912 plug colour

[bones]

JUst got an email about the colour of the plugs in a 912;

 

"I removed the spark plugs for inspection and found them quite dirty with a sooty black coating"

 

Obviously i feel these are the result of running abit too rich as the egt's are low also, question is, what is the easiest fix smaller main jets or will dropping the needles fix this without the need for new main jets?

 

 

[Guest Maj Millard]

The 912 spark plugs can end up sooty and oily for a number of reasons, in my experience. If the engine is new (less than a 100 hrs) the rings may not have seated fully, and you could be getting oil up past them which is then burnt. They don't start to loosen up till about 50hrs and can take up to 100 before things settle down, and you get a good colour. Those Nikasil cylinders are tough as chrome, and can take a while to seat in, but once they do they'll last forever, or close to it .

 

Factory setting on the needle is second notch from the bottom. I have people say to me my EGT is great, only 850 F. Wrong, you are running too rich and burning unnecessary fuel, and your plugs will show it. Rotax recommends normal operating EGT at 1470F with a max at takeoff of 1620F. I like to run around 1250 F myself. Below that your just burning excess fuel.

 

Are you using a Rotax recommended oil ?. I have seen some non-recommended oils cause a sooty burn. Remember a small film of oil remains on the bore during normal conbustion and is burnt along with the fuel.

 

How about your idle mixture control screw, is it set per Rotax recommendations. If it is further out than recommended, it will cause a rich mixture.

 

Plugs ?, are they the correct type, and gap. And last but not least, are your air filters nice and clean. The 912 gulps a lot of air to produce 100 hp when it's up and running. If your not getting that air it'll run rich.You should not have to mess around changing main jet sizes on the 912 to get the right plug colour, this is a four stroke remember, not a two stroke.........................................................................Maj...

 

 

[BigPete]

That's 100% grade AAAA advice right there :thumb_up: (and it was FREE).

 

regards

 

 

[bones]

Yep thanks Maj, and yep was aware of most of it, just never thought about the new motor bit, and yep its only just done 25hrs so i'l just tell him to keep flogging it for another 75 then look again, the egt were abit low from memory, but this is not too much of a concern yet, we will just see how it goes for awhile, thanks again.

 

Or nd that 914 machine be in my hangar next week endso it be down your way soon i guess.

 

 

[eastmeg2]

If your engine's at 25hrs, make sure you nip up all your coolant hose clamps as part of your 25hr service. I was really suprised how loose mine had become when I did mine.

 

 

[Guest Maj Millard]

Thanks Big Pete.......................................................................Maj...

 

 

[facthunter]

Reading plug colours.

 

It should be borne in mind that you are getting readings relating to the last bit of operation of the engine. This is usually a descent and sometimes LONG taxi where the engine internally is relatively cool and the mixture(s) are richer. To get a representative reading the engine would have to be run at high power for a fair while and the engine "cut". You then read the colour of the ceramic insulators. I have never seen this done in an aircraft situation as it obviously would be impossible to arrange in flight and most people wouldn't want to subject the engine to shock cooling. It doesn't take long to get a soot coating on plugs when the mixture is rich at idle. (as it always is). Nev

 

 

[Outbackflyer]

Are you reaching the recomended oil and water temps each flight? low opperating temps can cause a lower combustion temp/ dark plug colour too.

 

That oil needs to reach 100c once per day to cook off all the moisture these engines produce in the oil system. FYI.

 

Cheers John.

 

 

[bones]

Yep thanks been doing that

 

 

[pylon500]

OK, a quick one then.

 

I'm running an 80hp 912 on 100LL, with good temps indicated, but have recently been having some starting problems.

 

Unsure if I'm flooding or still dry, I've removed plugs to inspect, and found them to have a noticeable yellow colour?

 

Didn't think to photograph.

 

Is this typical of 100LL?

 

 

[Guest Maj Millard]

If your running a lot of 100LL you'll get a bit of lead build up on plugs. It's sort of a whitish yellow colour. Just need to clean plugs a bit more often, otherwise it won't cause you too much bother.............................................................................Maj...

 

 

[pylon500]

Why don't you use MOGAS unleaded?

It's not my plane and used to be VH registered.

 

It also comes with a fuel card

 

It's sort of a whitish yellow colour.

I guess that's my answer, it did start better after the plugs were cleaned.

 

Doing more flying lately, will have to clean more regularly.

 

Arthur.

 

 

[Outbackflyer]

Hi, sorry for the long delay in replying, I have just returned from collecting a microlight in Qld,

 

The yellow colour is common on the 912 running 100LL fuel, I think it is a reaction / residue that favours the porcelain part of the plug , I don’t think it indicates any abnormality.

 

Try running the minimum (cold weather) gap, as per the service manual.

 

There are several things you can check re the poor starting, a full dismantle and servicing of the carbs is a good start,( float levels?) then synchronising the carbs, doing this ensures that you are starting from the right place.

 

Make sure the bowl vents are not obstructed, you can check this by putting your mouth to the open end of the small tube that is secured alongside the bowl retaining clip and blowing on it for 5 seconds, you will notice fuel being discharged from the vent port at the bottom of the carb intake, (no fuel? then its blocked)

 

During servicing the “choke carburettor” should be cleaned and new O rings fitted there is often grime / moisture in there, take care to re fit the choke assembly correctly.

 

Its usually the carbs, hope this helps.

 

 

[pylon500]

Good info, thanks :thumb_up:

 

Arthur.

 

 

[bones]

getting back to this thread, will dropping the needles, raise the egts, or do you need to change the main jets?

 

 

[Guest Maj Millard]

The needle positions on the jet-needle tend to effect the amount of fuel throughout the engines mid-range. (2nd from bottom is recommended posn). Once the needle is out of the needle-jet (around 3/4 throttle on up) it's the main jet alone that meters the amount of fuel for full power. Changing the needle clip position will have minimal effect on EGTs.

 

The exhaust gas temperature indicates the combustion temperature, and is used to show if the fuel/air mix is correct. The best fuel/air mix often referred to as the 'sthionotich' mix is around 12-14 parts air, to one part fuel.

 

Too much fuel your EGTs will read low. Too much air they will read high.

 

Also as the various parts in the 912 carb wear, the tendancy is for the mixture to richen up.

 

Have you checked the condition of your air-cleaners lately ??...............Maj...

 

 

[bones]

Maj,

 

I only getting about 800* sposed to be around 900*, with 950* max, so im using a bit more fuel than i could be, though i wont worry about it for awhile till the motor gets over 50 odd hrs.

 

I figured as much thou it was free to ask thanks :)

 

Need to go for a fly again to just enjoy flying, sick of going round and round and round, startiing to understand how a student can get sour i guess.