High idle after mechanical synchronization

[Eriselle]

Hi VAF.

I am experiencing a rather high idle speed out of the gate after doing the mechanical carb synchronization according to the RV-12 MM. It's sitting at about 2500 cold, and then 2600 as it warms up. It takes a full CCW turn of the idle stop screws on both sides to get to 1800, and another 1/8 ish of a turn to get to 1650. The RV-12 MM is a little ambiguous whether the limit of 1/2 turn on the idle stop screws only applies for making the two sides synchronized at idle, versus also applying to the idle speed.

I am also experiencing some variation in the off-idle synchronization over the range of the throttle. If I synchronize at 2500, it is off at cruise RPM. If I synchronize for cruise, it's off at 2500.

The carbs were recently overhauled. I took them apart twice now after seeing this behavior to see if everything was reassembled correctly. I did not find any issues, everything seemed exactly how it should be. The carbs are seated in the sockets properly. The sockets are brand new. The socket clamps are tight to the spacers, and the carbs are snug in the sockets. The air filters were recently cleaned and re-oiled, and they are snug and seated well on the carbs.

I've redone the entire synchronization procedure exactly according to the MM four times now and I'm getting the same result every time.

Maybe I'm being too picky. Maybe this is all totally normal? It's not grossly off, but it is noticeable. And the idle thing seems to be totally fine if I just make the adjustment to where I want it and ignore that 1/2 turn thing for setting the speed.

Unfortunately, I do not know whether this was occurring before the carbs were worked on.

Any thoughts on this, anyone?

 

[seagull]

This is what I do and it has worked well for me for 1200 hrs. I balance at @ 3000 rpm, then balance and set the idle at @1750 using the idle stop on the carburetors. This process is adjusting each carb idle screw to have the proper rpm and balance. I may go back and forth a few times to get them exact. After that is good I set the cable stops so I can’t pull hard on the throttle and bend the carburetor stops. The cable stops are there to prevent this.

 

[Eriselle]

You should be balancing at @ 3000 rpm, then balance and set the idle at @1750 using the idle stop on the carburetors. This process is adjusting each carb idle screw to have the proper rpm and balance. You may go back and forth a few times to get them exact. After that is good you MUST set the cable stops so you can’t pull hard on the throttle and bend the carburetor stops. The cable stops are there to prevent this.

The RV-12 MM is not specific about what RPM you should sync at- it says "through the middle of the range". The Rotax Line Maintenance Manual is also not specific about an RPM. Lockwood taught me during the Rotax iRMT Service/Maintenance classes to do 2500 RPM. My LSRM-A course taught me to do it at 2500 RPM. Where are you getting 3000 RPM from?

1750 is also not an exactly correct idle value, because there is not one exact correct idle value. There are valid reasons to set the idle lower, and it is dependent on how much you can rely on the operator to run it appropriately and not cause excessive gearbox wear. A lot of schools set it higher, such as 1800, specifically because you cannot trust student operators to be kind to the gearbox.

The issue that I'm describing is not that I have no idea how to do synchronization. I'm stating that I have to clock the idle stop screws beyond the allowable range in order to achieve an acceptable idle speed. I am stating that the off-idle synchronization is not the same at different RPMs. Synchronizing at any given rpm, whether 2500 or 3000, does not address my question, because which side is reading higher manifold pressure, and by how much it is higher, is changing throughout the rpm range. I'm asking if this is normal to some degree, or if it is an indication of a problem and needs to be fixed.

 

[seagull]

Eriselle, I edited my answer so it isn’t directed specifically at you. Maybe other readers will find it useful.

 

[RV12_AL]

I rebuilt my carbs and re-positioned my vent lines to the air filters as per seagull did and my mechanical sync was so good, that I didn't have to make any adjustments except for idle on my last sync.

Below is from the great Roger Lee at rotax-owner.com. He is very wise on the Rotax engines!

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 Sync

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

 

[DanH]

The usual error is not realizing the job entails both an idle stop sync and an off-idle sync using the throttle cable ferrules.

Might consider finding a skilled motorcycle mechanic. Some of us were setting up CV Bings on BMW boxer twins all the way back in the 70's, and practically all have done inline 4's. Same thing. The 912 is nothing special.

 

[bobg56]

I use the electronic Carb Mate. I pinch off the balance tube at both ends instead of one. I do the off idle at around 2500 rpm or slightly less. It's usually just a minor adjustment unless you had the carbs or cables disconnected. I then set the idle to 1650 and sync using the idle screws. I used 1650 because after the balance tube is unrestricted the idle rises and also taking account of Flight Idle. During warm up I stay above 1800...going above 2500 when sycing I've heard that the idle circuit switches to the main jets. I've tried doing 3500 but my idle sync was off, I was chasing it. The balance tube will smooth out slight carb irregularities. The engine spends most of its time in the 5000 range. Carb sync is most beneificial during the warm up period. This is what I've read and have been told through out the years and conversations with Lockwood techs. My engine runs very smooth and starts easily so I must be doing something right.