be removed.

The throttle cables were installed on both carburetors and all that is left to do is install the choke cables and position and tighten the Adel clamps the control cables are passing through.

Instillation Of Throttle And Choke Cables Begins

Van’s instructs the builder to install the Mc Farlane Vernier-Assist  throttle cables first, feeding them through the grommet on the firewall and then through an Adel clamp installed on the engine mount. For both the throttle and choke cable pair’s one cable is longer than the other … the longer of the two is routed over to the right carburetor.  I quickly discovered the wires for the GPS 2020 antenna previously installed were somewhat in the way. There was not enough slack firewall forward to move the wires underneath the big Adel clamp mounted to the engine mount and the wires would have ended up wedged between the throttle and choke cable bundle and the big Adel clamp … this was not to my liking. Fortunately, when the Dynon GPS 2020 antenna was installed, the wires were cut so a little extra length was available behind the instrument panel. The down side is that the upper forward fuselage skin needed to be removed to gain access to the slack. So another minor setback was incurred and the upper forward fuselage skin was removed so the instrument panel is now opened up again. Actually, that is not a bad thing because I have an EAA tech inspection scheduled for next weekend … so they will now be able to have a look at the instrument wiring. After pulling enough slack forward to route the GPS 2020 wires under the large Adel clamp and out of harm’s way, the instillation of the throttle cables resumed.

Feeding the Mc Farlane Vernier-Assist throttle control’s two cable ends through the lock washer and locknut for the throttle control as it is being inserted into the panel hole.


With the throttle cables fished through the firewall grommet and into the engine compartment with the longest cable of the two closest to the right carburetor, the locknut for the throttle was tightened. This is a 1" nut and I quickly realized I did not have a wrench that size, so took a look in Mike’s hangar and fortunately there was a 1” wrench sitting on the table waiting for me to borrow it for a few minutes … thanks Mike. At this point, I realized it would be in my best interest if the autopilot disconnect switch were installed now because it would be easier to get a wrench onto the locknut without the choke control in the way … so it was rounded up and installed along with the associated Molex connector the wires slip into.


With the autopilot disconnect switch now installed, the choke control was installed next. The choke cable routing is through the grommet in the firewall and UNDER the throttle cables with the longest cable positioned closest to the right carburetor. The handle on the choke control locks in a closed position, so the cable is to be installed with the handle locked so it is in a vertical position.

Tightening the locknut on the choke cable assembly … the handle needs to be in the locked “off” position and vertical when the choke control is secured to the panel. The red autopilot disconnect switch and associated Molex connector can also be seen.


With the throttle and choke controls now tightened down, attention was given to the Adel clamp the four cables need to pass through. Unfortunately, in order to get all the cables through the clamp it was necessary to remove the bolt. This created a small problem of trying to get the screw installed again while fighting two clamps. The solution was to use a piece of safety wire around the Adel clamp attached to the engine mount and twist it tight enough to bring the ends of the clamp together so the bolt could be inserted through the smaller Adel clamp and slipped into the hole in the other clamp without a fight.

The mounting screw for the Adel clamps was left lose so once the four cables are connected to the carburetors, the angle of the clamps can be adjusted to prevent the cables from rubbing on the engine mount they are sitting on now. The grey and orange wires with the do not remove label on them are the GPS 2020 antenna wires which were routed so they now pass below both Adel clamps.


The next step will be installing the throttle and choke cables to the Rotax 912's carburetors ... which will hopefully be accomplished during the next work session.

T-1211 Fuel Filler Neck Assembly & Fuel Tank Venting Installed

To complete the fuel system the only steps remaining was installing the T-1211 fuel filler neck assembly and the new T-1219C vent pipe which was installed and the fitting tightened. The holes and nutplates were installed on the T-1211 long ago and the mating area on the turtle deck around the filler neck was masked off to minimize the fuel tank sealant mess. All that was required now to complete the job was mixing up a small batch of fuel tank sealant and slather it in onto the flange on the fuel filler neck then mate the assembly to the turtle deck.


Van’s plans suggests using car wax as a release agent so the filler neck assembly can be easily separated from the turtle deck ... which will be necessary for fuel tank removal. I elected to slather Vaseline on the turtle deck skin as the release agent. The assembly went fast, prior to mixing the tank sealant, the T-1212 rubber fuel neck collar was slipped up on the fuel filler neck and two band clamps were placed on the fuel tank. After the tank sealant was mixed, the flange on the fuel filler neck was given a liberal slathering of sealant and the fuel filler neck was carefully mated to the turtle deck with six screws. The screws are tightened until the tank sealant begins to ooze out evenly around the filler and turtle deck junction. The screws are not to be tightened fully because the idea here is to make a gasket that becomes part of the flange on the T-1211 fuel filler neck and is reusable.

Tightening the band clamps for the T-1212 rubber fuel neck collar.

Completed instillation of the fuel tank filler neck and new fuel tank venting system. The RV-12’s fuel system is now 100% completed.

Propeller Re-pitched - Correctly This Time

A few days ago after completing the oil purge process, the propeller was pitched to 71.4 degrees and photos of the setup were posted on the DOG Aviation Blog and fortunately caught the eye of a fellow builder. One of the perks of sharing the building process of the Van’s RV-12 via the DOG Aviation Blog is there are many eyes watching the build, producing feedback from around the world. After posting the photo covering the process of using a digital level to set the pitch of the propeller, I received an Email from Nick, a fellow RV-12 builder in Colorado, saying one of us pitched the propeller wrong and would be saying “crap” …. Well, that would be me …. Crap!


Nick referenced a photo on a page 47-07 in the latest version of the plans, which is not in my plans. I had viewed the new page online and the use of the digital level seemed simple enough … but I managed to mess it up by rotating the prop blades the wrong way to obtain the required 71.4 degrees … DUH! The DOG Aviation RV-12 would have had the first RV-12 in the fleet that could back into a hangar under its own power. Many thanks Nick for the sharp eyes and taking the time to write and get me pointed “pitched” in the right direction so to speak.

This photo is incorrect … the photo was posted a few days earlier on the Blog covering use of a digital level to set the initial pitch to 71.4 degrees. What Nick spotted as being incorrect is the level is leaning in the wrong direction … good eyes Nick! The level should be leaning to the left or aft and NOT forward as it is here.


One unfortunate aspect of pitching the propeller blades the other way is the gap seals were shaped when the blades were in the opposite position, so it required many hours of filing with a jeweler’s files to get the shape acceptable. I did not want to remove the blades by myself or bother friends, yet again, for the same task. So with only a very tiny gap to work with, blue masking tape was placed on the prop for protection and the gap seals were slowly reshaped using lots of careful filing until there was approximately 1/16" gap between the gap seal and propeller when the blades were positioned correctly … an allowance was made in case the blades need to be pitched shallower because the 71.4 degrees is just a starting point. Builders will adjust the pitch to best fit their needs based on many factors such as field elevation, runway length and type of flying they mostly do.

Carefully filing one of the gap seals using a jeweler’s file to obtain the necessary clearance between the gap seal and the prop blade when the propeller blade is positioned correctly.


After filing both gap seals, the propeller blades were adjusted to 71.4 degrees (correctly this time) and the bolts were tightened one flat at a time and the angle rechecked and tweaked if necessary. After a couple of cycles of tightening the blades by one flat on the bolt, the blades were pretty much locked in position. At this point the propeller bolts were tightened in four stages of torqueing checking the blade angles after each stage ... 75 inch pounds, followed by 10 ft. lb., 14 ft. lb. and final torque of 19 foot pounds. The final blade angles ended up at 71.4 degrees and 71.35 degrees. Time to put the spinner back on and move on.

This is the correct orientation of the digital level when using the TOOL-00002 pitch tool with a digital level to set the propeller blade pitch to 71.4 degrees. The level should be leaning aft as it is in this photo.

The final result after torqueing the bolts to 19 foot pounds.