912 running rich?

James Stout wrote:

Check the plugs to see if they're black and sooty. Maybe the carbs need be synced.

Good call. Reading the plugs will tell if it’s really running rich or not. 

Nicholas Wilcox wrote:

James Stout wrote:

Check the plugs to see if they're black and sooty. Maybe the carbs need be synced.

Good call. Reading the plugs will tell if it’s really running rich or not. 

It seems to me that most Rotax 912's have black(ish) plugs. when inspected.

The 912's are not designed to run efficiently at low RPM (no ignition/retard/advance mechanism) so in the time that you cut power  Downwind, to flap deployment speed, Base, Final and taxi in, the plugs have all gone black.

You are unlikely to learn much from the plug colour.

Sean Griffin wrote:

Nicholas Wilcox wrote:

James Stout wrote:

Check the plugs to see if they're black and sooty. Maybe the carbs need be synced.

Good call. Reading the plugs will tell if it’s really running rich or not. 

It seems to me that most Rotax 912's have black(ish) plugs. when inspected.

The 912's are not designed to run efficiently at low RPM (no ignition/retard/advance mechanism) so in the time that you cut power  Downwind, to flap deployment speed, Base, Final and taxi in, the plugs have all gone black.

You are unlikely to learn much from the plug colour.

Good point. Although I’ve seen a difference in plugs from carbs with a leak and plugs without. Rainbow Aviation has a good write up on reading plugs that has visuals of plugs that are sooty and aren’t. 

Don't get hung up over dry black soot plugs IF YOU HAVE YOUR PLUGS GAPPED PROPERLY, CARBS SYNCED PROPERLY AND THINGS LIKE AIR FILTERS ARE CLEAN, ECT.....

If you sit and idle then forget reading your plugs. The imbalance of air will cause some plugs to have that dry black soot. The fuel burn from the plugs on the bottom of the cylinders aren't quite as efficient as the top. The black soot burns off at higher rpms. As mentioned if you are at idle all way way down wind to touch down then that's just like sitting on the tarmac and idling. You want to know what the burn is like at your normal flight rpm because that's where you spend most of your time and where it's more important to know what's going on.

If you want to get a real read on the plugs then run the engine up to at least 4K rpm or more and sit there for 5-10 minutes then immediately shut it down with no idling.

p.s.

You will not have dry black sooty plugs after any high rpm runs. This only happens at the lower idle rpms. This is normal and not a concern.  When you warm up the engine be up around 2500 + rpm especially in cold weather and not just because of plugs, but it will save your gearbox over the many hours and years. You are not stressing the engine at these warm up rpms.

Although very unlikely another source of "...unburnt fuel staining fuselage..." could be leakage of raw fuel from the mechanical fuel pump's vent outlet.  Many ROTAX-powered aircraft have the discharge tube for this vent located under or at the bottom of the fuselage near the firewall.  See if any of your fuel staining is associated with this discharge tube.  Again, unlikely but easy to check.