912 plugs

[barandbrew]

Just replaced the plugs on my 912 No. 1/2/3 clyinders light brown colour but No.4 were light grey suggesting lean mixture

 

Any idears????

 

John

 

 

[facthunter]

PLUG colour

 

Check for air leaks, manifold/head joint, Inlet valve guide wear (unlikely, but it happens) compression test cyls on that side. Listen for leaks on inlet valves thru carb.with throttle open and slide lifted. (Air cleaner or airbox hose off of course) When checking compression with air hose connected to cyl.,make sure the piston is on top dead centre, with both valves closed. Beware the prop. may move under the effect of the compressed air. so keep clear. N...

 

 

[barandbrew]

Thanks Facthunter

 

We have just completed a 100 hrly I didn't see any sign of leaks on the motor at all.Compression was simular on all four pots,. Would I see some residue of some sort if I had a leak in the intake manifold ? I would have thought the other plugs on that side would have shown the same condition if air was being sucked into the manifold?

 

Also do you believe this is a condition that could cause problems with the motor if we continue to use the aircraft while trying to sort it out ?

 

Thanks in advance

 

John

 

 

[facthunter]

plug colour

 

Barandbrew, I don't want to get you too worried, so howabout we go back to getting a VALID plug colour.

 

I would suggest,

 

a After flying your A/C, let the engine sit for about 5 minutes.

 

b In a suitable spot ( clear of aircraft & hangars ), restart your engine & run it with both chokes on (once started) to coat the plug insulators with soot (Lightly)Keep the engine at low revs, then

 

c Run the engine at full power for a couple of minutes,& shut it down without prolonged idling. Some theorists will think this will damage the motor due to thermal shock, but a small capacity motor such as this with seperate heads (watercooled ) I don't think it's a problem. If you're worried run it at high power for a shorter time

 

d Remove the plugs or One from each cyl. only and check for evenness of colour

 

Remember that once you have taxied for a while the plug responds to the engine idling situation.

 

The idle manifold vacuum is at a maximum, while the flow is at a minimum, so any leak will have most effect at idle. which won't hurt your engine,except to make it run rougher,.

 

The fault is not likely to transfer across to the other bank of cylinders,as it's not really a common system. The balance tube is very small

 

With Y branches such as in this system, some variations in mixture are difficult to eliminate. The high power plug check is the only one to take any notice of. N...

 

 

[Guest brentc]

I used to ask Wal at Bert Flood's about the colour of my plugs at which point I was assured that the rear cylinders run leaner and that I should not be at all concerned.

 

I do realise that it's hard to trust advice like that so it would be hard to know whether something is wrong or not.

 

A compression / leak-down test should guide you further if you are worried about the engine.

 

 

[facthunter]

Plugs

 

That is quite likely the case. The motor runs pretty wild cam timing & there is most likely interference N....

 

 

[barandbrew]

I have been told that the rear clyinders run leaner than the front too however only no4 looks lean . I will have to wait till the weekend then I will do as facthunter suggests . I will post the results next week

 

Many thanks again

 

John

 

 

[barandbrew]

Hi All

 

Completed the plug test on Saturday as facthunter suggested and the results were all plugs showing a nice light brown colour. rear plugs only slightly lighter( thank God )We have a fairly long taxi from the strip to the hanger so this could be why I had such differing results orginally.

 

Thanks again

 

John

 

 

[Guest Maj Millard]

Pretty damn good engines those 912s, just fly them a lot, and keep an eye on oil and plugs. No need to tinker with them much. Have flown the 912s with well over 2000hrs on them and they run just like a little turbine. Like most new Rotax engines they seem to have had some initial weaknesses (especially the 912 S, for a while there the S stood for suspect) but in true Rotax fashion they seem to have elimated the problems without admitting there was ever a problem. Carbs tend to require a rebuild kit around 700 hrs or so, they will richen up when due. Costs about $250-300 including diaphragms, and takes about 1/2 hour each carb to do, depending on how dirty your carbs are. Interesting that they put the turbo on the 80hp not the 100hp and made the 914. Like the 503 and 582 before them the 912 is another classic and should be around for a long time.

 

 

[markendee]

Maj,

 

100 at 10.5:1 is too high for turboing (is there such a word?)

 

80 at 9:1 just nice.

 

Mark

 

 

[Guest Maj Millard]

Yes thank you, why didn't I think of that. I'm sure that's the reason.

 

 

[phvdw]

hi,

 

has someone experience with injections systems on the 912S iof this old carbs?

 

 

[Guest Maj Millard]

Hey keep it simple, stick with the carbs, they give great reliable service if you set them up properly, and don't fiddle with them. Filters need to be cleaned periodically to maintain good service. I do feel that we will soon see fuel-injection on offer though. Unfortunatly all these u-beut things just keep upping the price of engines and aeroplanes, and I'm maxed out in that respect now. Also electronic fuel injection would be another electronic device to die suddenly, and expensivly, when you least expect it............