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PROCEDURES FOR MECHANICAL AND PNEUMATIC SYNCHRONIZATION OF THE 912 CARBS:
Carb sync for beginners. :)


This will get you through the basics and after you do a sync or two it will get easier and you will get better. This is not an official document and is only meant for your edification and will get you through a carb sync with a little better understanding. Some experienced people that do carb syncs on a 912’s may vary some, but the basics ideas are the same.


Mechanical Synch of the Carburetors – Do this First

Bowden cables adjustment screws (cable housing adjuster) should be centered, but not mandatory. By having the Bowden cable adjustments in the middle of their adjustment range will give you a little more adjustment when you get to that part of the procedure. Having these adjusters all the way out or in may hamper you adjusting needs.

Adjust throttle cables so that both carb throttle arms move simultaneously to both extremes (open and closed).

Ensure Idle Mixture Control Screw is properly set (on the bottom of each carb). Screw them all the way in then back out 1½ turns.

Reverse the throttle arm springs so they pull toward “closed” throttle just to facilitate mechanical sync.

Unscrew the Idle Stop Screw. Make sure the throttle is closed in the cockpit. With a .004” feeler gauge between the screw and the arm tighten the screw until it just touches. There should be minimal friction on the feeler gauge.

Remove the feeler gauge and screw the Idle Stop Screw in 1 turn. This is just a starting point and will most likely change.

Return throttle springs to original configuration (i.e., pulling the throttle arms toward the “open position”).

Connect the Pneumatic Sync Tool

There is more than one way and location to connect your vacuum lines.
It is imperative to have the engine up to the operating temperature. Remove one end of the rubber crossover compensating tube between the two intake manifolds or connect to the top of the intake manifolds or use the cross over tube rubber hose pinch off method shown in Rotax owner videos. All these attachment methods will work.

Hook up gauges or electronic sync tool. This is the setup I use. Secure one end of a gauge or electronic device to the rubber end of the compensating tube and the other to the gauge at the 90 degree fitting coming out of the intake manifold where the compensating tube hose was removed. Secure both attachments with a hose clamps to prevent leaks. Leaks will affect the results of your sync process. You can hook up your sync tool at the small screw on top of the intake manifold, but then you have to pinch off the rubber hose between the carbs on the cross over compensating line. I find this is just more work and you need to make sure the rubber hose on the compensating tubes are long enough (usually newer engines) and you don’t allow any leaks between the carbs. Many people have to cut the aluminum tube shorter and make the rubber hose longer to pinch it if you have an older engine with short rubber connecting tubes. I think it is easier and faster by just pulling the hose off as originally described and it facilitates complete carb separation.
Make sure you have good brakes, use wheel chocks and or if you have not so good brakes or no wheel chocks then tie your plane down and always have someone in the cockpit. Never do this alone while running an engine and no one in the cockpit..

“On Idle” Synch
NOTE* Many people like to do the high rpm or off idle adjustment first and then do the idle set.
I would do the high rpm sync first because it will affect the idle sync.

Note*
The gauge or carb with the lower vacuum number is getting more air/fuel and the gauge or carb with the higher vacuum number is getting less air/fuel.

Bring the cockpit throttle lever to idle stop. Ideal idle RPM is approximately 1700-1800 rpm depending on your specific aircraft and needs. Adjust idle setting to this value. The 912UL 80 HP can handle a little lower idle rpm over the 912ULS 100 HP due to compression ratio.

If the idle is high (say, 1900), adjust the carb with the lower vacuum number on the gauge because that is the one getting more air/fuel. Retard the Idle Set Screw until the vacuum numbers are the same. Example: If one gauge is at 15” of vacuum and one at 14” of vacuum retard the lower 14” until it drops to 15” to equal the other carb which will now lower your idle rpm.

If idle is low (say, 1600), adjust the carb with the higher value, it is getting less air/fuel. Advance the Idle Set Screw until the vacuum readings are the same. Example: If one carb is at 15” and one at 14” turn the idle stop screw in on the carb with 15” of vacuum to match the one with 14”. This will raise the overall idle rpm.

Needles should now match.


“Off Idle” or high rpm Synch

If while running the engine the gauge needles shake then you will need to install or close down the inline needle valves until the gauge needles stop shaking and pulsating. Now the needles will likely show different vacuum readings. Do this sync before the idle sync because this sync usually will affect your idle sync set point.

Run engine at approximately 3500 RPM. Do the “Off Idle” synchronizations at this rpm. I prefer 3500 rpm to start since that is closer to where you run the engine. You may find that setting the balance at lower rpms doesn’t stay very accurate and that as you move up in rpm to 4000-4500 rpm they are out of sync again. Set them at the higher rpm or at least double check the higher rpm when you think you are done. There is a good chance they won’t be in sync if you used a low (<2500) rpm to set them.

These adjustments will be made only at the brass Bowden cable adjustment for the higher rpm sync, where the cable housing meets the carb adjuster on top of the carb, do not touch the idle set screws.

Note*
The gauge or carb with the lower reading is getting more air/fuel and the gauge or carb with the higher reading is getting less air/fuel.

Compare vacuum gauge readings. Normally, but not always, adjust the carb (retard) with the LOWER vacuum reading (getting more fuel) back down towards the carb with the higher vacuum reading. Adjust the Bowden cable screw outward (toward reducing the air/fuel) so the throttle arm backs off (decreased RPM) to bring vacuum reading to a higher vacuum number.


When vacuum readings are equal, lock it down.
If vacuum readings are the same, bring throttle lever back to idle stop then advance it again to 3500 rpm. Verify vacuum readings are still the same for both carbs. If readings are different adjust the carb’s Bowden cable with the lowest gauge reading to match the higher gauge. Once the gauges are aligned move the throttle back to idle and back to 3500 rpm a time or two to make sure the carbs stay equal. Throttle to idle. Re-adjust the Idle Stop Screw if necessary at this point. You are now done.
Remove the gauges and hook the carb compensating tube back up. The engine will run slightly smoother after the compensating tube is reconnected and sometimes the idle will be slightly higher after the cross over balance tube is reconnected. If you know this then while doing the sync set the idle 50 rpm lower so when the balance tube is reconnected you be right on your personal target.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

  • Re: Carb sync for beginners

    by » 10 years ago


    Hi Roger, firstly many thanks for all your advice on this topic over the years! I'm obviously being a bit thick and missing something fundamental here so would just like to clarify:
    I have sync'd my 912uls for many years using your technique. I check/set at 3500 RPM (your advice) and it holds the sync over the full rpm range which is good. However I just don't get the bit about subsequently setting the idle synch. Surely the only place one can adjust the carb synch is the cable adjusters and once it is set (at high rpm) it is set? Are you referring to adjusting the throttle arm stops on each carb individually as required to achieve a perfect idle synch? If so it just seems to me that adjusting the throttle arm stop(s) individually to perfect the idle synch must put a strain somewhere in the system if the pilot subsequently pulls the throttle right back to idle, ie on one cable/ throttle arm/ throttle arm stop etc? I actually have never used the throttle arm stops (only to help with initial mechanical synch). I have them set clear of the throttle arms at idle and solely use the cockpit throttle idle stop, which is adjustable during maintenance to set the idle speed. To me this eliminates any risk of bending or breaking anything by pulling the throttle back to hard.
    Have I missed something really basic here (very likely!). Cheers.

  • Re: Carb sync for beginners

    by » 10 years ago


    Hi Jonathan,

    Carbs are usually out of sync at idle and the higher rpm. Carbs should be set at idle sync and the best way is to use the stop screw. Engine throttles should have a stop on it so it can not pull back so far as to bend the idle stop plate. Some people have a throttle stop in the cockpit and some have it out on the carb. For an example I have some with a small brass cable end stop or lock on the cable between the Bowden adjuster and the the throttle arm. When I get the carbs synced at idle with the idle stop screw I slide this device up against the Bowden cable adjuster right were the cable exits and snug it down with its set screw. This way the idle is synced, but the pilot can not pull the throttle back any more and bend the idle stop screw plate. I would bet if you put your gauges on you can or are synced at the higher rpms, but if you don't sync the carbs at idle or use the idle stop screw then they are out of sync at idle. What are the two rpm levels we use the most? We use high rpms for cruise (should be 5K+) and we sit down at the lower idle rpms during warm ups, approaches or waiting to take off. If we think it is important to sync carbs at the higher rpms why not the idle rpms. It is easy to do. Next time I get a plane in with a Vernier throttle cable I'll try and remember to take a picture to show where the cable lock is placed. I have only seen 1 major shop here in the US that does not sync carbs at idle and only the high rpms. I personally don't like 2500 rpm as it describes in the manual because I think it is too low and close to idle settings. It just doesn't flow enough air and fuel to have a good representation of the higher rpm settings we use when at cruise. This is why I like 3500 rpm for the higher rpm sync. The gauge needles tend to stay together much better too if synced there.

    Here is the Rotax carb sync video. Chapter 4 close to the end of the video shows idle stop screw set and throttle stop in the cockpit. If you have a Vernier throttle then using a cable stop on the cable out at the carb is easier.

    http://www.rotax-owner.com/all-videos/rotax-minute/408-cs-4247

    I hope this helps or makes sense. Some times I may not be as clear in my writing as I would like to be. :)

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell


  • Re: Carb sync for beginners

    by » 10 years ago


    Many thanks for the reply Roger (very clear thanks!). I will do this next time.

  • Re: Carb sync for beginners

    by » 10 years ago


    I found a picture. See the little brass cable stop on the throttle cable. That will keep you from pulling back too far. keep it loose until you get the idle set with the stop screws. Then put this up against the bowden cable end and tighten the set screw. Now your Vernier throttle system has a stop that can't come back too far and over power the idle stop screw plate and bend it.





    Verniercablestop.pdf (You do not have access to download this file.)
    Verniercablestop.jpg (You do not have access to download this file.)

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell


  • Re: Carb sync for beginners

    by » 10 years ago


    HI. I just got my carbs from overhaul rotax center. I'm trying to synchronize them pneumatic but on low rpm (2000) I can see higher vacumm on left carb and on 4000rpm I see higher vacumm on right carb. What characteristic is changing ?

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