Rough running 912ULS

Check for inlet manifold leak - suspect area between new carburettor "rubber"  flange  (No's 15, 14, 20, 19) & aluminium manifold

 For diagram, refer to  Parts catalog, Section 73-10-00, Page 2

Sean Griffin wrote:

Check for inlet manifold leak - suspect area between new carburettor "rubber"  flange  (No's 15, 14, 20, 19) & aluminium manifold

 For diagram, refer to  Parts catalog, Section 73-10-00, Page 2

Carb sockets are new ( 1 year old) and checked for leaks - all good   

If you think it could be a leak in the Manifold/intake system you can rev the engine slightly until it starts running rough, then use WD 40 or other spray and spray it over the engine intake manifold area in little squirts, if you get a sudden increase in RPM you have a leak. You can narrow it down from there.

Hello Don,

When I got my new engine installed (workshop) I also had a rough engine running.

I've gone through all the possibilities, tried and optimized everything would go too far here. It was all very small steps but probably not that true cause.

Ultimately, I only have one tip for you if I consider that the engine runs rough after the soft start and you can rule out that there are other causes than the carburetor or the cables that control the carburetor.

So my experience is that the factory setting of the BING carburettor regarding the idle mixture control screw (unscrew 1 1/2 turns - ROTAX/BING recommendation) is not correct. I had to unscrew a whole 2 or 2 1/4 turns. This was the only way to set the mechanical and pneumatic synchronization of the carburetors (throttle valve) in a reasonably stable manner. If the idle mixture is too lean during the first synchronization, then you don't have a chance because you probably have to open the throttle valve too far when idling so that enough fuel gets into the intake air (see below - you need air and fuel).

If the first synchronization has been done to some extent (idling probably still a bit too rich), then adjust the idle mixture control screws piece by piece so that the spark plugs of cylinders 2 and 4 (it is sufficient if you look at the upper spark plugs) on the right and left are fawn to slightly dark brown after a long idle (several long idle approach flights).

The idle rev will probably change after adjusting the idle mixture control screw (you will probably have to screw in the idle mixture control screws in 1/8 turns, depending on the spark plug image on the right or left, initially probably on both sides). If there are major changes in idle rev, repeat (finetuning) the synchrinization (throttle) until the spark plug color is correct (right and left same color) and the idle is as desired.

This may take several attempts - at least that's how it worked for me.

And now very important: The bowden cables or linkages of the throttle valve must run absolutely parallel when you open the throttle. The linkages sometimes have play or they run jerkily or or. And finally: check whether the chokes on both sides are closed again after the start. If one doesn't close completely, then the mixture is unequal.

Remember: the sync has nothing to do with the mixture setting. The synchronization only guarantees that the same amount of air is drawn in on both sides. How much fuel is in the air cannot be adjusted with the synchronization. To do this, you have to turn the idle mixture control screws while idling and when the throttle is open, it is the main jet (needle jet position 3) that provides the necessary fuel in the air. Professionals with the appropriate CO measuring devices can see from the exhaust gas how well the mixture is set.

Basically, we know: sucked air alone does not cause combustion, only the proportion of fuell makes an ignitable mixture and that should be identical on the right and left.

Just to close out this issue - 

Had an excellent ROTAX certified tech check out the ignition system, which ended up being all ok.  No manifold leaks, nothing mechanical in the engine.....

The prop was set wrong!  The prop angle when measured was correct on both, but one blade was set the wrong direction! One leading edge was forward, the other back.  Set the prop so many times before, it was dismissed that it could not be a problem but a good set of eyes saw the issue (actually heard it on a test run) and now all is good. engine runs great!