Carb sync issue

Okay, I guess you are right.

May I ask a question regarding the pneumatic sync?

When you adjust the idle speed while mechanically syncing, you adjust both idle stops. In our airplane, the two bowden cables are attached to the same lever in the cockpit- if I was to adjust only one of the idle stop while pneumatically syncing... I see that the other carb, who is still resting on the stop screw, prevents the other to retard the amount you won't to. I guess that's the reason to only adjust 1/2 turn, to prevent stress.

But that seems to be a strange procedure?

I found and article from a magazine, ( https://www.xairireland.com/912%20carb%20balance.pdf )where the person suggest to back up both stops, only to adjust the stop in the cockpit, and then when the carbs are synced- just gently put the stops the same as the stop on the lever in the cockpit.

I guess that's not exactly what Rotax suggests... but he kinda have a point?

And last, shall I expect synced carburetors from 2500 all up to max power, when all is working?

Hi Tage,

Carb syncs are easy and quick once you understand what the gauges tell you  Carb sync's are easy and that's why I cover them in such detail in my class and we do a real time carb sync with everyone participating. It takes me  10-15 minutes once the engine is warm and the gauges connected. Use gauges because you can diagnose with them , but not a Carb Mate type instrument. Your article person is doing it improperly. You're welcome to call me if you want at 520-349-7056.

Here is a simple quick basic carb sync.

1. Make sure the mixture screws on the bottom of the carb are equally 1.5 turns out. Once done they won't move so if you know these are good don't worry about them.

2. Mechanical sync:

 Now back out the idle stop screws so they do not touch the stop plate under it. Pull the throttle in the cockpit all the way back to idle position then check to make sure each carb throttle arm is all the way back in the closed position. If not adjust the cable length to make sure they both are fully closed. Now take a .004 feeler gauge and screw down the idle stop screw until it touches the feeler gauge / stop plate and then remove the feeler gauge and turn the idle stop screw down 3/4 turn. This will slightly open the carb to around 1700 - 1750 rpm. The manual says to use 1.5 turns, but I find that sets the idle higher than I want it to be.

3. Now warm up the engine to operating temp then shut it down and connect your gauges. Start the engine and signal whomever is in the cockpit to run it up to 3300 - 3500. Most of us use this rpm range now because 2500 rpm is too far into the idle circuit and if you sync them there the needles on the gauges will split again if you advance the throttle. You'll get a far better sync at this higher rpm. Now go to the carb with the lowest vacuum as that's the one getting more fuel and that's the one you want to screw the bowden cable adjuster backwards to slightly increase this vacuum and as you do this you can watch that gauge vacuum come up to the other gauge's vacuum. Once these are equal tighten the bowden cable nuts on that carb and now the high rpm sync is done.

4. Now sync the idle rpm. Here you get to decide what rpm you want. If you want 1900 rpm or 1700 rpm that's your choice, Just know when you set lower idle rpm's that is not a place to just sit and idle. So if your rpm is 1900 and you want to lower it then go to the carb with the lower vacuum because it's supplying more fuel and back off the idle stop screw until that lower vacuum goes up to where you want it and it's equal vacuum to the other carb. If you want to increase idle rpm then go to the carb with the higher vacuum and screw the stop screw down so your idle rpm comes up, the vacuum number goes down and the two vacuum readings are the same.

Now I double check my sync job by advancing the throttle up one more time to make sure they stayed equal and then back down to idle to make sure those are equal. If all is good you're done.

You can diagnose with gauges too. Like if at idle one gauge reads 14 inches of vacuum and the other says 24 inches then shut down and clean that higher gauges idle jet out. That said I always clean idle jets during an annual anyway because I find about 60% either partially clogged or fully clogged. We do this in class too.

Another thing to look for is when you start the engine and see what carb you have to adjust and then look at the bowden cable adjuster and there is no more adjustment left you need to shut the engine down and screw the bowden cable adjuster way in to have some adjustment and then shorten the throttle cable at the throttle arm. If you don't do this you can't adjust that carb.

These are the basics and should get it done.

Carb syncs are easy and what makes them easy is understanding what the gauges are telling you and not trying to re-invent the wheel. One last thing. Out of the 35 planes I annual a year all, but maybe two will be out of sync. Never neglect to do a carb sync. 

I hope this helps.