Hi John,

The things that tend to cause the carbs to be rough at idle are heavy floats (they need to be weighed to rule these in or out even if you just did it 10 hours ago). The idle jet is clogged, but that should show up on a carb sync with a really high vacuum on the offending carb. An air leak around the rubber intake sockets for the carbs or at the air intake manifold where they screw down on top of the heads. Usually from someone taking these off and failing to make sure the "O" rings aren't pinched. This can and does cause detonation. Tweaking the mixture screw on the bottom by 1/4 turn should not cause a roughness. The 1.5 turns out is usually good for almost all people unless you live in an extreme condition. A choke being partly open and not closing all the way or a cable hanging up and not in sync with the other side.


Last week at CPS and the Heavy maint. Rotax class out of the way.:)

It has been 2 months now since the last post regarding the new Sensenich 3 blade test prop installation at Van's. I thought it might be worth resurrecting this old post, hoping for a performance update. Anyone have a progress report?

Tom

I have no insider information, but it seems to me if there is an update on this topic it will likely be announced at Oshkosh :)

I beg to differ with you....mid range synch @ 3800 rpm (called off idle) is where you synch the needle valve circuit of your carb (done first before you synch idle setting. Done right...these carbs function well.

Following the Rotax 9 Series IRMT class that Jolly teaches....I have been called to help synch several different types of 912 uls installations as well as RV-12’s. What i’ve Found to be the root cause of guys having problems getting a good synch accomplished is incorrectly installed/adjusted carb cables (one or both with to much slack)...basic carb installations stuff..

I disagree with your method and the training given at the Rotax Service Specialty classes does as well.

The critical detail to having good gear box longevity is carbs that are WELL synced at the low end of the RPM range.

If they are after you do your adjustment in the middle of the range then great. If they aren't, the low end should never be compromised in an effort to get the middle exact.
I guarantee that doing so will cost you $$$$ and down time.

No disagreement Scott...idle synch is done following “off idle adjustment” as per Van’s maint manual...it just wasn’t clear imho...until my Rotax class...the fact that there are 3: distinct fuel flow circuits in these carbs wasn’t clear until the class...idle and off idle (mid-range controlled by needle valve circuit) are the two synched. The cruse/take-off power circuit (main jet) is not synched.

The fact that you disagree with what was taught...does not make you right!

As Scott says? the critical point for synching the carbs is at idle and just-off-idle when the engine is making low RPM and low power. At low speeds power pulses are no dampened well by the prop (read flywheel) and the gearbox gets hammered. Synching the carbs is really matching power settings for this critical phase of operation. The Rotax gearbox cannot tolerate power imbalance at low speeds ? this is why a minimum of 1800 RPM (Green Arc) is required. Slower speeds than this and you?ll kill the gearbox in short order. Power imbalance at mid-range and highspeed is tolerated well because the crankshaft, gearbox, and prop are all running fast and have inertia and mass to dynamically dampen power pulses.

It?s imperative to understand the difference between a conventional horizontally opposed engine (Cont. & Lycoming) and a Rotax. Conventional engines have a single carburetor that feeds all cylinders relatively equal. All cylinders pretty much get the same fuel/air mixture and the engine hums along well at all speeds. No gearbox to worry about, so slow speeds aren?t a problem.

The 4-cylinder Rotax design is really two (2) separate power sources that share a common crankshaft and gearbox. What makes the engine behave this way is each cylinder bank is controlled by its own carburetor. Both left and right cylinder banks must produce similar power at slow speeds for the reasons stated above.

For clarity,
When I am talking about sync at idle, I do not mean at the idle stops. That can be tweaked a bit with the stop screws, but only after the sync is good at 1700 - 1800 RPM
That should be a focus point since that is the minimum you should be idling at for anything other than very short periods of time.
If you first sync at mid range throttle, there is no adjustment you can make to sync at 1800 and have it not effect the mid range.
At 1800 RPM, the carb's are still in the idle circuit range.

And I don't disagree with what I was taught. I was taught to focus on the sync while the carbs were operating in idle circuit mode.

The 912 engine suffers from rather serious torsional vibration below about 1400 rpm and a propeller on a geared engine does not act as a damper like a flywheel inserted between the engine and the gearbox might. Quite the opposite in fact as larger a MMOI here almost always exacerbates TV issues in this rpm range on most 4 cylinder engines and a reason why Rotax specifies a maximum MMOI for propellers.

I wish I'd shot some video of our EFI 912 on a test stand transitioning through this range years ago. Truly scary how much the TV would flex the entire test stand and a clear picture of why Rotax warns against operation in the low rpm ranges. It's there even with perfectly synched carbs or a single TB in our case.

Balancing carbs is vitally important but not the cause of the TV issues at low rpm on this engine.

Scott....again, could not agree more. I, when I first started flying my -12, I adjusted my idle to 1550 rpm (which you are correct stating from there to correct synch you would have to choose using “off-idle” compromising toward idle jet synch to effect smooth low rpm operation). As I gained experience flying my -12; I have found that setting idle to around 1775-1800 rpm {thanks to Roger aka Jolly} (synching using idle stop screws); then using off-idle for mid-range synch...you wind up with gauge needles staying dam near perfect....and a very smooth running engine.

The difference between 1550 and 1775 rpm, having very little difference in landing performance...once set up on stabilized approach (a slightly higher nose attitude)...just my two cents. I say again...pay attention to cable set up on carbs!

Edit: Sorry for the thread drift guys...just my passion for “Smooth Running Engines”...didn’t want to get your knickers in a twist Scott!