Rotax 912ULS exhaust pipe colour ? (MOGAS only)

[RFguy]

What is the range of exhaust outlet  pipe colour and characteristics of powder/dust people see on their well behaving Rotax 912ULS running MOGAS ? (and flying at mostly less than 5500') 

 

Glen

:-)

 

 

[Yenn]

What you will see is not necessarily what happens at 5000'

You are seeing what is deposited at idling, taxying speeds, unless you cut the engine at 500'.

[skippydiesel]

Hi Glen - basically black. If you squint, one might suggest, very dark brown. Texture - a little sooty. As for the altitude  - I fly from 3 -10 thousand depending on forecast winds, cloud,  terrain/ and controlled airspace clearance. Only exception is an occasional beach run at 500 ft.

[RFguy]

Yenn-- agreed, what I meant was all ops < 5000'

 

Hi Sean, ok basically black. little brown..

After float replacement, readjustment, it was still a bit rich. front pair of cylinders fouled, rear set looked OK. according to forums, seems a common problem. due to low gas velocity at 1500-2500, front set of cylinders (from different carbies) goes rich ..... previous person before me must have sat it idling for ages, and apparently lengthy ground  idling is a bad idea (I read) for this carb setup on the rotax). No problem at usual flying RPMs. 

float levels got readjusted, problem mostly gone away, . when flying solo, 30% fuel, my descent rpm is quite LOW (2500-2700)  and a little roughness begins to appear on successive circuits at same location (low RPM in late base) . full rpm blast fixes it. needs a bit more of a tweak.

making static now though... 

 

Oh what a difference  sanding the back of the prop smooth made. (LAMEs) WOW its like I have an extra 300 rpm on the prop. WOW !

Like a different aeroplane !

 

 

 

 

 

 

 

Edited September 29, 2020 by RFguy

[kgwilson]

My comments relate to the Jab 3300.

 

Running with Mogas the plug porcelain insulator stays quite white and the electrodes black. The exhaust is always black, a bit sooty in fact. This is after landing and then taxiing back to the hangar at idle speed for about 5 minutes. Part of the cooling of a Jab engine is from fuel with the mixture fairly rich. If Avgas is used the exhaust can vary from brown to black but less sooty and the plugs are usually brown plus dark grey lead deposits. The same type of Bing carburettor is used but the 3300A has one whereas the 912 ULS has 2 & if they are tuned properly both sides should be the same.

 

Mogas 98 RON does keep the pistons, valves and heads much cleaner though and IMHO way better than Avgas. Results may be different if you are using Mogas 91 RON

[RFguy]

Hi KG. OK on all that.  right. OK so MOGAS doesn't generate a nice grey exhaust due to the lack of really good mixture control by the systems. 

 

So a piece of the puzzle revealed.

did 1.6 yesterday late arvo, 3.3 today :

 

The choke is working its way out !!!! about 2mm per minute and gets quite active !!   

push it back in, power increases.

 

no wonder this richness is a problem (remember this is after carb float replacement in the rotax) - system disturbed. ha ha the old thing, new faults from maintenance.

.

I will pull the cowling off in the morning before tomorrows flight and just check nothing is really bad happening to the bowden cable...... likely a spring. Its OK, I'll have the total focus of the LVA there ! lots of cooks.

 

-Glen

Edited September 29, 2020 by RFguy

[tillmanr]

Mogas will not give a grey indication due to the lack of lead additives.

[RFguy]

mmmm all my ICEs for motor vehicles I have seen  (admitidly EFI) are all mid grey exhausts,

[Thruster88]

Just now, RFguy said:

mmmm all my ICEs for motor vehicles I have seen  (admitidly EFI) are all mid grey exhausts,

My Brumby (ute) with a carb ea81 is always black😎

[M61A1]

3 hours ago, RFguy said:

The choke is working its way out !!!! about 2mm per minute and gets quite active !!   

push it back in, power increases.

Normally Rotax 912 Bing carbs have a return spring that prevents this from occurring.

I have seen some that didn't return fully due to poor design, but never ones that creep out.

You may not notice the difference much at high power settings, but it will idle poorly, foul plugs, and use significantly more fuel.

Unless it was excessively lean in the top end to begin with, it will also be down on power.

It should be rectified immediately.

 

[kgwilson]

Years ago before the advent of aromatic laced 98 octane, a car exhaust would be light grey to almost white after a long run if the engine was well tuned. Now the exhaust is almost always black.

[facthunter]

The choke set up on a Bing works only at near throttle closed. It's an enrichened by pass system. The white exhaust on motors with lead in the fuel is white (when lean enough) due to a bromine additive used to remove most of the build up. It like lead is toxic so maybe a bit less bromine these days. Nev